Patent Application
20210405209
The present invention relates to an apparatus and a method for receiving satellite positioning signals, such as, for example, signals coming from the artificial satellites of the GPS, GALILEO, GLONASS, QZSS constellations or from any other satellite navigation system.
As is known, the availability of low-cost global positioning devices has resulted in the development of commercial services and products on a large scale, thus making it possible to locate people and vehicles throughout the world. These low-cost devices cannot, however, be used in applications wherein the position needs to be known with a precision of less than one meter, because the signal received from the satellites does not have a sufficiently high carrier-to-noise ratio (CNR or C/N) to estimate with a sufficient level of precision the time of flight of each positioning signal transmitted from the satellites.
In fact, the positioning device is configured for determining its own position (latitude, longitude and height) by solving the following system of equations:
where the terms Xu, Yu, Zu and Cu are the unknown terms to be determined, respectively representing the latitude, the longitude, the height, and the deviation of the clock of the device from the clock of the satellites, which are all mutually synchronized, the terms X1, Y1, Z1; . . . ; X4, Y4, Z4 are the positions of the four satellites (necessary for determining the position of the device) and are determined by said device on the basis of part of the data contained in each positioning signal (i.e. the ephemerides of the satellite transmitting the signal and the time instant at which transmission has occurred), while the terms R1, . . . , R4 are determined as follows:
R
i
=c·T
i 1≤i≤4 (2)
where c is the speed of light and Ti is the time of flight of the positioning signal emitted by the i-th satellite, wherein the time of flight Ti is the result of the arithmetic difference between the time instant at which the signal has been received (determined by means of the internal clock of the device) and the time instant at which the transmission of said signal has occurred (determined on the basis of the information contained in the signal itself).
From this brief summary it is possible to appreciate how the carrier-to-noise ratio can affect the precision with which the time instant of reception of the satellite positioning signal is determined; in fact, the higher the carrier-to-noise ratio, the higher the precision with which the device can determine the time instant of reception of the signal, because the signal can be detected more effectively by the receiver of said device.
In order to increase the level of positioning accuracy, one known solution makes use of two or more positioning apparatuses, positioned at a known distance from each other, and a processing device (e.g. a PC, a smartphone, or the like) configured for determining the position by using the information coming from each one of said positioning apparatuses and by taking into account the distance that separates said receivers, so as to be able to remove a large part of the effects of noise. This solution cannot however be easily exploited in applications where space and energy consumption are important factors, because the required use of two or more positioning apparatuses results in doubled energy consumption and space occupation.
Another source of noise is due to the effects of the multipath and/or of interfering (jamming) sources and/or signal sources trying to simulate satellite positioning signals by inserting fake signals intended to cause wrong positioning (spoofing). As described in American patent application publication US 2014/375500 A1 by ELECTRONICS AND TELECOMMUNICATIONS RESEARCH INSTITUTE, the effects caused by these sources of noise can be mitigated by using a plurality of receivers connected to one another by means of a beam-forming network, the primary task of which is to reduce the noise (also artificial noise) coming from a certain direction by selectively turning off those receivers which introduce the most noise in the positioning device. It is apparent that this solution is not suitable for increasing the carrier-to-noise ratio, since it prevents signals from entering the positioning devices in particular situations.
Furthermore, American patent application publication U.S. Pat. No. 5,952,968 by ROCKWELL INTERNATIONAL CORPORATION describes a solution for reducing the effects of interfering (jamming) sources by using a beam-forming network operating downstream of frequency converters (down converters), each one of which is positioned downstream of an antenna. Nevertheless, this solution requires a modification to the receiver apparatus, and cannot therefore be used in association with existing receivers. Moreover, such a solution suffers from a number of criticalities as far as reliability is concerned, because the presence of the converters decreases the mean time between failures. In addition to this, the position calculated by the positioning device is disadvantageously affected by a (constant) minimum positioning error beyond which it is impossible to go, since the converters introduce both background noise and a phase delay which cannot be exactly quantified and which is due to the presence of oscillator circuits that are inevitably affected by thermal phenomena.
The present invention aims at solving these and other problems by providing an apparatus and a method for receiving satellite positioning signals.
The basic idea of the present invention is to use a plurality of antennae for receiving satellite positioning signals, wherein each one of said antennae is (directly) connected to a phase shifter, and wherein the outputs of said phase shifters are connected to one another by means of an output collector, in which constructive interference can be generated between the signals coming from the satellites (and received by the antennae) and/or disruptive interference can be generated between the reflections of said signals and/or between signals coming from sources located in positions other than those of the satellites of a constellation of artificial satellites (such as, for example, fake signals transmitted from a ground or sea station).
In this way it is possible to increase the carrier-to-noise gain of the signal, which can then be acquired by a positioning device positioned downstream of said phase shifters, thereby also reducing the multipath effects, because constructive interference will only be produced between the signals directly received from the satellites, and not between the reflections of said signals, which will interfere with one another in an advantageously disruptive manner.
Further advantageous features of the present invention are set out in the appended claims.
These features as well as further advantages of the present invention will become more apparent from the following description of an embodiment thereof as shown in the annexed drawings, which are supplied by way of non-limiting example, wherein:
Any reference to “an embodiment” in this description will indicate that a particular configuration, structure or feature is comprised in at least one embodiment of the invention. Therefore, the phrase “in an embodiment” and other similar phrases, which may be present in different parts of this description, will not necessarily be all related to the same embodiment. Furthermore, any particular configuration, structure or feature may be combined in one or more embodiments as deemed appropriate. The references below are therefore used only for simplicity's sake and do not limit the protection scope or extent of the various embodiments.
With reference to
The apparatus 1 comprises an antenna array 2 comprising the following parts:
In this way, constructive interference can be generated between the satellite positioning signals, which are received by the antennae 21a-22d and (appropriately) phase-shifted by the phase shifter 22a-22d, and/or disruptive interference can be generated between the reflections of said satellite positioning signals, thereby increasing the carrier-to-noise ratio and consequently reducing the positioning error of a positioning device.
Each phase shifter 22a-22d comprises a control input that can receive a control signal over a control line 24 (represented in
As an alternative to the above, the control line may be shared by two or more phase shifter, e.g. by using an I2C bus or another type of bus that allows directing the data being transmitted over said control line.
In the preferred embodiment, the antenna array 2 is preferably implemented by using four antennae 21a,21b,21c,21d, wherein the output collector 23 is then connected, via a transmission line, preferably a coaxial one, to a satellite positioning device 4, which may be of a type known in the art (such as, for example, a UBX-G7020, UBX-G8020 receiver or the like).
It must be pointed out that the outputs of the phase shifters 22a-22d illustrated in
In addition to the above, the apparatus 1 may also comprise a control unit 3 (which will be further described hereinafter) configured for controlling the phase shifters 22a-22d according to at least one control logic, so as to increase the carrier-to-noise ratio of at least one of the signals received by means of the antennae 21a-21d.
Also with reference to
As an alternative to the communication bus 37, the processing means 31, the memory means 32, the reception means 33 and the control means 34 can be connected by means of a star architecture.
As aforementioned, the control means 34 may preferably comprise a programmable voltage regulator, and therefore the control signal may be at least one constant-voltage electric current; in fact, as will be further described hereinafter, the phase delay generated by each phase shifter may be dependent on the voltage value of the control signal.
It must be pointed out that the person skilled in the art will be able to use control means 34 generating control signals of a type other than the one just described (e.g. control signals compliant with the I2C communication protocol), without however departing from the teachings of the present invention.
Also with reference to
In this way, it is advantageously possible to increase the carrier-to-noise ratio, thereby improving the positioning precision that can be obtained by the satellite positioning device 4.
The processing unit 31 can be configured for generating (during the control generation phase) a control signal on the basis of a plurality of carrier-to-noise ratios of a plurality of satellite positioning signals received by the antennae 21a-21d, e.g. all the positioning signals that can be received by said antennae or used or usable by the positioning device 4 in order to calculate the position. In other words, in the course of the control generation phase the reception means 33 determine a plurality of carrier-to-noise ratios, wherein each carrier-to-noise ratio concerns one satellite positioning signal received by the antennae 21a-21d, and said control signal is generated on the basis of said carrier-to-noise ratios. This control logic can be used to advantage for reducing the time required by said device to determine its own first position (the so-called Time To First Fix—TTFF).
As an alternative to the above-described control logic, the processing unit 31 may be configured to generate (in the course of the control generation phase) a set of control signals, wherein each control signal is generated on the basis of a carrier-to-noise ratio of a single satellite positioning signal received by the antennae 21a-21d. In other words, during the control generation phase the reception means 33 determine at least one carrier-to-noise ratio concerning a satellite positioning signal received by the antennae 21a-21d, and said control signal is generated on the basis of said carrier-to-noise ratio. This control logic can be used to advantage for improving the precision of the position determined by the positioning device 4; in fact, by cyclically optimizing every single positioning signal receivable, it is possible to have the device 4 determine a time of flight of the signal with a higher level of precision than any other solution according to the prior art, because said device 4 will internally receive a signal having a higher carrier-to-noise ratio than in any one of said prior-art solutions. When a sufficient number of times of flight of signals have been determined (i.e. four), the device 4 will be able to determine the position on the basis of said times of flight.
As an alternative to or in combination with the above, the processing unit 31 may be configured to carry out, during the control generation phase, the following steps:
The position of a satellite relative to the apparatus 1 (or the antenna array 2) can be defined by angular quantities such as the azimuth and the height above the horizon.
It must be pointed out that the method according to the invention is carried out by the apparatus 1 in real time, so as to optimize at best the signal entering the device 4. In particular, the processing unit 31 can be configured for determining (in real time) at least one pointing datum by executing the following sub-steps:
In combination with or as an alternative to the above, the pointing data may be associated with at least one position of a satellite relative to the receiver; to this end, the processing unit 31 can be configured for determining at least one pointing datum by selecting it from a set of pointing data contained in the memory means 32. In fact, the phase information can advantageously be determined when the apparatus 1 is not in an operating condition. For example, a first set of pointing data can be determined by means of a series of laboratory tests, during which a signal source (simulating the signal of a positioning signal) is located in a particular position relative to the apparatus 1 and the delays generated by the phase shifters are varied until a sufficiently high carrier-to-noise ratio is obtained, after which the signal source can be moved and a second set of pointing data can be determined in the same manner; this process can be repeated until a number of positions is obtained which allows attaining a sufficiently high spatial selectivity of the apparatus 1.
As already mentioned, during the control generation phase P2 the position of said emission source relative to said apparatus (1) is determined on the basis of the following elements:
In this way it is possible to advantageously increase the carrier-to-noise ratio, thereby improving the positioning precision obtainable by the satellite positioning device 4; moreover, it is possible to reduce the effects of the presence of any fake signals emitted by stations located in positions other than those of the satellites of the constellation of artificial satellites C, e.g. on the terrestrial surface or at sea.
Also with reference to
The beam-forming network 25 is preferably implemented on a substrate of IS400 material having a dielectric constant of approx. 4.46, a thickness of approx. 1.55 mm, and a dielectric perturbation/loss coefficient of approx. 0.0163.
The beam-forming network 25 is preferably so designed that the points supplying power to the antennae 21a-21d, i.e. the ports P2-P5, are separated by a distance of less than approx. half the wavelength of the satellite positioning signals (to be received), which is equivalent to, for a maximum scan angle of 50 sexagesimal degrees, approx. 85 millimeters for the L1 band of GPS (centered at 1575.42 MHz), so as to maintain a low level of coupling between the antennae and make it possible to obtain a maximum spatial selectivity (i.e. the maximum rotation of the radiation lobe) of about 50 sexagesimal degrees. In this way it is possible to increase the carrier-to-noise ratio of at least one of the satellite positioning signals received, thereby reducing the positioning error of the positioning device 4.
Moreover, the beam-forming network 25 is preferably so designed that the antennae 21a-21d can be positioned over said network 25 at a distance equal to the length (in the air) of approx. a quarter-wave of the satellite positioning signals (to be received), which is equivalent to approx. 47.5 millimeters for the L1 band of GPS, so as to obtain a rear lobe with a maximum gain of approx. −19 dBi. In this way, the disturbances caused by the signals reflected by the ground can be reduced, and it is possible to increase the carrier-to-noise ratio of at least one of the satellite positioning signals received, thereby reducing the positioning error of the positioning device 4.
The combination of these technical features concerning the geometry of the signal beam-forming network 25 allows obtaining remarkable spatial directivity; in fact, the main lobe has a maximum directivity of approx. 12 dBi, while the secondary lobes (i.e. those at +45 and −45 sexagesimal degrees relative to the main lobe) have gains of 15 dB less than the maximum directivity value of the main lobe. When the phase shifters are configured to cause the main lobe to take an angle of approx. 50 sexagesimal degrees, said main lobe has a maximum directivity of 11 dBi, whereas the secondary lobes have a directivity of 5 dB less than the maximum directivity value of the main lobe. Based on these values, one can understand how the apparatus 1 can reduce the disturbances due to the signals reflected by the ground, so as to increase the carrier-to-noise ratio of at least one of the satellite positioning signals received. In this way it is possible to reduce the positioning error of the positioning device 4.
The beam-forming network 25 may also comprise one or more couplers 26, preferably a 90° branch coupler controlled by means of a pair of direct currents of voltage V1,V2 generated by the control unit 3, wherein said coupler 26 is in radio-frequency signal communication with microstrip lines, each one of which is in communication with one of the ports P2-P5 (through one of the phase shifters 22a-22d), and wherein said microstrip lines have the same electric length. Also, the coupler 26 comprises four additional phase shifters similar to the shifters 22a-22d, which further increase the capacity of introducing phase shifting in the network, so as to reduce the positioning error of the positioning device 4.
Each phase shifter 22a-22d is a 4-port RF device comprising two varactor diodes D1,D2 having very similar electric characteristics, which, being controlled by voltage, allow the input signal to be delayed within the range of 0-150 sexagesimal degrees; moreover, each phase shifter 22a-22d comprises also a pair of ceramic capacitors C1,C2 respectively connected in series to one of the ports P2-P5 in signal communication with one of the antennae 21a-21d and to the output port of the phase shifter 22a-22d in signal communication with the port P1 (through the coupler 24). These capacitors D1,D2 insulate the port P1, the coupler 24 and the antenna 21a-22d from the direct components caused by the presence of one of the control voltages V1,V2, while each one of the diodes D1,D2 is inversely polarized by said control voltage V1,V2, so as to create an electric capacity (necessary for delaying the output of the incoming signal) having a value dependent on the inverse polarization voltage. As aforementioned, the electric characteristics of the diodes D1, D2 must be as similar as possible, for the purpose of avoiding any distortion of the signal exiting the phase shifter; such diodes D1,D2 are preferably of the SMVA1248-079LF type available from Skyworks, since they offer very similar characteristics within the same production lot and require relatively low voltages V1,V2. In order to prevent the received signal from exiting along the lines that supply the power voltages V1, V2, the beam-forming network 25 may comprise, for each phase shifter 22a-22d, an RF stop filter 27 of the type well known to the man skilled in the art.
The input/output relationship of any one of the phase shifters 22a-22d can be described as follows:
OutSignal=InSignal·e−jΦ(V
where Vdc is the (constant) voltage V1 or V2, and, since the value of Vdc is a real (non-complex) number, said voltage Vdc defines, by means of a known function that is characteristic of the phase shifter, the phase delay that is imposed by the phase shifter 22a-22d on the incoming signal Insignal.
It must be pointed out that the antenna array 2 never changes the amplitude of the signals entering through the antennae 21a-21d; on the contrary, it only makes phase variations imposed by the phase shifters 22a-22d, so that the signal exiting through the port P1 will be the result of constructive interference between at least one of the signals directly received from the satellites of the constellation C and/or of disruptive interference between the reflections of said signals.
Of course, the example described so far may be subject to many variations.
A first variant comprises an apparatus according to the invention, which is similar to the apparatus 1 just described, wherein the control unit of said first variant of said apparatus is configured not only for executing the operations described herein for the main embodiment, but also for determining the position of said apparatus, i.e. for executing the functions carried out by the positioning device 4.
A second variant comprises an apparatus similar to the apparatuses of the two previous embodiments, wherein said apparatus can also operate on mobile means, such as, for example, boats, aircraft (e.g. multirotor aircraft, aeroplanes, helicopters, etc.), terrestrial vehicles, or the like.
More in detail, the processing unit of the control unit of said second variant of the apparatus is configured for acquiring, during the signal acquisition phase, also an orientation signal, preferably generated by an accelerometer (which may be comprised in said apparatus), wherein said signal contains at least orientation information defining the spatial orientation (e.g. relative to the gravity vector) of the antenna array, which is usually integral with the frame of the mobile means whereon it has been mounted; furthermore, said processing unit is also configured for generating the control signal, during the control generation phase, also on the basis of the orientation information. In this way it is possible to configure the antenna in an optimal manner not only on the basis of the position of at least one of the satellites of the constellation C, but also on the basis of the orientation of said antenna array, so as to increase the carrier-to-noise ratio of at least one of the satellite positioning signals received even when the apparatus of the invention changes its own inclination along at least one axis of rotation.
In addition, this variant turns out to be particularly interesting when, for example, the device is to be mounted on aircraft flying at a distance of just a few meters from buildings, i.e. in the presence of many reflections of the positioning signals; in such a situation, in fact, it is very likely that a change in the trim of the aircraft, even by only a few tens of degrees (e.g. when making a turn), will cause a change in the carrier-to-noise ratio of at least one of the signals received and used for determining the position of the vehicle. With this embodiment it is possible to mitigate this effect by adequately reconfiguring the antenna array as a function of the orientation thereof.
Although this description has tackled some of the possible variants of the invention, it will be apparent to those skilled in the art that other embodiments may also be implemented, wherein some elements may be replaced with other technically equivalent elements. The present invention is not therefore limited to the illustrative examples described herein, since it may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the basic inventive idea, as set out in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102017000050784 | May 2017 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2018/053178 | 5/8/2018 | WO | 00 |
