Patent Application
20090079619
The present invention relates generally to millimetre imaging systems and in particular to a realtime millimetre imaging system for detecting millimetre wave radiation and generating a corresponding image.
Millimetre-wave imaging systems produce a picture of a scene by detecting thermally generated radiation in the 30-300 GHz range, which is emitted or reflected by objects in the field of view of the instrument. Such systems offer advantages over equivalent instruments detecting infrared and visible light, because the millimetre-wave radiation can penetrate low visibility and obscuring conditions (e.g., caused by clothing, walls, clouds, fog, haze, rain, dust, smoke, sandstorms) without the high level of attenuation that occurs at the other noted wavelengths. This is particularly the case in specific “windows” for atmospheric transmission of radio waves that occur between 90 and 110 GHz and between 210 and 250 GHz.
Millimetre-wave imaging systems may be used in a range of important applications such as: aids to aircraft landing; collision warning in air, land and sea transport; detection and tracking of ground based vehicular traffic; covert surveillance for intruders, contraband and weapons. In such applications, the availability of real-time, “movie-camera” like imaging is highly desirable. However, for such systems to find wide acceptance in the commercial market-place, the sensing instrumentation must be light in weight, small in size, and affordable in cost.
A range of millimetre-wave imaging systems have been reported, but fail to meet the size, weight, and cost requirements for wide commercial acceptance of the technology, while at the same time offering real-time moving images. Such systems use two distinct technologies: mechanical scanning of the beam of a single antenna, and two-dimensional arrays.
Mechanical scanning of the beam of a single antenna connected to a single receiving system is performed in a raster pattern over a scene to detect the emitted radiation and produce a map or image of the brightness. The angular resolution of the resultant image is determined by the width of the antenna beam, whereas the scan angle determines the field of view. Rapid real-time imaging is difficult or inadequate, because physically large and cumbersome antenna elements (required to achieve high angular resolution) must be moved quickly at high rates.
Two-dimensional arrays of electrically-small antennas and integrated receivers sample the magnitude of the received millimetre-wave signal at the focal plane of an antenna system. This information is then used to produce a snap-shot of the brightness in the field of view of the instrument. In any given plane, the angular resolution of the resultant image is determined by the number of elements across the array and the outer dimensions of the array. In contrast, the field of view is determined by the beam-width of the individual antenna-array elements. Rapid real-time imaging can be achieved with these systems. However, this occurs at the expense of large numbers (1000's) of millimetre-wave receiving sub-systems and complex electronic phase shifting and amplitude weighting networks. Because of the large number of receivers required, heterodyne systems are avoided (in view of the local oscillator distribution problems) in favour of direct detection systems, with the attendant problems of gain stability and poorer sensitivity. Coherent local oscillator distribution to such a large number of millimetre-wave heterodyne receivers presents significant difficulties.
Thus, a need clearly exists for an improved real-time millimetre-wave imaging system capable of producing real-time, movie-like imaging, in which the system is more compact, less complex, and less expensive to produce.
In accordance with a first aspect of the invention, an image is formed from millimetre waves. To do so, a field of view is scanned using two geometrically orthogonal, intersecting co-polarized fan beams to receive millimetre-wave radiation. The components of received millimetre-wave radiation from the two fan beams are cross-correlated. The polarizations of the electric fields of the two fan beams are arranged to be substantially parallel in alignment. This may be achieved by polarization rotation filtering of the millimetre-wave radiation received in one of the fan beams. The two fan beams may be scanned in azimuth and elevation defining a scan range. The intersection region of the two fan beams is able to cover any point in the scan range. The scan range determines the field of view and a beam width of each fan beam in the narrow direction determines an angular resolution of the image. The cross-correlated output is measured at each point in the field of view to produce a map of the brightness. The position of the two geometrically orthogonal, intersecting fan beams may be controlled to generate the cross-correlated output at each fan beam intersection point in the field of view. Preferably, the scanning is implemented using a dual fan-beam antenna. The dual fan-beam antenna may have two modified pill-box antennas and a polarization rotator to change the direction of the incident polarization for one of the modified pill-box antennas. An image may be formed from millimetre waves of a different polarization by having a polarization rotator to change the direction of the incident polarization for a different modified pill-box antenna, only one polarization rotator being used at any time.
In accordance with a second aspect of the invention, millimetre-wave radiation is received. A field of view is scanned using a fan beam to receive millimetre-wave radiation. Polarization of incident millimetre-wave radiation is rotated through 90 degrees, and the field of view is scanned using another fan beam to receive the polarization-rotated millimetre-wave radiation. The fan beams intersect and are geometrically orthogonal to each other, yet the radiation is co-polarized. The fan beams are provided by respective fan-beam antennas. Each such antenna may include a modified pill-box antenna. Preferably, the modified pill-box antenna includes: a metal housing with an elongated aperture in at least one side of the housing, a curved primary reflector surface located within the housing and opposite the aperture, a feed horn within the housing, and one or more sub-reflectors for coupling the feed horn to the primary reflector surface. At least one of the sub-reflectors is designed to rotate, providing one-dimensional beam scanning in the narrow direction of the fan beam. The polarization rotation for a fan beam may be implemented using a polarization rotating transreflector. Preferably, the transreflector includes: a planar metallic reflector, and a grid of closely spaced wires. The wires are preferably spaced n×λ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimetre-wave radiation. The polarization rotating transreflector may be positioned at a 45 degree angle relative to the aperture of the second fan-beam antenna and at a substantially 45 degree angle relative to the direction of incident millimetre-wave radiation. The polarization rotation for a fan beam may be switched by exchanging a polarization rotating transreflector and a planar metallic reflector, both aligned in the same way. An exchange may be effected by turning a polarization rotating transreflector by 180 degrees to use its back surface as a planar metallic reflector. An exchange may be effected by making the wires of a polarization rotating transreflector out of a material that has a switchable conductivity.
In accordance with a third aspect of the invention, millimetre wave radiation is received for generating an image. To do so, millimetre wave radiation is received in accordance with first and second fan beams. The first and second fan beams are geometrically orthogonal to each other and intersecting. The millimetre wave radiation received in accordance with the second fan beam is co-polarized with the millimetre wave radiation received in accordance with the first fan beam. Components of the millimetre wave radiation received in accordance with the first and second beams are downconverted to generate respective intermediate frequency (IF) signals. The IF signals are cross-correlated. The resulting cross-correlated signal is filtered to provide a value proportional to brightness at each point in the scene. The received millimetre wave radiation may be amplified in accordance with the first and second beams prior to the step of downconverting.
In accordance with a fourth aspect of the invention, millimetre-wave imaging is disclosed. To do so, millimetre-wave radiation is received. The receiving includes: receiving millimetre-wave radiation by scanning a field of view using a fan beam, rotating the polarization of incident millimetre-wave radiation through 90 degrees, and receiving the polarization-rotated millimetre-wave radiation by scanning a field of view using another fan beam. The fan beams intersect and are geometrically orthogonal to each other. The received millimetre-wave radiation is processed. The processing step includes: receiving components of millimetre-wave radiation from the antenna received in accordance with the fan beams, downconverting respective components of the received millimetre wave radiation received to generate respective intermediate frequency (IF) signals, cross-correlating the IF signals; and filtering the resulting cross-correlated signal. The filtered, cross-correlated signal is proportional to the brightness at each point in the field of view as the antenna beams are scanned. In this way, an image of the scene may be built up. The scanning of each fan beam may be independently controlled as required so that the image can be generated from the filtered, cross-correlated output signal which provides a value proportional to the brightness of the scene at each point in said field of view.
A small number of embodiments are described hereinafter with reference to the drawings, in which:
A method and an apparatus for forming an image from millimetre waves, a method and an antenna for receiving millimetre wave radiation, a method and an apparatus for receiving millimetre wave radiation for generating an image, and a method and a system for millimetre wave imaging are disclosed. In the following description, numerous specific details are set forth. In the other instances, details well known to those skilled in the art may not be set out so as not to obscure the invention. It will be apparent to those skilled in the art in the view of this disclosure that modifications, substitutions and/or changes may be made without departing from the scope and spirit of the invention.
The embodiments of the invention involve improved imaging methods, antennas, and systems that enable the realization of a simple, low-cost instrument, capable of real-time imaging of moving targets. In broad terms, the embodiments produce a map or image of the millimetre-wave brightness in the field of view of the instrument by cross-correlating the signal received from two orthogonal, intersecting fan-beams.
An antenna with a fan-beam radiation pattern detects radiation from a region in the field of view that is of narrow angular extent in one direction only, while possessing a broad pattern in the orthogonal plane. Typically, a fan-beam can be generated by an antenna, or array of antennas, which is essentially one-dimensional (e.g., a long narrow slot, a linear array of slots, or a linear array of patch antennas). The width of the beam in the narrow direction is inversely proportional to the electrical length of the aperture or array. In contrast, the beam-width in the broad direction is inversely proportional to the width of the aperture or an individual element of the array. The angular position of the fan-beam in the narrow direction may be scanned across the field of view by producing a varying linear gradient in the phase of the electrical excitation across the aperture or across the elements of the array.
In accordance with embodiments of the invention, two such fan beams are arranged so that the beams intersect at right angles in the field of view of the instrument.
A significant component of the imaging system is the receiver, which takes the output from the antennas, amplifies the signals, and then down-converts the amplified signals to a convenient intermediate frequency at which the cross-correlation can tale place. There are a number of possible implementations for such receiving systems, depending upon the design of the fan-beam antenna.
An imaging receiver system 200 in accordance with an embodiment of the invention shown in
The respective block down converters 232, 234 produce respective intermediate frequency (IF) signals that are both provided to a correlator 240. The output of the correlator 240 is provided to a low pass filter 250, which produces the output signal 260. A map of the millimetre-wave brightness at each point in the field of view is produced by scanning the antenna beams over the field and at each field point measuring the cross correlation between the receiver outputs using a broadband analogue multiplier 240.
A polarization rotating filter (not shown) may be placed in front of one of the antenna apertures so that both fan beams operate in the same polarization.
In accordance with an embodiment of the invention, a simple, inexpensive implementation uses a multiple reflector “pill-box” style antenna 300 shown in
In a conventional “pill-box” antenna, a parabolic cylinder is used as the reflector. The “pill-box” is formed by two parallel planes which cut through the parabolic cylinder perpendicular to the cylinder elements. Typically, the focal line of the cylinder is positioned in the center of the aperture formed by the open ends of the parallel plates. When a feed horn is placed at the focal line, the feed horn blocks a significant portion of the aperture, resulting in large sidelobes in the far-field pattern of the antenna as well as standing waves within the “pill-box” itself.
Much improved performance can be obtained when an offset feeding arrangement is used, so that only one side of the “pill-box” is illuminated. The arc of the parabola does not include its vertex, and the feed horn points to illuminate this arc. Even though the illumination is asymmetric, good sidelobe performance is obtained. Alternatively, the “pill-box” antenna may be symmetrical about the axis of the parabola, but arranged as a folded lens to avoid blockage. Such an antenna, however, is more difficult to manufacture than an unfolded design.
The millimetre-wave fan-beam antenna 300 shown in
The antenna 300 uses one or more sub-reflectors 320 to couple the feed horn 330, 332 in an offset “pill-box” structure. The primary reflector 334 is shaped away from the traditional parabola to provide enhanced off-axis scanning angle with good sidelobe performance over the widest possible range of scan. The primary reflector 334 is coupled to the single feed-horn 330 via one or more sub-reflectors 320, which are also designed to have a profile that enhances the scan performance of the complete antenna assembly 300. One of these secondary mirrors 320 is arranged so that this sub-reflector 320 rotates, providing main beam scanning as the sub-reflector 320 spins. With careful mechanical and electrical design, in which the rotating sub-reflector 320 rotates about its center of mass, high speed scanning can be achieved.
For the imaging system, a pair of independently-scanned, orthogonally-oriented fan beams are required, with the sense of electric polarization aligned in each beam. Two “pill-box” antennas 410, 420 of the type shown in
The passive reflecting screen 430, 440 is generally configured at an angle of 45° relative the surface of the fan-beam antenna 410 having the aperture 414. The passive reflecting screen preferably has a planar metallic reflector 430 spaced apart by a multiple of a quarter wavelength (n λ/4) from a closely spaced, fine wire grid 440. The grid 440 is located between the reflector 430 and the antenna 410. The wires of the grid 440 are aligned at 45° to the direction of incident field polarization. This arrangement 400 results in orthogonal polarization in the far-field, if a standard plane reflector 430 is used.
Another way to achieve a co-polarized far-field response may be to modify the feed for the “pill-box” antenna 410, 420, so that the E-field vector is rotated through 90 degrees and aligned parallel to the long direction of the aperture. For this configuration, small variations in the surface quality and spacing of the metallic walls may cause significant degradation in antenna performance. However, for this arrangement, the polarization rotating filter 430, 440 is no longer required to be included. The preferred way to achieve co-polarization is by the use of a “transreflector” 430, 440. The transreflector 430, 440 consists of the wire grid 440, with wires aligned at 45 degrees to the incident electric field vector, backed by the planar metallic mirror 430 spaced away by an odd-multiple of a quarter wavelength at the operating frequency. The wire spacing and wire diameter must both be small compared to the operating wavelength. Over a limited bandwidth determined by the spacing between the grid 440 and the reflector 430 (the higher the number of quarter wavelengths, the narrower the bandwidth), this arrangement results in a rotation of the polarization of the incident wave through 90 degrees, without significantly altering the far-field radiation pattern of the antenna system.
Two “pill-box” antennas 510, 520 of the type shown in
The antenna 510 has an aperture 530 oriented lengthwise in a horizontal sense, while the other antenna 520 has an aperture 540 oriented lengthwise in a vertical sense, as depicted in
The E-plane output is provided to a low noise amplifier 612 and the H-plane output is provided to a different low noise amplifier 614. In turn, the low noise amplifiers 612, 614, acting as RF amplifiers, are coupled to respective mixers 620, 622. Further, a local oscillator 630 is coupled to both of mixers 620 and 622. The respective outputs of mixers 620 and 622 are provided as inputs to IF amplifiers 640, 642. The output of the IF amplifiers 640, 642 are provided to a cross-correlator 652.
The output of the cross-correlator 652 is provided to a base band filter 660. The base band filter 660 provides the output signal for the system. The output of the base band filter 660 is provided to an analogue to digital (A/D or ADC) converter 670. The ADC 670 produces digital data from the output signal that is provided as input to a computer 680. The computer 680 using hardware and/or software can produce a computer image 682 using the digital data from the ADC 670. In turn, using the digital data, the computer 680 can provide scan control signals 690 (indicated by dashed lines) to the dual fan-beam antenna 610. As shown in
The embodiments of the invention have various advantages including one or more of:
Use of a “pill-box” antenna to implement a scanned-beam imaging system;
A “pill-box” antenna in which the beam is scanned in one dimension using a rotating sub-reflector;
Use of a wire-grid transreflector to achieve a dual-scanning-beam system with co-polarized far-field response;
Use of two wire-grid transreflectors, exchangeable for planar metallic reflectors, to achieve switchable polarisation of the far-field response.
Use of a mechanically scanned beam so that only a single heterodyne receiver per beam is needed.
Use of two intersecting fan beams so that each antenna is required to scan only in one direction.
Thus, a method and an apparatus for forming an image from millimetre waves, a method and an antenna for receiving millimetre wave radiation, a method and an apparatus for receiving millimetre wave radiation for generating an image, and a method and system for millimetre wave imaging have been disclosed. In the light of this disclosure, it will be apparent to those skilled in the art that modifications, substitutions and/or changes may be made without departing from the scope and spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002950196 | Jul 2002 | AU | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10518758 | Jul 2005 | US |
| Child | 12155051 | US |
