Patent Application
20130285864
1. Field of the Invention
The present invention relates to wireless communication systems, in particular, to an antenna and communications subsystem that may be used on passenger vehicles.
2. Discussion of Related Art
Many communication systems involve reception of an information signal from a satellite. Conventional systems have used many types of antennas to receive the signal from the satellite, such as Rotman lenses, Luneberg lenses, dish antennas or phased arrays. However, these systems may suffer from limited field of view or low efficiency that limit their ability to receive satellite signals. In particular, these conventional systems may lack the performance required to receive satellite signals where either the signal strength is low or noise is high, for example, signals from low elevation satellites.
In addition, many conventional systems do not include any or sufficient polarization correction and therefore cross-polarized signal noise may interfere with the desired signal, preventing the system from properly receiving the desired signal. Further, locating such systems on a fuselage of an aircraft for transmission or reception of signals poses a number of issues that must be addressed for such systems.
There is therefore a need for an improved communication system, including an improved antenna system, which may be able to receive weak signals or communication signals in adverse environments, and which can be located at least partly on the fuselage of an aircraft.
Aspects and embodiments are directed to a communications system including an antenna array and electronics assembly that may be mounted on and in a vehicle. The communication system may generally comprise an external subassembly that is mounted on an exterior surface of the vehicle, and an internal subassembly that is located within the vehicle, the external and internal subassemblies being communicatively coupled to one another. As discussed below, the external subassembly may comprise the antenna array as well as mounting equipment and steering actuators to move the antenna array in azimuth, elevation and polarization (for example, to track a satellite or other signal source). The internal subassembly may comprise most of the electronics associated with the communication system. Locating the internal subassembly within the vehicle may facilitate access to the electronics, and may protect the electronics from the environment exterior to the vehicle, as discussed in further detail below. Embodiments of the communication system provide numerous advantages over prior art systems, including being of relatively small size and weight (which may be particularly advantageous for a system mounted on an aircraft), and having excellent, broadband RF performance, as discussed further below.
According to one embodiment, an antenna array comprises a plurality of horn antenna elements, a corresponding plurality of dielectric lenses, each dielectric lens of the plurality of dielectric lenses being coupled to a respective horn antenna element of the plurality of horn antenna elements, and a waveguide feed network coupling the plurality of horn antenna elements to a common feed point, wherein the plurality of horn antenna elements and corresponding plurality of dielectric lenses are shaped and sized such that the antenna array is tapered at either end of the antenna array.
In one example, the plurality of horn antenna elements are arranged in one or more parallel rows, wherein, in examples where there are two or more rows, the parallel rows may be offset from one another along the length of the antenna array by one half the width of one of the plurality of horn antenna elements. In another example, the plurality of horn antenna elements may include an interior horn antenna element, a third horn antenna element, a second horn antenna element, and an end horn antenna element, wherein the third horn antenna element is smaller than the interior horn antenna element and is located closer to an end of the antenna array than the interior horn antenna element, wherein the second horn antenna element is smaller than the third horn antenna element and is located closer to the end of the antenna array than the third horn antenna element, and wherein the end horn antenna element is smaller than the second horn antenna element and is located at the end of the antenna array. In another example, the plurality of dielectric lenses elements may include an interior dielectric lens, a third dielectric lens, a second dielectric lens, and an end dielectric lens, wherein the interior dielectric lens is coupled to the interior horn antenna element, wherein the third dielectric lens is smaller than the interior dielectric lens and is coupled to the third horn antenna element, wherein the second dielectric lens is smaller than the third dielectric lens and is coupled to the second horn antenna element, and wherein the end dielectric lens is smaller than the second dielectric lens and is coupled to the end horn antenna element. The antenna array may further comprise a plurality of horn inserts, each one of the plurality of horn inserts being located within a respective one of the plurality of horn antenna elements. In one example, the horn inserts located within the end horn antenna element and the second horn antenna elements are made of a radar absorbent material. In another example, each dielectric lens is fastened to the respective horn antenna element with a fiberglass pin.
Another aspect is directed to a method of calibrating a vehicle-mounted antenna array. In one embodiment, the method comprises determining an RF center of a beam pattern of the antenna relative to a location of a position encoder mounted on the antenna array or gimbal assembly, calculating a first pitch offset and a first roll offset of the antenna array, gimbal assembly or other component of the external sub-system, relative to the location of the position encoder, and storing the calculated first pitch and roll offsets in a local memory device. In another embodiment, the method further comprises receiving data representative of a vehicle pitch and vehicle roll of a host vehicle upon which the antenna array is mounted, sensing with the position encoder, an antenna pitch and antenna roll, calculating an second pitch offset between the vehicle pitch and the antenna pitch, calculating a second roll offset between the vehicle roll and the antenna roll, and storing the calculated second pitch and roll offsets in the local memory device. In one example, method further comprises storing the calculated second pitch and roll offsets in a remote memory device. In another example, the method further comprises correcting the second pitch and roll offsets based on the first pitch and roll offsets, and storing the corrected second pitch and roll offsets in the local memory device. The method may further comprise storing the corrected second pitch and roll offsets in the remote memory device. In one example, the method further comprises receiving data representative of a vehicle heading of the host vehicle, pointing the antenna array at a selected satellite signal source, determining an antenna heading based on a signal lock with the selected satellite signal source, calculating a heading offset between the vehicle heading and the antenna heading, and storing the heading offset in the local memory device. The method may further comprise storing the heading offset in the remote memory device. In one example, receiving data representative of the vehicle pitch and vehicle roll of the host vehicle includes receiving the date from a navigation system in the host vehicle.
According to another embodiment, a communications system comprises a first sub-system comprising an antenna array configured to receive and transmit signals, a gimbal assembly configured to mount the antenna array a host platform and to move the antenna array in azimuth and elevation, a first memory device, and at least one position encoder mounted to the antenna array, and a second sub-system communicatively coupled to the first sub-system and comprising a second memory device, and a control unit configured to control movement of the antenna array in azimuth and elevation, wherein the at least one position encoder is configured to detect a pitch and roll of the antenna array relative to a factory-calibrated level position of the antenna array and to provide a first antenna data signal representative of the detected pitch and roll of the antenna array, wherein the first and second memory devices are communicatively coupled together and are configured to receive and store the antenna data signal. In one example, the first and second memory devices are further configured to store identifying information about the first and second sub-systems.
According to another embodiment, a vehicle-mounted communications system comprises an external sub-system mounted to an exterior surface of the vehicle, the external sub-system comprising an antenna array configured to receive and transmit signals, a gimbal assembly configured to mount the antenna array to the vehicle and to move the antenna array in azimuth and elevation, a local memory device, and at least one position encoder mounted to the antenna array, and an internal sub-system communicatively coupled to the first sub-system and comprising a control memory device, and a control unit configured to control movement of the antenna array in azimuth and elevation, wherein the control unit is configured to receive data representative of a pitch and roll of the vehicle upon which the antenna array is mounted, wherein the position encoder is configured to sense a pitch and roll of the antenna array, wherein the control unit is configured to calculate a pitch offset between the pitch of the vehicle and the pitch of the antenna and a roll offset between the roll of the vehicle and the roll of the antenna, and wherein the control memory device is configured to store the calculated pitch and roll offsets.
In one example, the local memory device is configured to store the calculated pitch and roll offsets. In another example, the local and control memory devices are further configured to store identifying information about the internal and external sub-systems.
Another aspect is directed to a communications system comprising an antenna array including a plurality of antenna elements each adapted to receive an information signal from a signal source, and a feed network coupling the plurality of antenna elements to a common feed point, and a polarization converter unit coupled to the common feed point, the polarization converter unit configured to compensate for polarization skew between the antenna array and the signal source. In one embodiment, the polarization converter unit comprises a rotary orthomode transducer configured to receive two orthogonally polarized component signals making up the information signal and to provide a polarization-corrected output signal, a drive system coupled to the rotary orthomode transducer configured to receive a control signal representative of a desired degree of rotation of the rotary orthomode transducer, and a motor configured to provide power to the drive system to rotate the rotary orthomode transducer to the desired degree of rotation.
In one example, the polarization converted unit is mounted to the antenna array. In another example, the plurality of antenna elements and the feed network are arranged to provide a cavity between the feed network and the plurality of antenna elements, wherein the polarization converter unit is mounted at least partially within the cavity. In another example, the plurality of antenna elements are horn antenna elements, and the feed network is a waveguide feed network.
According to one embodiment, an antenna array comprises a plurality of horn antenna elements, a corresponding plurality of dielectric lenses, each dielectric lens of the plurality of dielectric lenses being coupled to a respective horn antenna element of the plurality of horn antenna elements, and a waveguide feed network coupling the plurality of horn antenna elements to a common feed point, wherein each dielectric lens is a plano-convex lens having a planar side and an opposing convex side, wherein each dielectric lens comprises a plurality of impedance matching features formed proximate an interior surface of the convex side, and wherein an exterior surface of the convex side is smooth.
In one example, the plurality of impedance matching features includes a plurality of hollow tubes. In another example, each dielectric lens further comprises a plurality of impedance matching grooves extending from a surface of the planar side into an interior of the dielectric lens. The plurality of dielectric lenses may comprise, for example, a cross-linked polystyrene material or, for example, Rexolite™.
In another embodiment, an antenna array comprises a plurality of horn antenna elements configured to receive an information signal, a corresponding plurality of orthomode transducers, each respective orthomode transducer coupled to a respective horn antenna element and configured to split the information signal into a first component signal and second component signal, the first and second component signals being orthogonally polarized, and a waveguide feed network coupling the plurality of orthomode transducers to a common feed point, the waveguide feed network configured to sum the component signals from each orthomode transducer in both the E-plane and the H-plane.
In one example, the waveguide feed network comprises a first path to guide the first component signal and a second path to guide the second component signal, wherein the first path sums in the E-plane the first component signals received from each orthomode transducer, wherein the second path sums in the H-plane the second component signals received from each orthomode transducer, and wherein the waveguide feed network is configured to provide at the common feed point a first summed component signal and a second summed component signal. In another example, the plurality of orthomode transducers comprises a first orthomode transducer coupled to a first horn antenna element and a orthomode transducer coupled to a second horn antenna element, wherein the waveguide feed network includes a waveguide T-junction having a first input configured to receive the first component signal from the first orthomode transducer and a second input configured to receive the first component signal from the second orthomode transducer, and an output configured to provide an output signal corresponding to a weighted sum of the two first component signals, and wherein the waveguide T-junction comprises a tuning element configured to bias the waveguide T-junction to produce the weighted sum of the two first component signals.
Another aspect is directed to a communications system mountable on a vehicle. In one embodiment, the communications system comprises an external sub-system, mountable on an exterior surface of the vehicle, comprising an antenna array configured to receive and transmit information signals, and a gimbal assembly configured to mount the antenna array to the exterior surface of the vehicle and to move the antenna array in azimuth and elevation, and an internal sub-system, mountable within the vehicle, comprising a control unit and a transceiver, the internal sub-system communicatively coupled to the external sub-system and configured to provide power and control signals to the external sub-system, wherein the control unit is configured to provide the control signals to the gimbal assembly to control the movement of the antenna array in azimuth and elevation, wherein gimbal assembly comprises a mounting bracket configured to mount the external sub-system to the exterior surface of the vehicle, an antenna mounting bracket configured to mount the antenna array to the gimbal assembly.
In one example of the communications system the mounting bracket comprises a central portion and four feet connected to the central portion by four corresponding arm portions; and wherein each of the four feet is positioned outside of a rotational sweep of the antenna array. In another example, the external sub-system further comprises a rotary joint positioned inside the central portion of the mounting bracket, the rotary joint coupling the external sub-system to the internal sub-system. In another example, the antenna mounting bracket grips the antenna array at two locations along the length of the antenna array, neither point being at an end of the antenna array. In another example, the gimbal assembly comprises an elevation drive assembly configured to receive a control signal from the control unit and to rotate the antenna array in elevation responsive to the control signal. The elevation drive assembly may include a push-pull pulley system. In another example, the gimbal assembly further comprises a polarization converter unit mounted to the antenna array and configured to move the antenna array in polarization responsive to a polarization
Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of various aspects and embodiments. Any embodiment disclosed herein may be combined with any other embodiment in any manner consistent with at least one of the objects, aims, and needs disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification.
Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. Where technical features in the figures or detailed description are followed by references signs, the reference signs have been included for the sole purpose of increasing the intelligibility of the figures and detailed description. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures:
Aspects and embodiments are directed to a communication system including an antenna array and electronics subassembly that may be mounted on and in a vehicle. The communication system may generally comprise an external subassembly that is mounted on an exterior surface of the vehicle, and an internal subassembly that is located within the vehicle, the external and internal subassemblies being communicatively coupled to one another. As discussed below, the external subassembly may comprise the antenna array as well as mounting equipment and steering actuators to move the antenna array in azimuth, elevation and polarization (for example, to track a satellite or other signal source). The internal subassembly may comprise most of the electronics associated with the communication system. Locating the internal subassembly within the vehicle may facilitate access to the electronics, and may protect the electronics from the environment exterior to the vehicle, as discussed in further detail below. Embodiments of the communication system provide numerous advantages over prior art systems, including being of relatively small size and weight (which may be particularly advantageous for a system mounted on an aircraft), and having excellent, broadband RF performance, as discussed further below.
It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiments. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to embodiments or elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality of these elements, and any references in plural to any embodiment or element or act herein may also embrace embodiments including only a single element. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, and upper and lower are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation.
Referring to
According to one embodiment, the internal sub-system 104 also comprises a power supply 118 that provides power to the various components of the internal sub-system 104 as well as to the external sub-system 102. It is to be appreciated that the power supply 118 may include a dedicated power supply that is part of the internal sub-system 104, or may include any necessary components to convert and supply power from the host vehicle's power supply to the components of the internal sub-system that require power. The internal sub-system 104 may further comprise a network management server 120. An inertial navigation reference system 122, which may be part of the internal sub-system 104 or separate therefrom and in communication therewith, may provide navigation data from the vehicle in which the communication system is installed, as discussed further below.
Referring to
According to one embodiment, the communication system is mounted on and in a vehicle, such as an aircraft or automobile. Referring to
As known to those familiar with the operation of satellites in many regions of the world, there exists a variety of satellites operating frequencies resulting in broad bands of frequency operations. Direct Broadcast satellites, for example, may receive signals at frequencies of approximately 14.0 GHz-14.5 GHz, while the satellite may send down signals in a range of frequencies from approximately 10.7 GHz-12.75 GHz. Table 1 below illustrates some of the variables, in addition to frequency, that exist for reception of direct broadcast signals, which are accommodated by the antenna assembly and system of the present invention. The signal source 110 may include any of these, or other, types of satellites.
Still referring to
Referring to
The antenna array 106 may be mounted to the gimbal assembly 108 using an antenna mounting bracket 208. As illustrated in
Still referring to
According to one embodiment, the external sub-system may be covered by a radome that may serve to reduce drag force generated by the external subassembly as the vehicle/aircraft 132 moves. An example of a radome 202 is illustrated in
Referring to
Referring to
As discussed above, the gimbal assembly 108, and external sub-system 102, may be organized to mount to a host vehicle (or other host platform) and therefore may include a mounting bracket 214. An example of the mounting bracket 214 is illustrated in
The mounting bracket 214 may be fastened to the vehicle by fasteners, such as screws or bolts, through the feet 224. Referring to
Still referring to
Another view of an embodiment of the mounting bracket 214 is illustrated in
A perspective view of one example of the mounting bracket 214 is illustrated in
According to one embodiment, the mounting bracket 214 is constructed to attach to the host vehicle using four attachment pads. An exploded view of one example of a portion of the mounting bracket 214 and an attachment pad 232 is illustrated in
According to one embodiment, at least portions of the external sub-system 102 (e.g., the antenna array 106 and at least some parts of the gimbal assembly 108) are moveable in any or all of elevation, azimuth and polarization to facilitate communication with the signal source 110 from a plurality of locations and orientations of the vehicle. Accordingly, the gimbal assembly 108 may be designed to accommodate such movement. According to one embodiment, the gimbal assembly 108 is constructed to rotate in the azimuth axis about an axis or rotation which coincides with the center of the mounting bracket 214. In one embodiment, the central portion 220 of the mounting bracket 214 may accommodate a hub feature, also called an azimuth assembly 402, which defines the center of azimuth rotation, and which is used to interconnect the gimbal assembly 108 to one or more bearings to enable rotation in the azimuth axis. The azimuth assembly 402 may include, for example, a rotary joint that may penetrate the vehicle shell (e.g., the shell of aircraft 132) to allow cables to pass through the vehicle shell between the internal sub-system 104 and the external sub-system 102. In one example, the azimuth assembly 402 may include the rotary joint and a slip ring, as discrete parts or as an integrated assembly 446. The axis of rotation also is coincident with the axis of rotation of the rotary joint and the slip ring, shown in
According to one embodiment, the gimbal assembly 108 provides control signals to move the antenna array 106 over a range of angles in azimuth and elevation to perform beam-steering and signal tracking. Referring again to
To move the antenna array 106 in azimuth, the azimuth motor drive 408 may be coupled to the azimuth hub assembly 402. In one example, the azimuth hub assembly 402 is coupled, via a wire 412, to an azimuth pulley 428 that encircles the central portion 220 of the mounting bracket 214. The azimuth motor drive 408 may also include control circuitry and may receive control signals from the antenna control unit 112 (see
According to one embodiment, the elevation motor drive 404 is coupled via a flexible coupling 414 to the elevation motor 406. In one example, using flexible couplings, such as flexible coupling 414, to interconnect various components may add to the ease of manufacture of the external sub-system 102 by absorbing tilt and/or angle tolerances in connections and removing or reducing strain on the connections. The elevation motor 406 is mounted to an elevation motor support 416 and may be housed within housing 418. In the illustrated example, mechanical elevation drives 420a and 420b are coupled to the antenna mounting bracket 208 and are mounted to the azimuth hub assembly 402, thereby mechanically coupling the antenna array 106 to the azimuth drive system. As shown in
Referring to
According to one embodiment, the elevation drive system uses a pulley system to move the antenna array 106 in elevation. An example of a push and pull pulley system is illustrated schematically in
Referring to
In one embodiment, the mounting bracket 214 is attached to the gimbal assembly 108 along a center of rotation normal to the azimuth plane. The structure of the gimbal assembly 108 supports the antenna assembly including the antenna array 106 and low-noise amplifiers, and may also support a polarization converter unit (PCU) 128, as discussed further below. The gimbal assembly 108 may include a frame 442 which may provide support for various components of the gimbal assembly 108 as well as providing handles or lifting points for the gimbal assembly. In one embodiment, the antenna assembly is mounted on one side of the aforementioned centerline of the mounting bracket 214 and gimbal assembly 108, and mounted on the opposite side of the centerline is the azimuth motor, the drive train of the azimuth motor drive 408, the elevation motor 406, the elevation motor drive mechanism 404, and a gimbal connector unit 444 along with its associated cabling. In this embodiment, the weight of the entire external sub-system 102 is distributed, to the extent possible, by locating equipment about the azimuth axis of rotation. In another embodiment, the slip ring and rotary joints each rotate concentric to the azimuth axis of rotation and are each located above the mounting bracket 214 and are supported by the structure of the gimbal assembly 108. Other embodiments of the system permit the distribution of electronics to be supported by the structure of the gimbal assembly 108, such as, but not limited to, the polarization control unit, for example, as discussed further below.
Referring again to
As discussed above, the gimbal assembly may transfer signals, via cables, between various components of the internal sub-system 104 and the antenna array and/or other components of the external sub-system 102. In one embodiment, the mounting bracket 214 is configured with cable routing troughs and clamps to provide an efficient mechanism for routing cables between the internal sub-system 104 (via the central portion 220 of the mounting bracket) and components of the external sub-system 102. The cable routing mechanism incorporated into the mounting bracket 214 may minimize holes in the host platform (e.g., in the fuselage of aircraft 132) and maintain a horizontal relationship of RF and control cabling, as discussed further below.
Referring to
In one example, the conduit 244 is attached to the leg 222 using a clamp 246. In the example illustrated in
Referring to
According to one embodiment, the mounting bracket 214 may be formed with various grooves, indentations, channels, cavities and/or troughs to accommodate various components of the gimbal assembly 108 or external sub-system 102. Referring to
Thus, referring to
As also illustrated in
As discussed above, according to one embodiment, the antenna array 106 comprises a plurality of antenna elements 308, such as horn antennas (see
Referring to
In general, each horn antenna element 308 may receive incoming electromagnetic radiation though an aperture 310 defined by the sides 313 of the antenna element, as shown in
As discussed above, according to one embodiment, the external sub-system 102 may be mounted on a vehicle, such as an aircraft 132 as illustrated in
One result of the use of low-height, wide aperture horn antennas as the antenna elements 301 is that the antenna elements may have a lower gain than might be preferable. This lower gain results because, as shown in
According to one embodiment, the antenna array 106 is tapered to further facilitate sidelobe reduction in the beam pattern of the antenna array. In one example, the outer three horn antenna elements 301 at each end of each row of antenna elements are smaller than the remaining antenna elements, which may be substantially identical in size and shape. In embodiments of the antenna array 106 that include dielectric lenses 304, the dielectric lenses 304 associated with these tapered horn antenna elements 301 may be correspondingly smaller than the lenses associated with the remaining antenna elements. This tapering of the antenna array 106 can be seen with reference to
As discussed above, some embodiments of the tapered antenna array 106 may include any of one to eight rows of 32 antenna elements 308, in one example, horn antenna elements. For example, the antenna array 106 may include a 1×32, 2×32, 3×32, 4×32, 5×32, 6×32, 7×32 or 8×32 array. In some examples, the number of tapered elements may vary depending on the number of rows of antenna elements 308 in the array, and on the number of antenna elements per row. It is to be appreciated that although some currently preferred embodiments use rows of 32 elements, other numbers of elements per row may be used.
As discussed further below, in some applications, such as where the communication system is mounted on an aircraft 132, the antenna array 106 may experience large variations in environmental conditions such as temperature, humidity and pressure. These changing conditions can cause moisture to collect on and in the various components of the antenna array 106, which can have an adverse effect the performance of the antenna array. Accordingly, in one embodiment, horn inserts 382 are placed inside the horn antenna elements 301 to prevent moisture from collecting inside the horn antenna elements. In one embodiment, the horn inserts 382 are made from an extruded polystyrene insulation. In another example, the horn inserts are made of Styrofoam. However, it will be appreciated by those skilled in the art that a variety of other materials may be suitable. In embodiments of the antenna array 106 that include dielectric lenses, the horn inserts 382 are placed inside at least some of the horn antenna elements 301, beneath the dielectric lenses 304.
Referring to
As discussed above, in one embodiment, the antenna array 106 is tapered, having smaller antenna elements 308 near the edges of the array, to reduce sidelobes in the beam pattern of the array. The smaller horn antenna elements 324, 326 and 330 have lower signal amplitude and contribute less than do the interior horn antenna elements 322 to the overall signal received or transmitted by the array. By appropriately sizing these antenna elements 324, 326 and 330 the signal contribution from these elements, and therefore the beam pattern of the antenna array can be adjusted to reduce sidelobes. In embodiments of the antenna array that include dielectric lenses, the dielectric lenses 318, 328 and 332 associated with the smaller antenna elements 324, 326 and 330 may be similarly smaller in size. In addition, as discussed further below, the feed network 302 can be designed to weight the signal contribution from different antenna elements 308 differently, thereby further controlling the beam pattern of the antenna array 106 and reducing sidelobes. In one example, horn inserts 382 may also be constructed to facilitate sidelobe suppression. For example, the horn inserts 382 for some or all of the outer horn antenna elements 324, 326 and 330 may be made from a radar absorbent material (RAM) to further attenuate the signal contribution of these antenna elements. Selected ones of the horn inserts 382 in the interior horn antenna elements 322 may also be made of RAM to further control the beam pattern.
Sidelobe reduction may be advantageous for several reasons including, for example, to improve the gain of the antenna array (having lower sidelobes means that more energy is captured in the main, useful, lobe of the antenna radiation pattern), and to meet certain performance goals and/or regulations (e.g., the FAA may set specifications for sidelobe suppression for applications such as satellite television or radio). For applications in which the antenna array 106 is mounted on a vehicle, such as an aircraft, the effect of the vehicle's movement on the antenna beam pattern may also be taken into account. For example, when the antenna array 106 is mounted on an aircraft 132, the beam pattern should be such that it meets sidelobe specifications (set, for example, by the FAA or other international authorities or regulations) not only when directly aligned with the signal source 110, but also when there is a polarization offset between the antenna array and the signal source due to movement of the aircraft. Thus, any or all of the size, shape, and arrangement (including taper and spacing) of the antenna elements 308, and optionally associated dielectric lenses 304 and/or horn inserts 382, and the arrangement of the feed network (discussed below), may be controlled to facilitate producing a beam pattern that meets sidelobe suppression standards for various orientations (polarization offsets) of the antenna array relative to the signal source or destination.
Referring again to
Referring to
Additional beam patterns for an embodiment of the antenna array 106 at various frequencies and with varying degrees of polarization offset, up to +35 degrees or −35 degrees, are illustrated in
As discussed above, in some embodiments, the antenna array 106 includes dielectric lenses 304 to enhance the gain of the array. According to one embodiment, the dielectric lenses 304 are plano-convex lenses that may be mounted above and/or partially within the horn antenna aperture 310. For the purposes of this specification, a plano-convex lens is defined as a lens having one substantially flat surface and an opposing convex surface. The dielectric lens 304 may be shaped in accordance with known optic principals including, for example, diffraction in accordance with Snell's Law, so that the lens may focus incoming radiation to the feed point 305 of the horn antenna element 301.
Referring to
Reflections of the signal incident on the convex surface 338 of the dielectric lens 320 may typically result from an impedance mismatch between the air medium and the lens medium. The characteristic impedance of free space (or dry air) is known to be approximately 377 Ohms. For the dielectric lens 304, the characteristic impedance is inversely proportional to the square root of the dielectric constant of the lens material. Thus, the higher the dielectric constant of the lens material, the greater, in general, the impedance mismatch between the lens and the air. The dielectric constant of the lens material is a characteristic quantity of a given dielectric substance, sometimes called the relative permittivity. In general, the dielectric constant is a complex number, containing a real part that represents the material's reflective surface properties, also referred to as Fresnel reflection coefficients, and an imaginary part that represents the material's radio absorption properties. The closer the permittivity of the lens material is relative to air, the lower the percentage of a received communication signal that is reflected.
The dielectric material of the dielectric lenses 304 may be selected based, at least in part, on a known dielectric constant and loss tangent value of the material. For example, in many applications it may be desirable to reduce or minimize loss in the antenna array 106 and therefore it may be desirable to select a material for the lens having a low loss tangent. Size and weight restrictions on the antenna array 106, at least in part, determine a range for the dielectric constant of the material because, in general, the lower the dielectric constant of the material, the larger the lens may be. In some applications, it may be desirable to manufacture the dielectric lenses 304 from a material having a relatively high dielectric constant in order to reduce the size and weight of the lens. However, reflections resulting from the impedance mismatch between the lens and the air may be undesirable.
Accordingly, in one embodiment, the dielectric lenses 304 have impedance matching features formed in either or both of the convex surface 338 and the planar surface 336. Referring again to
The magnitude of the reflected signal may be significantly reduced by the presence of impedance matching features at the lens surfaces. With the impedance matching holes 340, the reflected signal at the convex surface 338 may be decreased as a function of ηn, the refractive indices at each boundary, according to equation 1 below:
A further reduction in the reflected signal may be obtained by optimizing the diameter of the holes 340 such that direct and internally reflected signals add constructively. In one example, the holes 340 are substantially similarly sized and have a diameter of about 0.129 inches.
It is to be appreciated that although the above discussion of the impedance matching features of the dielectric lens referred primarily to the interior dielectric lenses 320, the discussion applies equally to the tapered dielectric lenses 318, 328 and 332. The number of impedance matching holes 340 and/or impedance matching grooves 342 formed in each of the tapered lenses 318, 328 and 332 may vary with respect to the interior dielectric lenses 320 due to the smaller size and altered shape of the tapered lenses 318, 328 and 332. In addition, the “groove pocket” or area of the planar surface 336 in which the impedance matching grooves 342 are formed may be smaller for the smaller lenses, as discussed further below. Referring to
Referring to
Referring again to
The lenses may be created by, for example, milling a solid block of lens material and thereby forming the convex-plano lenses. The impedance matching holes 340 and/or grooves 342 may be formed by milling, etching, or other processes known to those skilled in the art. It is to be appreciated that the terms “holes” and “grooves” are merely exemplary and are not intended to be limiting in terms of the shape or size of the features.
It is to be appreciated that there are numerous variations for the size, shape and structural features of the dielectric lenses 304 and the invention is not limited to the use of dielectric lenses having the sizes, shapes and structural features of the above-discussed examples. For example, referring to
The grooves 358 may be formed in many different configurations including, but not limited to, parallel (horizontal or vertical) lines, an array of discrete indentations, a continuous, back and forth line, a series of regularly spaced holes or indentations spaced, for example, every one half wavelength, etc. There may be either an even or odd number of grooves, and the grooves may be regularly or irregularly spaced. In one example, the grooves 358 are evenly spaced, and may be easily machined into the lens material using standard milling techniques and practices. In one example, the grooves may be machines so that they have a substantially identical width, for ease of machining. In another example, each of the grooves 358 has a concave surface feature at a greatest depth of the groove where the groove may taper to a dull point on the inside of the lens structure. As discussed above, in embodiments where the lens 356 is a plano-convex lens, the lens has a greater depth of lens material near the center of the lens as compared with the edges of the lens. Accordingly, in at least one embodiment, the depth of the grooves 358 varies with location on the lens surface. For example, the depth to which each of the grooves is milled may increase the farther a groove is located from the apex, or center 360, of the convex lens surface. In one example, the grooves may penetrate the surface by approximately one quarter-wavelength in depth near the center axis and may be regularly spaced to maintain the coherent summing of the direct and internally reflected signals, becoming successively deeper as the grooves approach the periphery of the lens.
The width of the grooves 358 may be constant or may also vary with location on the lens surface. In one example, the grooves 358 may typically have a width 368 of approximately one tenth of a wavelength (at the center of the operating frequency range) or less. The size of the lens 356 and of the grooves 358 formed in the lens surface may be dependent on the desired operating frequency of the antenna array 106. In one specific example, the dielectric lenses 304 are designed for use in the Ku frequency band (10.70-12.75 GHz), having an appropriate height and length for this frequency band.
Still referring to
In the example illustrated in
In the example illustrated in
As discussed above, in many applications, the external sub-system 102, including the antenna array 106, is exposed to environmental conditions such as precipitation and varying humidity. In such environments, it is possible for moisture to collect within the grooves 358 on the convex surface 338 of the dielectric lenses 304 in those embodiments of the lenses in which the grooves are milled (or otherwise fabricated) on the external surface of the lens. Such collection of moisture in the grooves 358 may be highly undesirable as it may degrade the RF performance of the lens, for example, by changing the effective dielectric constant of the lens and adversely affecting the impedance match between the lens and the surrounding air. For example, build-up of water from condensation inside the grooves 358 of the dielectric lens may cause a reduction in signal power of about 2 dB. In addition, particularly in situations where the antenna array 106 is subject to wide temperature variations, any water collected in the grooves 358 can freeze and cause structural problems, such as cracking of the lens, due to expansion of the water when it turns to ice. It may be possible to reduce moisture collection in the external grooves 358 by covering the antenna array 106 with a radome 202 and, in some examples, coating the interior surface of the radome with a material adapted to shed water. One example of a coating material that may be used is fluorothane. However, it is to be appreciated that the invention is not limited to the use of fluorothane and other water-shedding materials may be used instead. However, even when the antenna array is covered with a radome coated with a moisture-shedding material, it may not be possible to completely prevent moisture from collecting in the grooves 358. In addition, dust particles and other material may also collect in the grooves 358, further affecting the RF performance of the lens and adding to environmental wear and tear on the lens. Accordingly, it at least some embodiments, it is presently preferable to provide the impedance matching features on the interior, rather than exterior, surface of the dielectric lens 304. For example, as discussed and illustrated above, the impedance matching holes 340 are provided on the interior of the dielectric lenses 304, such that the exterior convex surface 338 may remain smooth.
According to another embodiment, impedance matching between the dielectric lens 304 and the surrounding air can be achieved by forming the dielectric lens out of two or more dielectric materials having different dielectric constants. For example, the interior portion of the dielectric lens 304 can be made from one material, and another material with a lower dielectric constant can be used in bands along the convex surface 338 and planar surface 336. In this manner, the change in effective dielectric constant from the air to the outer portion of the lens and then to the inner portion of the lens, and back again, may be made more gradual, thereby reducing unwanted reflections. With the use of several materials with gradually decreasing dielectric constants, a dielectric lens 304 with a gradually changing effective dielectric constant can be created. In one example, an adhesive can be used to adhere together the various layers of different materials. In this example, care should be taken to ensure good adhesion between the different layers so as to avoid reflections that may occur as a result of pockets of poor adhesion, or minute spaces, between the different layers. In addition, particularly for applications in which the dielectric lenses 304 are likely to encounter a wide range of temperatures, it may be important to carefully select the different dielectric materials to have similar coefficients of thermal expansion, so as to avoid or minimize stresses on the boundaries between the different materials which could shorten the life of the dielectric lenses 304 and cause degradation in the structural integrity and/or RF performance of the lenses.
As discussed above, the dielectric lenses 304 may be designed to have an optimal combination of weight, dielectric constant, loss tangent, and a refractive index that is stable across a large temperature range. It may also be desirable that the dielectric lenses 304 do not deform or warp as a result of exposure to large temperature ranges or during fabrication. It may also be preferable for the dielectric lenses 304 to absorb only very small amounts, e.g., less than 0.1%, of moisture or water when exposed to humid conditions, such that any absorbed moisture will not adversely affect the combination of dielectric constant, loss tangent, and refractive index of the lens. Furthermore, for affordability, it may be desirable that the dielectric lenses 304 be easily fabricated. In addition, it may be desirable that the lens should be able to maintain its dielectric constant, loss tangent, and a refractive index and chemically resist alkalis, alcohols, aliphatic hydrocarbons and mineral acids.
According to one embodiment, the dielectric lenses 304 are constructed using a certain form of polystyrene that is affordable to make, resistant to physical shock, and can operate across the wide range of the thermal conditions likely to be experienced when the antenna array 106 is mounted on an aircraft. In one example, this material is a rigid form of polystyrene known as crossed-linked polystyrene. Polystyrene formed with high cross linking, for example, 20% or more cross-linking, may be formed into a highly rigid structure whose shape may not be affected by solvents and which also may have a low dielectric constant, low loss tangent, and low index of refraction. In one example, a cross-linked polymer polystyrene may have the following characteristics: a dielectric constant of approximately 2.5, a loss tangent of less than 0.0007, a moisture absorption of less than 0.1%, and low plastic deformation property. Polymers such as polystyrene can be formed with low dielectric loss and may have non-polar or substantially non-polar constituents, and thermoplastic elastomers with thermoplastic and elastomeric polymeric components. The term “non-polar” refers to monomeric units that are free from dipoles or in which the dipoles are substantially vectorially balanced. In these polymeric materials, the dielectric properties are principally a result of electronic polarization effects. For example, a 1% or 2% divinylbenzene and styrene mixture may be polymerized through radical reaction to give a crossed linked polymer that may provide a low-loss dielectric material to form the thermoplastic polymeric component. Polystyrene may be comprised of, for example, the following polar or non-polar monomeric units: styrene, alpha-methylstyrene, olefins, halogenated olefins, sulfones, urethanes, esters, amides, carbonates, imides, acrylonitrile, and co-polymers and mixtures thereof. Non-polar monomeric units such as, for example, styrene and alpha-methylstyrene, and olefins such as propylene and ethylene, and copolymers and mixtures thereof, may also be used. The thermoplastic polymeric component may be selected from polystyrene, poly(alpha-methylstyrene), and polyolefins.
A dielectric lens 304 constructed from a cross-linked polymer polystyrene, such as that described above, may be easily formed using conventional machining operations, and may be grinded to surface accuracies of less than approximately 0.0002 inches. The cross-linked polymer polystyrene may maintain its dielectric constant within 2% down to temperatures exceeding the −70 F, and may also have a chemically resistant material property that is resistant to alkalis, alcohols, aliphatic hydrocarbons and mineral acids.
In one example, the dielectric lens 304 so formed includes an example of the impedance matching features discussed above. In these examples, the dielectric lens 304 may be formed of a combination of a low loss lens material, which may be cross-linked polystyrene and thermosetting resins, for example, cast from monomer sheets & rods. One example of such a material is known as Rexolite®. Rexolite® is a unique cross-linked polystyrene microwave plastic made by C-Lec Plastics, Inc. Rexolite® maintains a dielectric constant of about 2.53 through 500 GHz with extremely low dissipation factors. Rexolite® exhibits no permanent deformation or plastic flow under normal loads. All casting may be stress-free, and may not require stress relieving prior to, during or after machining. During one test, Rexolite® was found to absorb less than 0.08% of moisture after having been immersed in boiling water for 1000 hours, and without significant change in dielectric constant. The tool configurations used to machine Rexolite® may be similar to those used on Acrylic. Rexolite® may thus be machined using standard technology. Due to high resistance to cold flow and inherent freedom from stress, Rexolite® may be easily machined or laser beam cut to very close tolerances, for example, accuracies of approximately 0.0001 can be obtained by grinding. Crazing may be avoided by using sharp tools and avoiding excessive heat during polishing. Rexolite® is chemically resistant to alkalis, alcohols, aliphatic hydrocarbons and mineral acids. In addition, Rexolite® is about 5% lighter than Acrylic and less than half the weight of TFE (Teflon) by volume.
As discussed above, the dielectric lenses 304 may be mounted to the horn antenna elements 301 and designed to fit over and at least partially inside the respective horn antenna element. Referring again to
Referring again to
According to one embodiment, retaining clips 306a, 306b and 306c (see
In another example, the dielectric lenses 304 are glued into the respective antenna elements 308 using an adhesive. Adhesive fastening may be used alone or in combination with any or all of the pins 372 and retaining clips 306a, 306b, 306c and 380 discussed above. In one example, the pins 372 and/or retaining clips 306a, 306b, 306c and 380 are used as secondary attachment means in conjunction with an adhesive to more securely fasten the dielectric lenses 304 to the respective antenna elements 308. This arrangement may be preferable, for example, where the antenna array 106 is mounted to an aircraft and must meet applicable safety standards.
As discussed above, the antenna array 106 includes a feed network 302 coupled to each of the antenna elements 308, and in one embodiment, the feed network 302 is a waveguide feed network, as illustrated in
According to one embodiment, the waveguide feed network 302 is a compressed, non-conforming (i.e., custom sized and shaped) waveguide feed that has a low profile and is designed to fit within a constrained volume. As discussed above, in some applications, the antenna array 106 will be mounted on a moving vehicle, such as an automobile or aircraft, and it may therefore be desirable for the antenna array to occupy as small a volume as possible, so as to have minimal impact on the aerodynamics of the vehicle and to be easily mountable on the vehicle. Accordingly, the feed network 302 may be shaped and arranged to occupy a reduced volume. In one embodiment, the feed network 302 performs signal summing/splitting in both the E-plane and the H-plane, a feature which contributes to the ability to provide a compressed, low-profile feed network, as discussed further below. In one embodiment, the feed network 302 may be designed to fit behind the rows of antenna elements 308, as illustrated in
Referring to
As discussed above, in one embodiment, the OMT 502 splits an RF signal received at the first port into two orthogonal RF component signals. One RF component signal has its E-field parallel to the long axis of the horn (designated here as vertical, V) and the other RF component signal has its E-field parallel to the short axis of the horn (designated here as horizontal, H). These RF component signals are referred to herein as the vertically polarized RF component signal, or vertical component signal (V), and the horizontally polarized RF component signal, or horizontal component signal (H). From these two orthogonal component signals, any transmitted input signal may be reconstructed by vector combining the two component signals.
Referring to
Another example of an OMT 502 is illustrated in
Referring to
Still referring to
In one example, the vertically polarized electromagnetic wave V of a basic mode such as TE01 is propagated from the port 508 of the OMT 502, through the waveguide OMT, bypasses the rectangular branching waveguides of 506, and is propagated in a basic mode such as TE01 to the port 504. During the transit of the vertically polarized electromagnetic wave V, each of spaces defined between upper and lower sidewalls of the rectangular branching waveguides in the OMT 502 may be designed so as to be equal to or smaller than a half of the free-space wavelength of the frequency band in use. Thus, the vertically polarized electromagnetic wave V may not propagate into port 506 due to the cut-off effect of those spaces with very low reflection characteristics. Thus, the vertically polarized electromagnetic wave V provided to port 508 may be efficiently transmitted to port 504 and provided at that port as the vertical component signal, while the OMT 502 suppresses the reflection to the port 508 and eliminates propagation to port 506. Similarly, the horizontally-polarized electromagnetic wave H in a basic mode TE10 propagates from port 508 through the OMT 502, bypassing the waveguide branch for port 504, and is provided at port 506 as the horizontal component signal.
It is to be appreciated, as has been discussed above that although the operation of the OMT 502 has been described with respect to the case where the signal flow is such that port 508 is an input terminal, and the ports 504 and 506 are output terminals, the OMT 502 can also be operated such that the ports 504 and 506 are input terminals for orthogonal component signals which are combined and provided at the output terminal, port 508. Further, it is to be appreciated that the OMT 502 may also contain substantially circular or elliptical waveguides and terminations.
According to one embodiment, the feed network 302 includes a first path coupled to the second port 504 of the OMT 502 that guides the vertically polarized component signal, and a second path coupled to the third port 506 of the OMT 502 that guides the horizontally polarized component signal. Each path is coupled to all of the antenna elements 308 in the antenna array 106. Thus, each of the two orthogonally polarized component signals may travel a separate, isolated path from the respective ports 504, 506 of the OMT 502 to a feed port where the signals are fed to the system electronics, as discussed below. For receive mode of the antenna array 106, the feed network 302 receives the vertically and horizontally polarized component signals from each antenna element and sums them along the two feed paths to provide at the feed port one vertically polarized signal and one horizontally polarized signal. For transmit mode of the antenna array 106, the feed network 302 receives a vertically polarized signal at the feed port and splits that signal into the vertical component signals provided at port 504 of each OMT 502. Similarly, the feed network 302 receives a horizontally polarized signal at the feed port and splits it into the horizontal component signals provided at port 506 of each OMT 502. In one example, the two paths are substantially symmetrical, including the same number of bends, T-junctions and other waveguide path elements such that the feed network 302 does not impart a phase imbalance to the vertical and horizontal component signals.
As discussed above, in one embodiment, the feed network 302 includes both a path in which signal summing is done in the E-plane, and a path in which signal summing is done in the H-plane. Summing in both the E-plane and the H-plane allows the feed network to be substantially more compact than a similar feed network in which summing is done only in one plane. In particular, using both the E-plane and H-plane allows the two paths 541, 542 of the feed network to interweave, as shown in
According to one embodiment, the feed network 302 includes a plurality of E-plane T-junctions and bends to couple all of the antenna elements 308 together in the E-plane path, and a plurality of H-plane T-junctions and bends to couple all of the antenna elements 308 together in the H-plane path. When the antenna array 106 is operating in receive mode, the T-junctions operate to add the component signals (vertical or horizontal) received from each antenna element 308 to provide a single output signal (in each orthogonal polarization) at the feed port. When the antenna array 106 is operating in transmit mode, the T-junctions serve as power-dividers, to split a signal from the single feed port (for each orthogonal component signal) to feed each antenna element 308 in the antenna array 106.
Referring to
According to one embodiment, the waveguide T-junctions include a notch 550 at the cross-point of the T that may serve to decrease phase distortion of the signal as it passes through the T-junction 544. In another embodiment, there is a stepped septum at the center of the H-plane waveguide T-junctions 544. In another embodiment, there is a “V” shaped septum at the center of the E-plane waveguide T-junction 544. For impedance matching, the waveguide short wall dimension on the two inputs to the E-plane T-junction may be approximately ½ the short wall dimension of the output waveguide section. In another example, a short conductive tuning cylinder 552 is provided at the tip of the septum, as illustrated in
In one embodiment, the position of the E and H-plane waveguide T-junction septums are located such that they are biased toward either one of the two input ports of the T-junction, so as to create an amplitude balance or imbalance. Referring to
S
out
=AS
1
+BS
2 (2)
where S1 and S2 are the signals received at inputs 560 and 562, and A and B are scaling factors determined by the biasing of the T-junction. Biasing of the T-junction 544 may also be achieved using the tuning element 566. If the tuning element 566 is centered in the T-junction 544, as shown in
For example, referring to
According to one embodiment, dielectric inserts may be positioned within the feed network 302 at various locations, for example, within the E-plane and/or H-plane T-junctions. The size of the dielectric insert and the dielectric constant of the material used to form the dielectric insert may be selected to improve the RF impedance match and transmission characteristics between the input(s) and output(s) of the waveguide T-junctions. In one example, the dielectric insert may be constructed from Rexolite®. The length and width of the dielectric insert(s) may be selected so that the dielectric insert fits snugly within the waveguide at the desired location. In one example, the dielectric insert may have a plurality of holes formed therein. The holes may serve to lower the effective dielectric constant of the dielectric insert such that a good impedance match may be achieved.
As discussed above, in one embodiment, the feed network 302, in receive mode, sums the vertical and horizontal component signals from each antenna element 308 in the antenna array 106 and provides at the feed port a summed vertically polarized signal and a summed horizontally polarized signal. In one embodiment, the two summed signals are recombined by the system electronics. Alternatively, in another embodiment, the feed network 302 includes a feed orthomode transducer (not shown) at the feed port that combines the two orthogonal summed signals in the same manner discussed above with respect to the OMT 502. In one example, the antenna OMT 502 and feed OMT may be orthogonally fed. Thus, the vertical component signal may receive a first phase delay φ1 from the antenna OMT 502, a path delay φp, and a second phase delay φ2 from the feed OMT. Similarly, the horizontal component signal may receive a first phase delay φ2 from the antenna OMT 502, a path delay φp, and a second phase delay φ1 from the feed OMT. Thus, the combination of the two OMTs, orthogonally fed, may cause each of the vertical and horizontal component signals to receive a substantially equal total phase delay, as shown below in equation 3,
Φ[(ωt+φ1)+φp+φ2]=Φ[(ωt+φ2)+φp+φ1] (3)
where (ωt+φ1) and (ωt+φ2) are the vertically and horizontally polarized component signals and which are phase matched at the output port of the feed OMT. It is to be appreciated that although the operation of the OMTs and feed network 302 have been discussed in terms of two orthogonal linearly polarized component signals, the invention is not so limited and the OMTs may alternatively be designed to split an incoming signal into two orthogonal circularly polarized (e.g., left-hand polarized and right-hand polarized) signals (and to recombine these component signals). In this case, the feed network 302 may be designed to guide the two orthogonal circularly polarized signals.
According to another embodiment, the two orthogonally polarized summed component signals from the feed network (V and H) are fed to a first feed OMT having a circular dual mode port. A circular rotary waveguide section may be connected to the circular dual mode port of the first feed OMT. A second feed OMT, also having a circular dual mode port, may be connected to the circular rotary waveguide, such that the second feed OMT may rotate on the axis of the circular dual mode port. Thus, in at least one example, the phase lengths of the V signal and the H signal from the feed network 302 through the circular dual mode port of the first feed OMT are effectively equal. Rotating the second feed OMT effectively creates two linear, orthogonally polarized signals for any slant angle at the output of the second feed OMT. In one example, the feed OMTs and circular rotary waveguide may be located off the antenna array. In this example, a flexible waveguide may be used to connect the final T-junction of the feed network 302 to the first feed OMT so as to accommodate movement of the antenna array.
According to one embodiment, the feed network 302 may be manufactured in component pieces that are then mechanically coupled together. As discussed above, the feed network 302 may comprise a plurality of symmetrical sections, forming a “tree-like” structure to couple each of the antenna elements 308 in the antenna array 106 to a single feed point. Thus, the structure of the feed network 302 may be conducive to separation into elements that can be individually manufactured and then coupled together. In one example, the feed network 302 is manufactured by casting metal into the required sections and then brazing the metal to finish it. The casting and brazing steps may be performed on sections of the feed network at a time, for example, sections that include four antenna elements. These finished pieces may then be coupled together to create the entire feed network 302. In another example, the antenna array, including the feed network 302 and the horn antenna elements 308, is arranged such that it is symmetrical along a center line taken along its length. Accordingly, in this example, the antenna array can be divided along this center line into two symmetrical sections, each of which can individually manufactured (e.g., by casting and brazing) and then coupled together. Dividing the antenna array 106 “longitudinally” may greatly shorten the manufacturing time, even though each of the two sections may be significantly more complex than the smaller four-element or similar sections that arise when the array is split as discussed above.
Satellite (or other communication) signals may be transmitted on two orthogonal wave fronts. This allows the satellite (or other information source) to transmit more information on the same frequencies and rely on polarization diversity to keep the signals from interfering. If the antenna array 106 is directly underneath or on a same meridian as the transmit antenna on the satellite (or other signal source 110), the receive antenna array and the transmit source antenna polarizations may be aligned. However, as discussed above, in some instances there may be a polarization skew between the antenna array 106 and the signal source 110 caused by the relative positions of the signal source 110 and the host platform of the antenna array 106. For example, for applications in which the antenna array 106 is mounted on an aircraft 132, the pitch, roll, yaw and spatial location (e.g., meridian or longitude) of the aircraft may result in a polarization skew β between the signal source 110 and the antenna array 106. Accordingly, in one embodiment, the external sub-system 102 includes a polarization converter unit 128 that is adapted to compensate for polarization skew between the information source and the antenna array. The polarization converter unit 128 may use electronic and/or mechanical mechanisms to perform the polarization compensation, as discussed further below. The PCU 128 may receive control signals via the gimbal assembly 108.
According to one embodiment, in a receive mode of the communication system, the antenna array 106 may be adapted to receive incident radiation from the information signal source 110 and may convert the received incident electromagnetic radiation into two orthogonal electromagnetic wave components using the OMT and feed network 302 discussed above. From these two orthogonal electromagnetic wave components, the PCU 128 may reproduce transmitted information from the source whether the polarization of the signals is vertical, horizontal, right hand circular (RHC), left hand circular (LHC), or slant polarization from 0° to 360°. A part of, or the complete, PCU 128 may be part of, or may include, or may be attached to the feed network 302 of the antenna array 106. The PCU 128 may receive the signals from the feed network 302 and provide a set of either linearly (vertical and horizontal) polarized or circularly (right-hand and left-hand) polarized signals. Thus, the antenna array 106 and the PCU 128 provide an RF interface for the external subsystem 102, and may provide at least some of the gain and phase-matching for the system. In one embodiment, the PCU 128 may reduce or eliminate the need for phase-matching for the other RF electronics of the system.
Referring to
According to one embodiment, the PCU 602 may provide the polarization-corrected signal to a low noise amplifier 604 which amplifies the signal and feeds it to the internal sub-system 104. As discussed above, the bulk of the signal processing and control electronics of the communications system may be included in the internal sub-system 104 and housed within the host platform so as to protect it from environmental conditions. However, as known to those skilled in the art, in many applications it is desirable to have the low noise amplifier 604 as close to the antenna feed as possible for signal-to-noise considerations. Accordingly, in one embodiment, the low noise amplifier 604 is part of the external sub-system 102. In the example illustrated in the
Referring to
According to one embodiment, the PCU 602 includes a rotary orthomode transducer (OMT) 608 that is responsible for the polarization skew correction, as discussed further below. The rotary OMT 608 is mounted to a spine 610 along which runs a cable 612 for the PCU drive. On end 614 of the cable 612 is coupled to the rotary OMT 608, and the other end 616 is coupled to a master pulley 618. A motor 620 supplies the power to drive the master pulley 618 and pulley 622 to rotate the rotary OMT 608 using the cable 612. The motor 620 may be supported by a motor mount 624. In one embodiment, the two summed component signals, vertical and horizontal, from the feed point of the antenna array 106 are fed to first and second waveguide ports 626, 628 of the rotary OMT 608. The two waveguide ports 626, 628 are coupled to rotatable section 630 of the rotary OMT 608. The rotatable section 630 rotates the received electromagnetic fields to compensate for polarization skew β between the signal source 110 and the antenna array 106. A polarization encoder 632 may be used to determine a degree of rotation of the rotary OMT 608, corresponding to a desired polarization correction factor. In one example, the PCU 602 receives control signals from the antenna control unit 112 (see
As discussed above and in more detail below, in one example, polarization skew compensation can be performed electronically. However, compensating for polarization skew β mechanically, using an embodiment of the PCU 602 discussed above, may have several advantages. For example, mechanical polarization skew compensation does not suffer from efficiency losses associated with converting an RF signal into an electronic signal (to be processed to compensate for the polarization skew) and back into an RF signal. In addition, the mechanical PCU 602 may be capable of handling very high power signals, particularly useful for compensating for polarization skew when the antenna array 106 is transmitting, whereas the electronics that may perform electronic polarization skew may require that the signals be relatively low power.
Referring to
In one embodiment, the first and second component signals on lines 704 and 706 may be amplified by low noise amplifiers 604 that may be coupled to the ports of the feed network 302 by a waveguide feed connection. The low noise amplifiers are coupled to directional couplers 714 via, for example, semi-rigid cables. The coupled port of the directional couplers 714 is connected to a local oscillator 716. The local oscillator 716 may be controlled, through the control interface 712, by the gimbal assembly 108. In one example, the local oscillator 716 may have a center operating frequency of approximately 11.95 GHz.
As shown in
Considering the path for circular polarization, lines 722 and 726 provide the second and fourth PCU signals to a 90° hybrid coupler 728. The 90° hybrid coupler 728 thus receives a vertically polarized signal (the fourth PCU signal) and a horizontally polarized signal (the second PCU signal) and combines them, with a phase difference of 90°, to create right and left hand circularly polarized resultant signals. The right and left hand circularly polarized resultant signals are coupled to switches 730 via lines 732 and 734, respectively. The PCU therefore can provide right and/or left hand circularly polarized signals from the vertically and horizontally polarized signals received from the antenna array 106.
Still referring to
A=10*log((tan(β))2
The value of the polarization skew β may be provided via the control interface 712. For example, if the input polarizations are vertical and horizontal (from the antenna array) and a vertical output polarization (from the PCU) is desired, no attenuation may be applied to the vertical path and a maximum attenuation, e.g., 30 dB, may be applied to the horizontal path. The orthogonal output port may have the inverse attenuations applied to generate a horizontal output signal. To generate a slant polarization of 45 degrees, no attenuation may be applied to either path and a 180 degree phase shift may be applied to one of the inputs to create the orthogonal 45 degree output. Varying slant polarizations may be generated by adjusting the attenuation values applied to the two paths and combining the signals. The BPM 740 may be used to offset any phase changes in the signals that may occur as a result of the attenuation. The BPM 740 is also used to change the phase of orthogonal signals so that the signals add in phase. The summers 742 are used to recombine the signals that were divided by second dividers 736 to provide two linearly polarized resultant signals that are coupled to the switches 730.
In one embodiment, the switches 730 are controlled, via lines 744, by the control interface 712 to select between the linearly or circularly polarized pairs of resultant signals. Thus, the PCU 702 may provide at its outputs, on lines 746, a pair of either linearly (with any desired slant angle) or circularly polarized PCU output signals. According to one example, the PCU 702 may include, or be coupled to, equalizers 748. The equalizers 748 may serve to compensate for variations in cable loss as a function of frequency—i.e., the RF loss associated with many cables may vary with frequency and thus the equalizer may be used to reduce such variations resulting in a more uniform signal strength over the operating frequency range of the system.
The PCU 702 may also provide phase-matching between the vertically and horizontally polarized or left and right hand circularly polarized component signals. The purpose of the phase matching is to optimize the received signal. The phase matching increases the amplitude of received signal since the signals received from both antennas are summed in phase. The phase matching also reduces the effect of unwanted cross-polarized transmitted signals on the desired signal by causing greater cross-polarization rejection. Thus, the PCU 702 may provide output component signals on lines 746 that are phase-matched. The phase-matching may be done during a calibration process by setting phase sits with a least significant bit (LSB) of, for example, 2.8°. Thus, the PCU 702 may act as a phase correction device to reduce or eliminate any phase mismatch between the two component signals.
According to one embodiment, the PCU 702 may provide all of the gain and phase matching required for the system, thus eliminating the need for expensive and inaccurate phase and amplitude calibration during system installation. According to one example, the PCU 128 may operate for signals in the frequency ranges of approximately 10.7 GHz to approximately 12.75 GHz and 14.0 GHz to 14.5 GHz, for receive and transmit. In one example, the PCU 128 may provide a noise figure of 0.7 dB to 0.8 dB over these frequency ranges, which may be significantly lower than many commercial receivers. The noise figure is achieved through careful selection of components, and by impedance matching all or most of the components, over the operating frequency band. Thus, polarization skew compensation, and optionally also phase balancing/matching, may be performed by the PCU 128, either mechanically using an embodiment of the PCU 602 discussed above or electronically using an embodiment of the PCU 702. A combination of electronic and mechanically polarization compensation can also be implemented.
Referring again to
Referring to
Referring again to
Referring to
According to one embodiment, the internal sub-system 104 comprises a down-converter unit (DCU) 148 that may receive input signals, e.g. the linearly or circularly polarized signals via the connector 140 and may provide output signals, e.g. linearly or circularly polarized signals, on lines 150, at a lower frequency than the frequency of the input signals received. The DCU 148 will be described in more detail below. The signals on line 150 may be processed by signal processing electronics 152. Similarly, in the transmit path, the internal sub-system 104 may include an up-converter unit 154. The transmit signal may be received by the internal sub-system 104 via connector 156 from a signal source, such as, for example, a passenger or user interface, processed by the signal processing electronics 152 and up-converted to the transmit frequency by the up-converter unit 154. As will be recognized by those skilled in the art, the up-converter unit 154 may operate in a similar manner to the down-converter unit 148, for example, by mixing the transmit signal with a local oscillator signal to change the frequency of the data signal, as discussed further below.
As discussed above, signals may be transmitted and/or received by the antenna array 106 over a wide range of frequencies extending up to several Gigahertz. For example, the vertical and horizontal component signals may be in frequency ranges of approximately 10.7 GHz-12.75 GHz or 14.0 GHz-14.5 GHz. Therefore, in some applications, particularly where the antenna array 106 may be receiving and/or transmitting at very high frequencies, it may be preferable to perform the down-conversion or up-conversion using two local oscillators. Accordingly, in at least one embodiment, the internal sub-system 104 may optionally include a second local oscillator to converts the signal of interest to a frequency useable by the modem 116. It is to be appreciated that the signal processing may occur before any down or up conversion, in between different down/up conversion stages, or after all down/up conversion has been performed. In receive mode, the down-converted and processed signals may be supplied via modem 116 and connector 156 to the passenger interfaces (e.g., seatback displays) for access by passengers associated with the host vehicle. Similarly, in transmit mode, the signals to be processed, up-converted and transmitted may be received from the passenger interface(s) via connector 156.
Referring to
Still referring to
Thus, the internal sub-system 104 may receive data, communication or other signals to be transmitted by the antenna array 106 from, for example, passenger interfaces within the host vehicle, may process these signals, and provide the transmit signal via connector 140 to the external subs-system 102. In the external sub-system 102, the polarization converter unit 602 may compensate for polarization skew β between the antenna array 106 and the desired destination of the transmit signal. The feed network 302 of the antenna array 106 may split the transmit signal into two orthogonally polarized component signals that are each split among all antenna elements 308 in the antenna array 106. Each antenna element 308 may include an OMT 502 that recombines the two orthogonal component signals into a signal that is transmitted by the antenna element 308. Similarly, the antenna array 106 may receive an information signal from a signal source via each antenna element 308 in the array. The feed network 302 may split the signal received at each antenna element 308 into two orthogonal component signals and sum the component signals, in each polarization, from all antenna elements to produce two orthogonal summed signals. These summed signals may be corrected for polarization skew β between the signal source 110 and the antenna array 106 and recombined into a received signal that is amplified by a low noise amplifier and passed, via connector 140 to the internal sub-system 104. In the internal sub-system 104, the received signal may be processed (e.g., down-converted) and supplied via connector 156 to passenger interfaces in the host vehicle.
According to one embodiment, the internal sub-system is contained within a housing that is mounted in the interior of the host vehicle. An example of such a housing 802 is illustrated in
Referring to
Referring to
Referring to
Referring again to
As illustrated in
As discussed above, in some embodiments, the signal source 110 is a satellite and the communications system is mounted on an aircraft 132. According to aspects and embodiments, an important design consideration for an aircraft-mountable antenna system is to prevent interference to adjacent satellites. Where the aircraft location and flight profile might impact the quality of service, the quality of service goals may be addressed through satellite selection. Embodiments of the antenna system and service offered therewith may prove extremely attractive and commercially viable. Similarly, although several aspects and features are discussed with respect to an aircraft-mounted satellite communications system, they may apply similarly to a communications system mounted on another type of vehicle or one that receives signals from a terrestrial source or other vehicle, rather than from a satellite.
The pointing accuracy of the antenna array 106 (i.e., how accurately the antenna array can be aimed at the signal source 110 or signal destination) may be a critical performance metric for the communications system. Pointing accuracy may be important both to prevent interference with neighboring satellites to the target satellite as well as to ensure good quality of service of the communications system. However, particularly where the communications system is mounted on a vehicle, such as aircraft 132, there are numerous conditions (e.g., shape and available mounting locations, environmental factors and mechanical tolerances) that can adversely affect the pointing accuracy if not accounted for. Accordingly, in one embodiment, a calibration procedure is used to correct for mechanical tolerances in the antenna array and structural tolerances in the host vehicle, and to automatically detect and adjust for replacement of components, as discussed further below. In one example, the calibration procedure may account for positional offsets and biases in the external sub-system relative to the vehicle's navigational system. The following discussion will assume that the vehicle is an aircraft, and refer to the aircraft's inertial navigation system 122; however, it is to be appreciated that the calibration procedure may be applied regardless of the type of vehicle on which the system is installed.
There are a number of degrees of freedom for an antenna array 106 with respect to pointing and alignment with a desired target satellite, including the antenna array alignment, the azimuth rotation axis of the antenna array, the elevation rotation axis of the antenna array and the polarization rotation axis of the antenna array. All satellite antennas must be oriented in azimuth, elevation, and polarization to point at the desired satellite. According to one embodiment, the antenna array 106 has a non-circular aperture with a beam pattern that is wider in elevation and therefore, it may be necessary to align the aperture with the target satellite orbital arc to prevent the contribution of the wider elevation beam pattern from causing interference with an adjacent satellite. In order to prevent the wider beam pattern in elevation from interfering with adjacent satellites, the major axis of the antenna may be aligned with the tangent to the geosynchronous arc at the target satellite point, to the extent required to meet specified off-axis EIRP (effective isotropic radiated power) criteria. Tangential alignment of the antenna array aperture with the orbital arc of the target satellite is referred to as antenna alignment or aperture alignment. In addition, the polarization of the feed should be aligned with the polarization of the satellite to prevent cross-polarization interference.
Since the orientation of the aperture of the antenna array 106 is fixed with respect to the fuselage of the aircraft on which it is mounted by the gimbal assembly 108, the antenna alignment will vary as the aircraft experiences orientation changes in pitch, roll and yaw during flight. Thus, in embodiments of the antenna system in which the antenna has a non-circular antenna aperture, independent consideration of the polarization axis and the alignment of the antenna aperture may be necessitated. Although the term “misorientation” is sometimes used to address errors in the aperture major axis orientation alignment with the geosynchronous satellite arc, this document will refer to this degree-of-freedom as aperture alignment, with a value of zero indicating perfect alignment (zero mis-orientation). Pointing error is limited to the angular difference between the main beam of the antenna and the true direction of the target satellite.
All four antenna axes (azimuth, elevation, polarization and major axis orientation) are impacted both by location (latitude and longitude) and orientation (roll, pitch, and heading) of the antenna mount. As the aircraft location (latitude and longitude) and position (roll, pitch, and heading) vary throughout the flight profile, the antenna control unit may drive and monitor the antenna in these axes to maintain accurate pointing of the antenna main beam towards the satellite and prevent adjacent satellite interference.
As discussed above, the antenna array 106 can be rotated in azimuth about the aircraft's yaw axis to point the main beam of the antenna array at the target satellite. Similarly, the antenna array 106 can be rotated in elevation to point the main beam toward the satellite of interest. Errors in the pointing of the azimuth and elevation axes are referred to as “pointing error.” As the aircraft orientation and position vary throughout the flight profile, the antenna control unit 112 may drive the antenna array 106 to maintain accurate pointing of the main beam of the antenna array at the target satellite. In typical circumstances, the aircraft may spend the large majority of its flight profile in straight and level flight. Accordingly, pointing error in the azimuth rotation axis may be the primary contributor to potential interference with adjacent satellites. Pointing error in the elevation rotation axis may couple with antenna alignment error to also contribute to potential interference with adjacent satellites. For example, if the antenna array 106 has an alignment error of zero degrees, any elevation axis pointing error is substantially perpendicular to the target satellite orbital arc and therefore may not contribute to interference with adjacent satellites. According to one example, aspects and embodiments of the calibration and/or tracking procedures discussed below account for pointing error and antenna alignment to reduce interference with adjacent satellites and improve quality of service of the communication system.
In addition, as discussed above, the polarization converter unit 128 may be used to compensate for polarization skew between the antenna array and the target satellite. For example, the linear polarization of the signal transmitted by (or received by) the antenna array 106 may be rotated clockwise or counter-clockwise about the main beam pointing vector using the polarization converter unit 128. In conventional dish antenna systems, polarization compensation is executed by rotating the linear feed horn on the mount structure in front of the dish. On conventional non-circular ground-mounted dish antennas the polarization rotation axis is fixed in alignment to the reflector such that polarization compensation and aperture alignment are identical with pointing corrections implemented by physical rotation of the elliptical reflector and attached feed horn. By contrast, according to one embodiment, antenna aperture alignment and polarization compensation are independent functions, with the polarization axis being driven by the antenna control unit 112 (using the polarization converter unit 128) to maintain beam alignment with the target satellite, while the antenna aperture alignment is a function of aircraft orientation (pitch, roll and heading) and location (latitude and longitude), as discussed further below.
According to one embodiment, the major axis of the aperture of the antenna 106 is fixed relative to the yaw axis of the aircraft 132, therefore the antenna alignment is a direct function of the aircraft orientation (pitch, roll, and heading) and will vary as the aircraft experiences geographical and orientation changes during flight. In one example, since the ACU 112 may not be able to drive this axis, this angle is calculated and monitored in order to prevent transmission in situations where the elevation antenna pattern would cause adjacent satellite interference, as discussed further below.
According to one embodiment, normal flight operations are defined to be conditions where pitch, roll, and heading vary at rates up to 7 degrees per second simultaneously for all three axes, up to 8.5 degrees per second simultaneously on two axes and 12 degrees per second on a single axis. These values were established by evaluating data collected from actual flight operations, including recorded ARINC data profiles from aircraft operations during taxi, take-off, climb-out, low- and high-speed holding patterns, descent, landing, and taxi. These profiles include turns with very high bank angles up to 40 degrees (well in excess of the bank angle encountered in normal operations) and pitch-up angles to 17 degrees, and turn rates of up to 8 degrees per second, roll rates of up to 13 degrees per second, and pitch rates of up to 4 degrees per second. For example, one airline presently considers it “very rare” for an aircraft to exceed 15 degrees of bank during the cruise stage of flight. To the extent that a bank of up to 30 degrees would be encountered during normal flight, it would typically occur shortly after take-off in areas where topographical conditions would require terrain avoidance.
According to one embodiment, the antenna system has the following characteristics: The ability to correct for aircraft pitch, roll, and yaw sufficient to prevent adjacent satellite interference; the degree of pointing accuracy required to prevent adjacent satellite interference; and the capability to shut down transmission within 100 milliseconds of exceeding 0.5 degrees of pointing error, as discussed further below.
According to one embodiment, a factor that may be considered when considering the ability of the antenna system to achieve a specified pointing accuracy is the accuracy of the airline-installed inertial navigation system 122. In one example, the inertial navigation system 122 is a Honeywell Laser-Ring-Gyro-based Air Data Inertial Reference Unit. The current ARINC characteristic for this style of unit lists absolute accuracies for Roll and pitch at 0.1 degrees and for heading at 0.4 degrees. According to one embodiment, the antenna system does not rely on the inertial navigation system 122 data alone for absolute accuracy, but rather a variety of measurements which together provide the required pointing accuracy. These provide compensation for long-term errors that negatively affect the absolute accuracy of the inertial navigation system 122.
Referring to
During operation of the system, information from the position encoders may be fed back to the antenna control unit 112 (See
In one embodiment, the communication system includes two memory devices, one memory device 130 located in the external sub-system 102 and the other in the internal sub-system 104. The memory device 130 in the external sub-system 102 is referred to herein as the antenna memory 130, and the memory device in the internal sub-system is referred to herein as the antenna control memory. In one example, the antenna memory is part of the gimbal measurement unit 460 discussed above. It is to be appreciated that the antenna control memory may be incorporated as part of the antenna control unit 112 or may be a separate device (not shown in
In one embodiment, the calibration data, such as the offsets calculated above, may be stored in both the antenna memory 130 and the antenna control memory. Any changes or updates to the calibration memory may similarly be stored in both memories. This dual-memory structure may provide several advantages, including redundancy of the data (i.e., if one memory is damaged, the data will not be lost as it is also stored in the second memory) and the ability to “swap out” either the external or internal sub-systems (or components thereof) and replace them with new/updated components without having to redo the factory calibration. For example, if the internal sub-system were to be replaced, the new antenna control memory may download the calibration data stored in the antenna memory 130, thereby avoiding the need to recalibrate the system.
Referring again to
Generally vehicles, including aircraft, do not have large flat surfaces upon which the external sub-system 102 can be mounted, but rather the surfaces may have some slant or curvature. Accordingly, when the external sub-system is mounted on such a surface, there will be some offset of the antenna array from level. Furthermore, given that it may be unlikely that the antenna array will be mounted very close to the aircraft's inertial navigation system sensors, there may also be an offset between the antenna array 106 and the inertial navigation system 122. The installation calibration procedure 910 may account for these offsets, as discussed further below. Conventional installation procedures may allow the external sub-system 102 may be accurately placed to within a few tenths of a degree to the know biases of the aircraft's inertial navigation system 122. However, if not compensated for, even this few tenths of a degree can cause the antenna array to not point at the satellite accurately enough for the onboard receivers to lock on the signal using only a pointing calculation, and thus may result in loss of signal for the passenger. Furthermore, accurate placement of the external sub-system 102 on the vehicle may be difficult and time-consuming. It may therefore be preferable to use an installation calibration procedure 910 that obviates the need for accurate placement of the external sub-system on the vehicle.
As discussed above, the external sub-system 102 may include one or more position encoders that may sense a pitch and roll of the antenna array 106 once it is installed on the vehicle. In one example, the pitch and roll of the antenna array may be calculated relative to the pitch and roll of the on-board inertial navigational system 122 (step 912). In one example, step 912 includes using on-board parameters to measure offsets between the antenna array frame-of-reference (measured by the position encoders and corrected using the stored factory calibration data) and the aircraft frame-of reference (measured using the inertial navigation system 122). This allows determination of pitch and roll offsets without time-consuming manual calibration and removes aircraft manufacturing tolerances. In addition, because all pitch and roll offsets can be accounted for by the calibration, there is no need to accurately place the external sub-system 102 on the aircraft. Rather, the error between the antenna array alignment and inertial navigational system alignment is simply stored in memory devices and compensated for by the antenna control unit 112 when it supplies pointing control signals to the antenna array 106. Thus, the installation calibration 910 may greatly improve the ease of installation of the system.
The aircraft's inertial navigation system 122 may typically have built-in accuracies as well as mechanical tolerances that arise from its installation. For example, a Laser-Ring-Gyro-based inertial navigation system available from Honeywell Corporation has absolute accuracies for roll and pitch at 0.1 degrees and for heading at 0.4 degrees.
Some factors which contribute to the absolute accuracy of the inertial navigation system 122 include latency, long-term drift, repeatability, and installation accuracy. Signal latency is a large contributor to orientation accuracy. Data has indicated that the maximum transport delay for heading is about 110 milliseconds (ms) while that for pitch and roll is about 50 ms. During a standard-rate turn of the aircraft of 3 degrees per second, this would amounts to 0.330 degrees in heading. Laboratory characterization of several flight-line inertial navigation units has shown that this latency value is very consistent at rates of turn from 3 to over 30 degrees per second, with a variation in latency of less than about 2 ms from unit to unit. In one example, latency correction in the antenna control unit 112 may reduce the relative error to less than 0.07 degrees. In another example, the processing used to correct for latency is also used to correct for latency in the processing and motor control loop, such that the actual antenna pointing vector does not lag the desired pointing vector.
Even with advanced filters, the inertial navigation unit 122 may experience a roughly 90-minute Schuler-cycle variance in the heading output, plus a 24 hour cyclic variation when stationary. In one example, the worst-case measured variation rate was 0.0008 degrees over 15 minutes and a total 24-hour peak-to-peak variation of 0.12 degrees. Each time an inertial navigation unit 122 is turned on and goes through its alignment process, the resultant orientation may change slightly. Variations, or lack thereof, in the orientation are referred to as repeatability of the unit. In one example, the worst-case measured heading peak variation was 0.035 degrees while the worst-case roll peak variation was 0.0325, and the worst case peak pitch variation was 0.0225 degrees. Conventional installation procedures require an installation accuracy of the inertial navigation unit of about 0.2 degrees for each axis. Using embodiments of the installation and calibration procedures disclosed herein, this installation accuracy requirement may be relaxed to several degrees, as discussed further below. These various errors and tolerances may significantly impact the absolute accuracy of the aircraft orientation provided by the inertial navigation system 122, even though the relative accuracy of the inertial navigation system remains high. In addition, as discussed further below, slow drift components may further negatively impact the accuracy of the inertial navigation system data. However, contrary to conventional systems, embodiments of the communication system do not rely on the inertial navigation data alone for absolute accuracy, but rather a variety of measurements which together provide the desired pointing accuracy. In one embodiment, neither the orientation of the aircraft's internal navigation system 122 nor the orientation of the antenna array 106 are assumed to be accurate, but instead are measured during the installation calibration, and optionally every time the system is powered up, so that effects of misalignment can be accounted for during the pointing process. In addition, drift terms in the inertial navigation system data may be compensated for, further improving the systemic pointing accuracy.
As discussed above, position encoders on the external sub-system 102 provide measured pitch and roll data which, as part of the calibration procedure, may be combined with data from the inertial navigation system 122 to calculate the frame of reference difference between the inertial navigation system and the antenna array 106, independent of whether this offset is caused by alignment errors and mechanical tolerances of the inertial navigation system installation or of the antenna array installation. In one example, at installation, and optionally every time the system is powered up on the ground, the true pointing vector to the satellite may be determined by a tracking subsystem. This vector may be combined with the pitch and roll frame-of-reference offsets to establish the true orientation of the antenna array 106 and of the inertial navigation system 122. As discussed above, this data may be verified and updated whenever the aircraft is stationary on the ground because the position encoders can measure a gravity vector when the aircraft is stationary on the ground. Accordingly, this data may be used to automatically correct for repeatability variations in the inertial navigation system 122.
Conventional antenna alignment processes are typically only performed during initial antenna system installation and are done by manual processes. Conventional manual processes usually do not have the ability to input delta roll, delta pitch and delta yaw numbers, so the manual process requires the use of shims. These shims are small sheets of filler material, for example aluminum shims, that are positioned between the attachment base of the antenna and the aircraft, for example, to force the antenna system coordinates to agree with the navigation system coordinates. However, the use of shims requires the removal of the radome, the placement of shims and the reinstallation of the radome. This is a very time consuming and dangerous approach. Only a limited number of people are authorized to work on top of the aircraft and it requires a significant amount of staging. Once the alignment is completed the radome has to be reattached and the radome seal cured for several hours. This manual alignment process can be very time-consuming and difficult. By contrast, the automatic installation calibration procedure 910 may be performed quickly and easily without the need to move the antenna array.
Referring again to
Thus, the installation calibration procedure 910 may be used to easily and automatically account for any bias or offset between the antenna array 106 and the aircraft's inertial navigational system 122. This allows the antenna control unit 112 (See
As discussed above, in one embodiment, the communication system is capable of automatically detecting replacement of various system components and adjusting for this replacement through the communication between the antenna memory 130 and the antenna control memory. In one example, at power-up, each of the antenna memory 130 and the antenna control memory may query the other to determine whether either memory device is new, using the shared and locally stored data. By comparing the existing data with any new data provided by the new memory device, the system can automatically calculate compensations for the potentially different tolerances and parameters of the new component identified by the new memory device. At each power-up, the system may determine whether conditions exist to re-evaluate the current calibration offsets. If such conditions exist, then the system may evaluate whether the current offsets remain valid. This provides for detection and correction of any airframe changes including replacement of the inertial navigation system 122. In addition, tracking updates during flight may address any slow drift from the inertial navigation system 122 and/or airframe mechanical changes as might be caused by hull pressurization and temperature effects.
According to one example, it has been found that the contribution of aircraft fuselage flex to pointing error is very small. This is because fuselage flex occurs primarily in the pitch axis which has almost no effect on pointing accuracy in the geosynchronous satellite orbital arc. In the yaw direction which may contribute to pointing error in the geosynchronous satellite orbital arc, aircraft flex is extremely limited. In one example, instrumented tail-mounted antenna array installations have recorded maximum measured flex contributions on the order of about 0.05 degrees. Accordingly, in one embodiment, the contribution of airframe flex is considered to be in the measurement noise.
According to aspects and embodiment, the above-discussed procedure may provide excellent antenna alignment. According to one embodiment, polarization rotation axis and antenna aperture alignment are separate. The aircraft location (latitude, longitude) and orientation (pitch, poll, and heading) are both used to calculate the antenna alignment, in one example, at a 10 millisecond rate. According to one example, when the calculated antenna alignment angle exceeds ±25 degrees with respect to the geosynchronous satellite arc for any reason, transmission is inhibited. This worst-case impact on alignment peaks only over a small range of heading angles. While some maneuvers may necessitate momentary blanking of transmissions, embodiments of the communications system are completely tolerant of such transmission blanking, simply pausing the connected session with no further consequence to any user. Further, for the public's use of the system, which may be limited to altitudes above 10,000 feet by FAA regulations, only a small number of relevant maneuvers occur in the course of a typical flight, meaning any inconvenience will be minor in comparison with the benefit provided.
In some applications, even after precise calibration, navigational data alone may be insufficient to keep the antenna array locked to a desired source within acceptable tolerance levels. Therefore, according to one embodiment, the antenna control unit 112 may implement a tracking algorithm that may use both navigational data and signal feedback data to track a signal source. The tracking algorithm may always be looking for the strongest satellite signal, thus if the inertial navigation data is slow, the tracking algorithm may take over to find the optimum pointing angle. When the inertial navigation data is accurate and up to date, the system may use the inertial data to compute its azimuth and elevation angles since this data will coincide with the peak of the beam. This is because the inertial navigation system coordinates may accurately point the antenna, without measurable error, at the intended satellite; that is, predicted look angles and optimum look angles will be identical. When the inertial navigation data is not accurate the tracking software may be used to maintain the pointing as it inherently can “correct” differences between the calculated look angles and optimum look angles up to about 5 degrees.
In one embodiment, the antenna array may be controlled to locate a peak of a desired signal from the information source. The antenna array may then be “dithered” about the signal peak to determine the beam width of the source signal (relative to the beam width of the antenna array). In one example, the tracking algorithm perturbs the antenna pointing vector by small known amounts and uses the resulting measurements to drive the antenna towards the actual peak. For example, the antenna control unit 112 may monitor the amplitude of the received signal may use the amplitude of the received signal to determine the optimum azimuth and elevation pointing angle by discretely repositioning the antenna from its calculated position to slight offset positions and determining if the signal received strength is optimized, and if not repositioning the antenna orientation in the optimized direction, and so forth. In one example, each tracking cycle update typically perturbs the antenna pointing vector from the current center point for a total of 2 seconds to validate and verify pointing accuracy. This subsystem maintains the pointing vector within +/−0.1 degrees of the actual peak, providing direct feedback of the actual satellite pointing vector as offset from the expected satellite pointing vector. All slow-drift pointing error contributions may be nulled by the tracking process, including passenger and freight loading, pressurization, and temperature effects.
As known to those experienced in the art, geometric calculations can be easily used to determine look angles to geostationary satellites from known coordinates, including those from aircraft. By locating and tracking three satellites, triangulation data can be used to further refine any biases between the antenna array look directions and the navigational system data. The refined error may then be stored in the antenna control memory and antenna memory 130 and used to facilitate accurate tracking of a desired signal source 110 during operation of the system.
Referring again to
In one example, the offsets may be maintained between tracking cycle updates with update cycles executed at a tunable period and whenever the aircraft completes a dynamic maneuver. This may ensure that all long-term drift elements to the pointing vector are removed from the pointing process while minimizing the potential impact of the typically +/−0.2 degree perturbation on the pointing error margin. In one example, the same feed is used for both the transmit and receive signal and no active phase shifting components are used. Accordingly, the offset between the transmit beam and receive beam is not a factor. Tracking may be performed in cooperation with the modem 116 to ensure the correct satellite is being used.
According to one embodiment, during normal flight operations the transmit frequency needs to be offset by the expected Doppler frequency change caused by the relative velocity of the aircraft 132 to the satellite. In one example, this phenomenon is addressed by calculating the Doppler shift caused by the relative velocity of the aircraft to the satellite. Onboard the aircraft, the system provides the velocity of the aircraft in three dimensional space. From that relative velocity the frequency can be calculated and the modem 116 on the aircraft is configured to compensate or adjust for the Doppler offset. As a result of the Doppler correction a 10 MHz reference signal that is normally created from the signal may be corrupted and therefore no longer useable. Accordingly, in one example, a separate, compensated 10 MHz signal is created that is used as the frequency reference for the whole system.
According to one embodiment, fault handling functions may serve to monitor pointing accuracy compliance, and any fault detected may result in direct inhibition of transmission through shutdown of the output power amplifier. In one example, shutdown is implemented via a discrete line to the high power transceiver 114, eliminating latency and preventing communications or software faults from preventing the shutdown. In one example, the system may validate that any pointing error is less than 0.2 degrees prior to allowing signal transmission to resume.
Mis-pointing faults can have various causes, including, for example, power loss, mechanical drive train failure, loss of motor control, loss of RF signal measurement, and inertial navigation system, or system data, failure. In one example, both input AC power and internal DC power are monitored for voltage and current. Any out of bound events may result in transmission shutdown. In another example, if AC power is lost to the antenna control unit 112 for more than a specified time period, e.g., over 50 milliseconds, transmission may be disabled. Mechanical failure is characterized by loss of continuity or impairment between the drive motor and the antenna load. In one example, since the antenna position is measured by the position encoders at the antenna and not at the motor, such a failure results in position errors being detected by the antenna control unit 112.
In another example, the antenna control unit 112 maintains a connection to the modem 116 in order to monitor RF signal level. Errors in this communication link may inhibit transmission. This measurement by the modem may prevent the antenna array from tracking or enabling transmission when pointed at an incorrect satellite. All data from the inertial navigation system 122 may be validated and monitored for errors. Loss of the data stream for any of the aircraft orientation labels may inhibit transmission. Some installations may allow for fallback and cross-verification between multiple inertial navigation data sources. To detect whether the inertial navigation system 122 is generating false data, the RF level may be monitored for a short-term drop indicating a pointing error of over 0.5 degrees. In addition, if the tracking subsystem detects a deviation indicating a pointing error of over 0.2 degrees, transmission may be disabled.
According to one embodiment, any faults detected will result in signal transmission shutdown, including failure of the antenna array 106 to follow the proscribed trajectory within tolerance, failure of the feedback signals measuring the antenna position, and failure of the motor feedback signals from the motor. In one example, all of these signals are monitored at rates better than 1 millisecond. Any faults in communications to the gimbal assembly may also result in transmission shut down. In one example, communications are monitored at a 10 millisecond rate. In one example, during normal aircraft dynamics, nearly all of the fault detection functions will be triggered long before a pointing error of 0.5 degrees can be achieved. In this manner, interference with satellites adjacent the target satellite may be avoided. Furthermore, in one example, transmission will be disabled before the antenna array is slewed to the new target satellite. The system may require that the new satellite signal be locked and pointing verified to less than 0.2 degrees by the tracking subsystem prior to transmission resumption, thereby also avoiding interference with adjacent satellites. In addition, as discussed above with reference to
Having thus described several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.
This application is a continuation of and claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 12/604,087, which issued as U.S. Pat. No. 8,427,087 on Apr. 23, 2013. U.S. patent application Ser. No. 12/604,087 claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/107,606, entitled “Communication System with Broadband Antenna” filed Oct. 22, 2008 and to U.S. Provisional Application No. 61/108,237 entitled “Communication System with Broadband Antenna” filed Oct. 24, 2008. U.S. patent application Ser. No. 12/604,087 is a continuation-in-part of, and claims priority to, PCT Application No. PCT/US08/76216 entitled “Communication System with Broadband Antenna” filed Sep. 12, 2008, which claims priority to U.S. Provisional Application No. 60/971,958 entitled “Communication System with Broadband Antenna” filed Sep. 13, 2007, and to U.S. Provisional Patent Application No. 60/973,112 entitled “Communication System with Broadband Antenna” filed Sep. 17, 2007, and to U.S. Provisional Patent Application No. 61/095,167 entitled “Communication System with Broadband Antenna” filed Sep. 8, 2008. Each of the above-identified application is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61107606 | Oct 2008 | US | |
| 61108237 | Oct 2008 | US | |
| 60973112 | Sep 2007 | US | |
| 60971958 | Sep 2007 | US | |
| 61095167 | Sep 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12604087 | Oct 2009 | US |
| Child | 13868657 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US08/76216 | Sep 2008 | US |
| Child | 12604087 | US |
