GPS system and method for deriving pointing or attitude from a single GPS receiver

What is claimed is:

1. A GPS pointing and attitude measurement system using a single GPS receiver and multiple antennas to derive pointing and attitude measurement using a selected PRN code recovered from GPS navigation signals, wherein azimuth and elevation are determined in pointing application and roll, pitch, and yaw are determined in attitude application, comprising:

a reference and at least one slave antenna mounted to a foundation, such that the separation is significantly less than the correlation interval for the selected PRN code;

reference and, for each slave antenna, slave precorrelation electronics for providing respective digital representations of the GPS signals received by said reference and slave antennas;

at least one replica carrier generator for generating a replica of the GPS carrier signal;

at least one replica code generator for generating a replica of the selected GPS PRN code signal;

reference correlation electronics, including a carrier mixer and a selected number of code correlators, responsive to the reference GPS signal, and to replica carrier and replica code signals, for generating reference I and Q correlation outputs;

for each slave antenna, slave correlation electronics including a carrier mixer and at least one code correlator responsive to the slave GPS signals, and to replica carrier and replica code signals, for generating slave I and Q correlation outputs; and

a GPS processor responsive to the reference I and Q correlation outputs for tracking with the reference antenna, and responsive to the reference and slave I and Q correlation outputs for computing pointing and attitude measurements.

2. The GPS single-receiver pointing and attitude system of claim 1, wherein the selected PRN code is C/A-code.

3. The GPS single-receiver pointing and attitude system of claim 1, wherein the selected PRN code is P-code.

4. The GPS single-receiver pointing and attitude system of claim 1, wherein pointing and attitude measurements are computed from differential carrier doppler phase measurements.

5. The GPS single-receiver pointing and attitude system of claim 4, wherein the carrier doppler phase measurement is computed using reference and slave prompt I and Q correlation outputs.

6. The GPS single-receiver pointing and attitude system of claim 5, wherein pointing and attitude measurements are based on computations of carrier phase differences between said reference and slave antennas according to the relationship:

TAN.sup.- ((I.sub.RP *Q.sub.SP -Q.sub.RP *I.sub.SP)/(I.sub.RP *I.sub.SP +Q.sub.RP *Q.sub.SP))

where I.sub.RP and Q.sub.RP are respectively in-phase and quadrature prompt correlator outputs for the reference antenna, and I.sub.SP and Q.sub.SP are respectively in-phase and quadrature prompt correlator outputs for the slave antenna.

7. The GPS single-receiver pointing and attitude system of claim 6, wherein differential code phase measurements are used as a coarse measurement for ambiguity resolution.

8. The GPS single-receiver pointing and attitude system of claim 1, wherein said reference correlation electronics includes early, prompt and late code correlators.

9. The GPS single-receiver pointing and attitude system of claim 8, wherein at least one additional code correlator is used for search/ acquisition operations, and after tracking is established with the reference antenna, said additional correlator is assigned to said slave correlation electronics.

10. The GPS single-receiver pointing and attitude system of claim 8, wherein said slave correlation electronics includes early, prompt and late code correlators, and wherein pointing and attitude measurements are computed from differential code phase measurements.

11. The GPS single-receiver pointing and attitude system of claim 10, wherein the pointing and attitude measurements are computed from the early and late reference and slave I and Q correlation outputs.

12. The GPS single-receiver pointing and attitude system of claim wherein the differential code phase measurements are computed according to the relationship:

(RLE-REE-SLE+SEE)/(RLE+REE+SLE+SEE)

where

REE=SQRT (I.sub.RE.sup.2 +Q.sub.RE.sup.2)

RLE=SQRT (I.sub.RL.sup.2 +Q.sub.RL.sup.2)

SEE=SQRT (I.sub.SE.sup.2 +Q.sub.SE.sup.2)

SLE=SQRT (I.sub.SL.sup.2 +Q.sub.SL.sup.2)

and where I.sub.RE.sup.2 /Q.sub.RE.sup.2 are in-phase and quadrature reference Early correlation outputs, I.sub.RL.sup.2 /Q.sub.RL.sup.2 are in-phase and quadrature reference late correlation outputs, I.sub.SE.sup.2 /Q.sub.SE.sup.2 are in-phase and quadrature slave Early correlation outputs, and I.sub.SL.sup.2 /Q.sub.SL.sup.2 are in-phase and quadrature slave Late correlation outputs.

13. The GPS single-receiver pointing and attitude system of claim 10, wherein the pointing and attitude measurements are computed from the early and late slave I and Q correlation outputs.

14. The GPS single-receiver pointing and attitude system of claim 13, wherein the differential code phase measurements are computed according to the relationship:

(1/SLOPE)*((L-E)/L+E))=0.5*((L-E)/(L+E))

where:

L=(I.sub.SL.sup.2 +Q.sub.SL.sup.2).sup.1/2

E=(I.sub.SE.sup.2 +Q.sub.SE.sup.2).sup.1/2

SLOPE=2 C/A CHIP.sup.-1

and where I.sub.SE.sup.2 /Q.sub.SE.sup.2 are in-phase and quadrature slave early correlation outputs, and I.sub.SL.sup.2 /Q.sub.SL.sup.2 are in-phase and quadrature slave late correlation outputs.

15. The GPS single-receiver pointing and attitude system of claim 1, wherein a single set of replica carrier and code generators is used in common in generating both the reference and slave I and Q correlation outputs.

16. The GPS single-receiver pointing and attitude system of claim 1, wherein each antenna includes an index, such that the antennas can mounted to said foundation with a selected relative orientation to minimize antenna phase center migration errors.

17. The GPS single-receiver pointing and attitude system of claim 1, further comprising external sensors are used to provide pointing vector boundary conditions to facilitate ambiguity resolution.

18. The GPS single-receiver pointing and attitude system of claim 1, further comprising, for each slave antenna, an intermediate slave antenna mounted asymmetrically with respect to the slave antenna, and an antenna switching circuit, such that said slave precorrelation and correlation electronics are switched between the slave and intermediate antennas to facilitate ambiguity resolution.

19. The GPS single-receiver pointing and attitude system of claim 1, wherein the GPS receiver is a P-code receiver that tracks both the Ll and L2 GPS signals, producing two different standing wave patterns that facilitate ambiguity resolution and which can be used to correct for differential multi-path error.

20. The GPS single-receiver pointing and attitude system of claim 1, wherein a single slave antenna is mounted on said foundation, and wherein the resulting reference and slave I and Q correlation outputs are used to compute pointing measurements.

21. The GPS single-receiver pointing and attitude system of claim 20, wherein said reference and slave antennas are mounted on a pointing fixture beam with a separation of about 1-5 meters.

22. The GPS single-receiver pointing and attitude system of claim 20, wherein at least one additional slave antenna is mounted co-linearly on said foundation, and wherein the resulting reference and slave I and Q correlation outputs are used to compute pointing measurements with an oversolution.

23. The GPS single-receiver pointing and attitude system of claim 1, wherein two slave antennas are mounted non-co-linearly on a planar foundation, and wherein the resulting reference and slave I and Q correlation outputs are used to compute attitude measurements.

24. The GPS single-receiver pointing and attitude system of claim 23, wherein at least one additional slave antenna mounted to the foundation, and wherein the resulting reference and slave I and Q correlation outputs are used to compute attitude measurements with an oversolution.

25. A GPS pointing and attitude measurement method using a single GPS receiver and multiple antennas to derive pointing and attitude measurements using a selected PRN code recovered from GPS navigation signals, wherein azimuth and elevation are determined in pointing applications and roll, pitch and yaw are determined in attitude applications, comprising the steps:

mounting a reference and at least one slave antenna mounted to a foundation, such that the separation is significantly less than the correlation interval for the selected PRN code;

for the reference antenna, establishing a reference antenna tracking loop to generate reference I and Q correlation outputs using a set of reference code correlators and the selected PRN code signals;

for each slave antenna, generating slave I and Q correlation outputs using at lest one slave code correlator and the selected PRN code signals; and

processing the reference and slave I and Q correlation outputs to compute pointing and attitude measurements.

26. The GPS single-receiver pointing and attitude method of claim 25, wherein the selected PRN code is C/A-code.

27. The GPS single-receiver pointing and attitude method of claim 24, wherein the selected PRN is P-code.

28. The GPS single-receiver pointing and attitude method of claim 25, wherein pointing and attitude measurements are computed from differential carrier doppler phase measurements.

29. The GPS single-receiver pointing and attitude method of claim 28, wherein the carrier doppler phase measurement is computed using reference and slave prompt I and Q correlation outputs.

30. The GPS single-receiver pointing and attitude method of claim 29, wherein pointing and attitude measurements are based on computing carrier phase differences between said reference and slave antennas according to the relationship:

TAN.sup.-1 ((I.sub.RP *Q.sub.SP -Q.sub.RP *I.sub.SP)/(I.sub.RP *I.sub.SP +Q.sub.RP *Q.sub.SP ((

where I.sub.RP and Q.sub.RP are respectively in-phase and quadrature prompt correlator outputs for the reference antenna, and I.sub.SP and Q.sub.SP are respectively in-phase and quadrature prompt correlator outputs for the slave antenna.

31. The GPS single-receiver pointing and attitude method of claim 30, further comprising the step of using differential code phase measurements as a coarse measurement for ambiguity resolution prior to computing pointing and attitude.

32. The GPS single-receiver pointing and attitude method of claim 25, wherein pointing and attitude measurements are computed from differential code phase measurements using early and late reference and slave correlation outputs.

33. The GPS single-receiver pointing and attitude method of claim 32, wherein the differential code phase measurements are computed according to the relationship:

(RLE-REE-SLE+SEE)/(RLE+REE+SLE+SEE)

where:

REE=SQRT (I.sub.RE.sup.2 +Q.sub.RE.sup.2)

RLE=SQRT (I.sub.RL.sup.2 +Q.sub.RL.sup.2)

SEE=SQRT (I.sub.SE.sup.2 +Q.sub.SE.sup.2)

SLE=SQRT (I.sub.SL.sup.2 +Q.sub.SL.sup.2)

and where I.sub.RE.sup.2 /Q.sub.RE.sup.2 are in-phase and quadrature reference Early correlation outputs, I.sub.RL.sup.2 /Q.sub.RL.sup.2 are in-phase and quadrature reference late correlation outputs, I.sub.SE.sup.2 /Q.sub.SE.sup.2 are in-phase and quadrature slave Early correlation outputs, and I.sub.SL.sup.2 /Q.sub.SL.sup.2 are in-phase and quadrature slave Late correlation outputs.

34. The GPS single-receiver pointing and attitude method of claim 31, wherein the pointing and attitude measurements are computed using early and late slave I and Q correlation

35. The GPS single-receiver pointing and attitude method of claim 34, wherein the differential code phase measurements are computed according to the relationship:

(1/SLOPE)*((L-E)/L+E))=0.5*((L-E)/(L+E))

where:

L=(I.sub.SL.sup.2 +Q.sub.SL.sup.2).sup.1/2

E=(I.sub.SE.sup.2 +Q.sub.SE.sup.2).sup.1/2

SLOPE=2 C/A CHIP.sup.-1

and where I.sub.SE.sup.2 /Q.sub.SE.sup.2 are in-phase and quadrature slave early correlation outputs, and I.sub.SL.sup.2 /Q.sub.SL.sup.2 are in-phase and quadrature slave late correlation outputs.

36. The GPS single-receiver pointing and attitude method of claim 25, wherein the reference and slave I and Q correlation outputs are generated using a single set of replica carrier and code signals.

37. The GPS single-receiver pointing and attitude method of claim 25, wherein a single slave antenna is used, and wherein the resulting reference and slave I and Q correlation outputs are processed to compute pointing measurements.

38. The GPS single-receiver pointing and attitude method of claim 37, wherein at least one additional slave antenna is mounted on said foundation co-linearly, and wherein the resulting reference and slave correlation outputs are used to compute pointing measurements with an oversolution.

39. The GPS single-receiver pointing and attitude method of claim 25, wherein two slave antennas are mounted non-co-linearly on a planar foundation, and wherein the resulting reference and slave I and Q correlation outputs are processed to compute attitude measurements.

40. The GPS single-receiver pointing and attitude method of claim 39, wherein at least one additional slave antenna is mounted to the foundation, and wherein the resulting reference and slave I and Q correlation outputs are processed to compute attitude measurements with an oversolution.

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TECHNICAL FIELD OF THE INVENTION

The invention relates generally to NAVSTAR Global Positioning Systems (GPS), and more particularly relates to such a GPS system and method for deriving precise pointing or attitude measurements using a single GPS receiver and a multiplicity of antennas (at least two for pointing, and at least three for attitude).

BACKGROUND OF THE INVENTION

The NAVSTAR Global Positioning System (GPS) is a Department of Defense satellite navigation system that uses a constellation of GPS navigation satellites in a space segment (SS) to transmit GPS signals and data from which a world wide User Segment (US) can derive accurate position, velocity and time.

A GPS Control Segment on the ground tracks the SS satellite constellation, and uplinks to each GPS satellite ephemeris data on its orbital characteristics and satellite clock correction parameters to precisely synchronize the on-board satellite atomic clock with reference to GPS system time. Each GPS satellite continually transmits a navigation signal that provides navigation message data--including time of transmission, satellite clock correction parameters and ephemeris data.

The navigation signal is transmitted over two carrier frequencies in the L band (L1 at 1575.42 and L2 at 1227.6 MHz), spread spectrum modulated With two pseudo random noise (PRN) codes: (a) a P-code (precision) with a seven day period (repeating its PRN sequence only once every seven days), providing for precision measurement of time, and (b) a C/A-code (clear/access or coarse/acquisition) with a one millisecond period, providing for rapid search and acquisition of the navigation signal from a given satellite (and hand-off to the more precise but harder to acquire P-code). Both L1 and L2 are modulated with the P-code (10.23 MHz), while only L1 carries the C/A-code (1.023 MHz), and each code modulation is further modulated with the 50 Hz navigation message data.

The problem to which the invention is directed is the design of an economic and high precision system of GPS measurements to permit accurate computation of pointing (azimuth and elevation) or attitude (roll, pitch/elevation and yaw/azimuth). More specifically, the design problem is to economically eliminate or control the distributed error sources that are typically encountered in GPS receivers, achieving precise performance necessary for accurate GPS pointing/attitude, in addition to the normal position, velocity and time computations. These error sources include satellite clock, satellite electrical path, receiver oscillator, code and carrier generators, receiver electrical path, receiver time bias error, satellite clock prediction, ephemeris radial prediction, ionospheric and other atmospheric signal paths.

For conventional GPS navigation (position and velocity in three dimensions), a User Segment GPS receiver tracks four GPS satellites, establishing synchronism with its navigation signal, and recovering the navigation message data. Ranges to the four satellites are determined by scaling the signal transit time by the speed of light, with the position of each satellite at the time of transmission being determined from the associated ephemeris parameters.

The need for a GPS receiver to include a precision clock (synchronized to GPS system time) is eliminated by the use of range measurements from four satellites. That is, the navigation problem for position is characterized by four unknowns--position in three dimensions and clock error (or fixed time bias)--requiring four user position equations to provide three estimated position coordinates and an estimate of the receiver time bias. In addition, velocity measurements are made by measuring the doppler shift in the carrier frequency of the navigation signal, with the error offset in the frequency of the receiver oscillator being compensated for by using four range rate (doppler) equations.

From each satellite, the GPS receiver is able to provide pseudo range and range rate (doppler) measurements that can be used for position and velocity computations. Pseudo range is the range measurement, with respect to the receiver clock, to any satellite being tracked based on signal transit time and satellite position before compensating for the fixed receiver time bias associated with any range measurement.

For GPS pointing applications, such as precision geodetic surveying, GPS interferometry techniques are used to define to a high degree of precision a baseline vector between a reference antenna (at a known location) and a second antenna (at an unknown location)--typically, this vector can be defined within a millimeter for relatively short baselines. The vector includes both the distance and the direction (azimuth and elevation) to the second antenna with respect to the reference antenna. Using this vector, the precise geodetic position of the unknown antenna can be computed by adding (in the same coordinate system) the measured vector to the known geodetic position of the reference antenna.

GPS interferometry uses differentially-processed carrier doppler phase measurements for the two locations to provide first order positioning of the second GPS receiver/antenna location--more precisely, the antenna phase center location--with respect to the reference GPS receiver/antenna location (References 1, 2 and 3, listed at the end of the Background). This technique can be extended to obtain attitude information using differential carrier doppler phase measurements for two GPS receivers/antennas at unknown locations arranged in a triangular pattern with respect to the reference antenna--the two measured vectors determine a plane characterized by roll, pitch and yaw.

In any GPS receiver design, tracking a GPS satellite requires synchronization with and demodulation of the carrier and PRN codes from the GPS navigation signals--a correlation process establishes carrier and code tracking loops that align selected GPS carrier and code (P or C/A) signals with corresponding replica carrier and code signals generated within the GPS receiver. In particular, the receiver measures apparent (pseudo range) transit time by measuring the phase shift between the GPS code signal and the receiver replica code signal--the receiver replica code is shifted until maximum correlation (within the error tolerance of the carrier and code tracking loops) is achieved between it and the received GPS code, with the time magnitude of the shift corresponding to measured pseudo range.

This tracking process of maintaining correlation between the P- or C/A-codes recovered from a selected incoming GPS navigation signal, and the corresponding receiver replica code, is a closed loop. For each GPS satellite being tracked, the selected GPS code/carrier signals are fed into the GPS inputs of the tracking channel's code and carrier correlators, while the replica inputs receive the receiver generated replica code and carrier. The resulting correlator outputs are split into in-phase (I) and quadrature (Q) signals, which are combined into code and carrier error signals. These error signals are fed into code and carrier tracking loop filters to generate corrections to the replica code and carrier generators, the corrected outputs of which are fed back to the code and carrier correlators as a closed loop.

The replica state for the tracking receiver changes as a function of two effects: (a) a time effect due to the signal state rate of change within the satellite as a function of time, and (b) a position effect due to the physical separation between the satellite and the receiver antenna. The physical separation between the satellite and the antenna is, in general, constantly changing due to the satellite orbital motion and, in the case of a dynamic user, due to the antenna motion, thereby producing a doppler effect on the code and carrier signals which is proportional to the net line of sight relative motion between the GPS antenna and the satellite antenna.

For a pointing application, if multiple GPS receiver/antennas are at fixed locations, or are rigidly attached to a single platform (such as on either end of a rigid beam), differential measurements can be made for the multiple antennas with respect to a single satellite. This differential measurement process eliminates the time effect (attributable to the satellite), so that only the relative position effect (attributable to the multiple antennas) remains. Measuring and processing relative position measurements yields the desired pointing vector (at least two antennas) or attitude vectors (at least three non-linear antennas).

This differential process for measuring relative position is commonly referred to as differential carrier doppler phase measurement. Differential carrier doppler phase measurement is not to be confused with range rate (doppler) measurements used for velocity computations--conventional range rate computations use fairly coarse doppler measurements that do not require precise measurement of the phase difference between the doppler shifted GPS signals arriving at different antennas. In contrast, to achieve significant pointing/ attitude accuracy, differential carrier doppler phase measurement must provide a highly precise measure of carrier phase difference with respect to the signals arriving at the different antennas.

Using separated GPS receivers for differential carrier doppler phase measurements inherently introduces differential receiver error sources that impact the precision of those measurements, and therefore, the pointing/attitude computations that use those measurements. These error sources include satellite clock errors, satellite electrical path errors, receiver clock errors, receiver electrical path errors, oscillator noise, code and carrier generator noise, time bias error in the range measurements, satellite clock prediction errors, ephemeris radial prediction errors and ionospheric errors. Several GPS processing techniques have been developed to eliminate the sources of receiver error caused by the introduction of separated GPS receivers (See references 4, 5, 6, listed at the end of the Background).

If analog multichannel GPS receivers are used, another significant error source is interchannel bias error--even if calibrated, interchannel bias changes as a function of both time and temperature. In current digital multichannel GPS receivers with precorrelation analog-to-digital conversion, the analog GPS signals from different satellites pass through a common analog RF/IF front-end, and are delayed by the same amount, producing a common interchannel bias because the analog circuit is common to all GPS signals. The differential digital signal delay paths can be precisely matched and are highly stable over time and temperature, so that the common interchannel bias is eliminated when the outputs attributable to different satellites (i.e., in different channels) are subtracted from each other in the pointing and attitude measurement process.

Even if these error sources can be eliminated or counteracted, three additional error sources significantly impact differential carrier doppler phase measurements used in pointing/attitude: (a) antenna differential phase center migration, (b) differential multipath, and (c) thermal noise.

An ideal GPS receiver antenna navigates the phase center of the GPS antenna when absolute positioning is performed because, ideally, all delay paths between the antenna phase center and the receiver correlators are equal and therefore cancel in the differential measurement process. However, in practice, the location of any GPS antenna phase center migrates as a function of the elevation and azimuth angle of each GPS satellite being tracked because of changes in the antenna gain and phase response at varying angles are not equal. These antenna phase center migration errors can be minimized by using phase matched antennas oriented in the same direction, such that any resulting errors cancel when the signals are differentiated.

Multipath errors arise because the direct-path GPS signal arrival is corrupted by associated multipath signals that arrive slightly later after reflecting from nearby reflecting surfaces. A significant source of multipath reflections is the location of reflecting surfaces near or below the horizon view of the GPS antenna. These multipath reflections can be minimized by antenna designs whose gain near and below the horizon is sufficiently low to reject multipath signals.

The precision of the differential carrier doppler phase measurements is ultimately limited by thermal noise, which is accurately predictable from the strength of the GPS signal, the noise figure of the receiver, and the bandwidth of the tracking loops. Thus the goal of GPS receiver design is to reduce the remaining sources of measurement error (including phase center migration and multipath) so as to be small compared with the thermal noise. In the case of the differential carrier doppler phase measurements used in pointing/attitude applications, an appropriate design goal is to make these error sources small compared to one degree of a carrier cycle at L-band--for the GPS L1 carrier at 1575.42 MHz, this corresponds to 0.5286 millimeters, and for the L2 carrier at 1227.6 MHz, this corresponds to 0.6784 millimeters.

Accordingly, a need exists for an economical GPS system that permits precise pointing or attitude measurements by eliminating or controlling distributed error sources.

REFERENCES

1. Bossler, J. D., and C. C. Goad, "Using the Global Positioning System (GPS) for Geodetic Positioning", Bulletin Geodesique, pages 553-563, 1980.

2. Fell. P. J. "Geodetic Positioning Using a Global Positioning System of Satellites", Reports of the Department of Geodetic Science, Report No. 299, Reference No. DMA PE63701B/3201/240, The Ohio State University Research Foundation, Ohio, Jun., 1980.

3. Remondi, B. W. "Using the Global Positioning System (GPS) Phase Observable for Relative Geodesy: Modeling, Processing, and Results", Center for Space Research, The University of Texas at Austin, May, 1984.

4. Ashkenazi, V., L. G. Agrotis, and J. H. Yau, "GPS Interferometric Phase Algorithms", Proceedings of the First International Symposium on Precise Positioning with the Global Positioning System--Positioning with GPS--1985, Vol 1, pages 299-313 Apr. 15-19, 1985, Rockville, Md.

5. Henson, D. J., E. A. Collier, and K. R. Schneider, "Geodetic Applications of the Texas Instruments TI4100 GPS Navigator", Proceedings of the First International Symposium on Precise Positioning with the Global Positioning System--Positioning with GPS--1985, Vol. 1, pages 191-200, Apr. 15-19, 1985, Rockville, Md.

6. Remondi, B. W., "Performing Centimeter Level Survey Results in Seconds with GPS Carrier Phase: Initial Results", Proceedings of the Fourth Geodetic Symposium on Satellite Positioning, Vol. 2, pages 1229-1250, Apr. 28-May 2, 1986, Austin, Tex.

SUMMARY OF THE INVENTION

The invention is a GPS single-receiver pointing/attitude system and method for deriving pointing/attitude measurements using multiple GPS antennas but only a single GPS receiver.

For pointing (azimuth and elevation), the minimum configuration is two GPS antennas attached to a rigid beam (or other foundation). For attitude (roll, pitch/elevation and yaw/azimuth), the minimum configuration is three GPS antennas arranged in a triangular pattern and attached to a rigid platform (or other foundation). In either case, for this invention, the spatial distance between antennas must be well within the correlation interval for the selected PRN code (plus/minus 29.3 meters for P-code, and plus/minus 293 meters for C/A-code). The invention includes all oversolution applications (i.e., more than two antennas arranged in line for pointing, more than three antennas arranged in a plane for attitude, and more than the minimum number of GPS satellites) to improve performance.

In one aspect of the invention, the GPS single-receiver pointing/attitude system derives pointing/attitude measurements from selected GPS carrier and PRN code (either P or C/A) signals recovered from GPS navigation signals. The system includes multiple antennas coupled to a single GPS receiver.

The antennas--a reference antenna and at least one slave antenna--are configured in a fixed spatial relationship, such that antenna separation is significantly less than the correlation interval for the selected PRN code.

The GPS receiver includes (a) reference precorrelation and correlation electronics, and (b) for each slave antenna, slave precorrelation and correlation electronics. In addition, the GPS receiver includes at least one replica carrier generator for generating a replica of the selected carrier signal, and at least one replica PRN code generator for generating a replica of the selected PRN code signal.

The reference and slave precorrelation electronics provide respective reference and slave digital representations of the GPS signal received by the reference and slave antennas from a selected satellite. The digital reference and slave GPS signals are respectively coupled to the reference and slave correlation electronics.

The reference correlation electronics includes a carrier mixer and a selected number of code correlators, which are responsive to (a) the reference GPS signals, and (b) replica carrier and replica code signals, for providing reference in-phase (I) and quadrature (Q) correlation outputs. The slave correlation electronics includes a carrier mixer and at least one code correlator, which are responsive to (a) the slave GPS signals, and (b) replica carrier and replica code signals, for providing slave I and Q correlation outputs.

A GPS processor is responsive to (a) the reference I and Q correlation outputs for tracking the selected satellite with the reference antenna (the slave antenna need not be tracked), and (b) the reference I and Q correlation outputs and the slave I and Q correlation outputs, which are phase shifted in proportion to the displacement (pointing) of the slave antenna with respect to the tracked reference antenna, for computing pointing/attitude measurements.

Thus, the single-receiver pointing technique involves: (a) establishing a conventional reference antenna tracking loop to obtain reference I and Q correlation outputs from a set of reference code correlators assigned to the reference antenna, (b) generating slave I and Q correlation outputs from at least one slave correlator assigned to the slave antenna; and (c) processing the reference and slave I and Q signals (using differential carrier doppler phase or code phase measurements) to determine phase differences from which pointing can be computed.

In more specific aspects of the invention, the reference correlation electronics can include Early, Prompt (or On-time) and Late (EPL) code correlators, while the slave correlation electronics can be implemented using only a Prompt code correlator. Pointing/attitude measurements are based on computations of carrier phase differences between the reference antenna and the slave antenna according to the relationship:

TAN.sup.-1 ((I.sub.RP *Q.sub.SP -Q.sub.RP *I.sub.SP)/(I.sub.RP *I.sub.SP +Q.sub.RP *Q.sub.SP))

where I.sub.RP and Q.sub.RP are respectively in-phase and quadrature Prompt correlator outputs for the reference antenna, and I.sub.SP and Q.sub.SP are respectively in-phase and quadrature slave prompt correlator outputs for the slave antenna. A reference antenna tracking loop is established using the EPL I and Q outputs from the reference code correlators, while the above carrier phase difference computation only requires the Prompt I and Q outputs from both the reference and slave correlators. Preferably, a single set of replica carrier and code generators is used in common for both the reference and slave antenna signals.

As an alternative to using differential carrier doppler phase measurements for pointing/attitude computations, the reference and slave I and Q correlation outputs can be used to compute reference/slave code phase difference measurements.

For GPS receivers in which each channel includes extra correlators (i.e., in addition to the EPL correlators typically used for tracking), the slave correlation electronics can be implemented using at least one of the extra correlators. In this implementation, the only additional hardware required is a mixer for each slave antenna. During search, all correlators can be used in a conventional manner to expedite acquisition. Once satellite lock is acquired, a conventional tracking loop is established for the reference antenna using the EPL I and Q outputs from the tracking correlators. The now-idle extra code correlator(s) can be coupled to receive the GPS signal from the slave antenna, through the slave mixer, along with an appropriate replica code--for the above carrier phase difference computation, at least the Prompt replica code would be used to provide Prompt I and Q outputs from a designated slave code correlator.

The technical advantages of the invention include the following. The single-receiver GPS design provides a method of GPS signal demodulation and measurement that is both economical (high commonality of hardware) and precise (elimination or control of distributed error sources likely to be encountered when using separate GPS receivers), permitting a highly accurate computation of pointing and attitude. The GPS single-receiver pointing/attitude system can be used in conjunction with an inertial guidance system, providing not only pointing/attitude measurements, but also an accurate source of calibration measurements, thereby allowing lower cost inertial guidance systems to be used.

The receiver tracks a reference antenna using conventional correlation hardware, adding for each slave antenna only an additional mixer and an additional code correlator (or correlators) to produce in-phase I and quadrature Q outputs for the slave antenna(s)--these slave I and Q outputs are phase shifted in proportion to the absolute displacement (pointing) of the slave antenna(s) with respect to the reference antenna, and can be used for pointing or attitude measurements. A common set of replica carrier and code generators can be used to minimize hardware duplication. Either P-code or C/A-code signals may be used for both tracking and pointing/attitude measurements.

The reference I and Q and slave I and Q outputs from the associated code correlators are processed differentially, minimizing common mode receiver error in the GPS observables, such as oscillator noise, code and carrier generator noise, and time bias error, such that the receiver error is largely characterized by thermal noise (which can be predicted). Pointing/attitude computations can be based on either differential carrier doppler phase measurements, or on code phase difference measurements.

Information available in the differential carrier doppler phase measurements can be used to solve for the precise antenna phase center separation (rather than relying on the initial a priori measurement of the antenna baseline), providing an accurate estimate of the antenna baseline separation which, in turn, improves the pointing or attitude measurement accuracy. In addition, an index can be added at precisely the same physical location on each of the multiple and identical GPS antennas (reference and all slaves), and used to orient the antennas identically on a platform, minimizing differential phase center migration which reduces the error contribution by the GPS antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary GPS antenna/receiver configuration of the single-receiver pointing system of the invention;

FIG. 2 illustrates an exemplary antenna structure for the GPS single-receiver pointing system;

FIGS. 3a-3c illustrate the phase difference relationship for GPS navigation signals received by reference and slave antennas from two GPS satellites, including illustrating the ambiguity problem caused by path differences that include an unknown integer number (M and N) of wavelengths in addition to the precisely measured phase difference within one wavelength;

FIG. 4 illustrates an exemplary precorrelation and correlation electronics block diagram for the GPS single receiver pointing system;

FIG. 5 illustrates in functional block diagrams the RF, Analog, Digital, and Control/Display modules for an exemplary GPS single-receiver pointing system;

FIGS. 6-9 respectively illustrate in further functional detail the RF Assembly, Analog module, Channel-on-a-Chip (COAC) used in the Analog module and the Digital module of an exemplary GPS single-receiver pointing system; and

FIGS. 10a and 10b illustrate the software functions performed by the GPS single-receiver pointing/attitude system.

DETAILED DESCRIPTION OF THE INVENTION

The Detailed Description of an exemplary embodiment of the GPS single-receiver pointing/attitude system of the invention is organized as follows:

1. GPS Single-Receiver Pointing Embodiment

2. Differential Carrier Doppler Phase

2.1. GPS Interferometry

2.2. Ambiguity Resolution

2.3. Computing the Pointing Vector

3. Single-Receiver Pointing Technique

3.1. Reference/Slave Correlation

3.2. Carrier Phase Difference Measurement

3.3. Code Offset Coarse Measurement

3.4. Pointing Observables

3.5. Error Sources

4. Exemplary GPS Receiver

4.1. Antennas

4.2. RF Assembly

4.3. Analog Module

4.4. Digital Module

4.5. Software

5. Conclusion

The exemplary embodiment of the invention is described in relation to a GPS single-receiver pointing system for deriving precise pointing measurements using two antennas (reference and slave) mounted to a rigid beam. This Detailed Description can be routinely extended to a GPS single-receiver attitude embodiment by providing a reference and at least two slave antennas mounted on a platform that defines a plane. In addition, the Detailed Description can be routinely extended to oversolution pointing embodiments with more than two slave antennas mounted co-linearly with the reference antenna to a beam, and to oversolution attitude embodiments with more than three antennas mounted on a platform, thereby improving performance through redundancy.

1. GPS Single Receiver Pointing Embodiment. The exemplary GPS single-receiver pointing embodiment of the invention is illustrated generally in FIGS. 1 and 2

Referring to FIG. 1, the GPS single-receiver pointing embodiment 10 includes an antenna structure 12 with two GPS antennas 14 and 15 mounted to a rigid beam 16. Antenna 14 is designated the reference antenna, and antenna 15 is designated the slave antenna.

The reference and slave antennas 14 and 15 should be proximate in that antenna separation should be significantly less than the correlation interval for the selected PRN code signal--58.6 meters for P-code, or 586 meters for C/A-code. An appropriate antenna separation would be 5 meters or less.

GPS navigation signals (Carrier, PRN Code and Navigation Message Data) are received by GPS antennas 14 and 15, and applied to RF/IF modules 21 and 22, which provide the L-band down conversion portion of the precorrelation electronics for both antenna signals.

An Analog module 30 receives the reference and slave IF GPS navigation signals into respective Ports 1 and 2. The Analog module provides the signal conditioning and analog-to-digital conversion for both ports, which completes the precorrelation electronics, and includes separate correlation electronics for the received reference and slave IF GPS navigation signals.

The reference and slave precorrelation electronics convert the corresponding analog GPS navigation signals to digital signals that are input to the correlation electronics. The reference and slave correlation electronics generates replica code and carrier signals, and include the mixers and code correlators that provide the in-phase I and quadrature Q outputs for the code and carrier loops.

A Digital module 35 receives the reference and slave I and Q correlator outputs (Early, Late and Prompt), and implements (a) conventional reference receiver tracking, and (b) differential carrier doppler phase processing to obtain pointing measurements according to the invention. For reference receiver tracking, the reference correlation electronics and the Digital module establish a conventional GPS satellite tracking loop using the GPS navigation signals received from a selected satellite by the reference receiver--the slave antenna need not be tracked to compute pointing/attitude according to the invention.

For the exemplary GPS single-receiver pointing embodiment, the digital module processes the Prompt I and Q outputs from the reference and the slave code correlators to measure the phase difference between the GPS navigation signals received by the reference and slave antennas from a selected satellite (see, Section 3). The Digital module then computes pointing using conventional differential carrier doppler phase processing.

For attitude determinations, the GPS receiver can be configured with an additional GPS antenna, associated precorrelation electronics and an additional port to the correlation electronics.

FIG. 2 illustrates an exemplary antenna structure 40, that includes reference and slave antennas 42 and 44 mounted on a rigid beam 45, separated by about one meter. The antenna structure can be configured for mounting on a tripod or vehicle.

2. Differential Carrier Doppler Phase. The GPS single-receiver pointing/attitude system of the invention implements conventional differential carrier doppler phase processing techniques used in GPS interferometry, including processing techniques for eliminating receiver error sources (referenced in the Background).

These error processing techniques have been developed in the context of geodetic surveying using widely separated GPS receiver/antennas, but also apply to the closely spaced reference and slave GPS antennas 14 and 15 mounted at the ends of the beam 16. As with geodetic surveying, pointing involves determining the azimuth and elevation angles for the baseline pointing vector established between the phase centers for the reference and slave antennas.

In contrast to geodetic applications using a reference antenna/receiver (known location) and a slave antenna/receiver (unknown location), for pointing applications using the GPS single-receiver pointing embodiment 10, the separation between the reference and slave antennas 14 and 15, i.e., the magnitude of the baseline vector, is known a priori fairly precisely because they are rigidly mounted and move together. However, the pointing angle of the baseline pointing vector defined by the direction of the beam in azimuth and elevation is unknown, and typically changing.

2.1. GPS Interferometry. The same vector computation techniques used in geodetic applications are used to determine the relative direction from the reference antenna to the slave antennas 14 and 15, and therefore, the desired pointing vector.

That is, conventional geometric principles establish the pointing angle for the line defined by the physical arrangements of the reference and slave antennas, and conventional GPS interferometry principles provide the differential carrier doppler phase measurement techniques for computing that pointing angle. In particular, the single-receiver GPS pointing technique of the invention uses these GPS interferometric principles to compute pointing from carrier doppler phase observables.

Geodetic interferometry techniques for pointing can be based on the differential processing of either (a) pseudo range observables alone, or (b) a combination of pseudo range and carrier doppler phase observables.

In dynamic point positioning, differential processing is applied to pseudo range observables, yielding a relative accuracy for the pointing vector of about 1 to 3 meters with the smoothing provided by a one second time constant filter (which can be improved with longer time averaging). With interferometry techniques, the continuously counted carrier doppler phase observables are also used, resulting in a differential accuracy for the same smoothing time constant of about 1 to 3 millimeters, which is about three orders of magnitude more accurate than dynamic point positioning.

2.2. Ambiguity Resolution. GPS interferometric techniques involve ambiguity in the solution space for the carrier doppler phase measurements, which must be resolved. Specifically, the carrier doppler phase measurement repeats every wavelength of the GPS carrier frequency, which is about 19 centimeters for L1.

Thus, the actual path difference for the reference and slave antennas to a selected satellite will be the distance corresponding to the precisely measured phase difference plus an unknown integer number of wavelengths. As a result, a number of path-difference solutions exist within the uncertainty region, creating an ambiguity problem at every integer wavelength. Dynamic point positioning may be useful in reducing the