Technique for accurate estimate of large antenna inertial two dimensional orientation using relative GPS spatial phase

A radar antenna has a reflector and maximum gain along its boresight. The reflector has a periphery, typically circular, rectangular or elliptical. A plurality of Global Positioning System (GPS) satellite signal receiving antennas are rigidly, mechanically attached to the reflector near its periphery. The plurality of GPS satellite signal receiving antennas are connected pairwise to a phase comparator for comparing a plurality of first phase differences induced by a first GPS satellite signal received concurrently between the plurality of GPS satellite signal receiving antennas. A Phase comparator measures the phase difference of the signal received at GPS satellite signal receiving antennas pairwise thus performing a differential phase measurement. This differential phase measurement is supplied to a computer for identifying an ambiguous boresight position using the phase differences measured by the phase comparator. The position of the GPS satellites is known with respect to the geo-location of the antenna. Thus, the boresight angle is derived from the phase difference of the carrier signal from the GPS satellite being received and the mechanical alignment information between the GPS satellite receiving antennas and radar antenna boresight stored during calibration/manufacture of the radar antenna. The ambiguity in the computed boresight position is resolved by making differential phase readings using the same GPS antennas from a second GPS satellite signal supplied by a second satellite.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention is in the field of antenna position location using signals from a satellite constellation part of the Global Positioning System (GPS).

2. Description of the Related Art

Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses. These pulses are from a relatively small antenna on a moving platform. As the platform moves, the information contained in the pulses is coherently combined to arrive at a high resolution SAR image.

The plurality of returns creating a SAR image generated by the transmitted pulses along a presumed known path of the platform make up an array. Theoretically, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. That is, the SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.

The usefulness of a SAR image is dependent on accurate knowledge of antenna orientation with respect to a local coordinate during the acquisition of the SAR image. Antenna orientation accuracy is critical to radar return motion compensation and map positioning within the local coordinate system. Motion compensation shifts the phase of each radar sample (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion in space of the moving antenna, with respect to a reference point. The SAR imaging process depends on the coherent, phase accurate summing of all radar returns expected within an array. These principles are detailed by W. G. Carrara, R. S. Goodman and R. M. Majewski inSpotlight Synthetic Radar, Boston, Artech House, 1995, incorporated herein in its entirety by reference.

Antenna azimuth and elevation with respect to a geo reference was measured in the prior art with a mechanical angle transducer. Such an approach dependent on mechanical gears having backlash is insufficiently accurate where a SAR display depends on antenna angle orientation, especially where the SAR map has resolution of a few feet at a range of 50 nautical miles.

A tool recently introduced in geo-locating a radar is the Global Positioning System (GPS). GPS provides a constellation of satellites, each transmitting a timing signal. Simultaneous receipt of two, preferably three or more of the GPS satellite timing signals yields a geo-position with an accuracy of 20-30 meters. This accuracy is sufficient for determining the general location of an antenna but insufficient to determine antenna orientation in typical radar applications. Even using a differential approach, for example as described in U.S. Pat. No. 6,559,793, dated Nov. 2, 2001 to Eschenbach, assigned to Trimble Navigation Limited, incorporated herein in its entirety by reference, the resulting position accuracy is insufficient to be used for antenna orientation purposes.

SUMMARY OF THE INVENTION

Accurate radar antenna orientation measurement is obtained by a radar antenna that has a reflector and maximum gain along the boresight. The reflector forms a curved surface, where the curved surface is generally perpendicular to the radar antenna boresight at the point of intersection between the boresight and the reflector. The reflector has a periphery, circular, rectangular or elliptical. The radar antenna comprises a plurality of Global Positioning System satellite signal receiving antennas. The plurality of Global Positioning System satellite signal receiving antennas are positioned on the reflector, preferably along the periphery, equidistantly spaced and straddling the boresight. The plurality of Global Positioning System satellite signal receiving antennas are connected pairwise to a phase comparator for comparing a plurality of first phase differences induced by a first Global Positioning satellite signal received concurrently between the plurality of Global Positioning System satellite signal receiving antennas.

A Phase comparator measures the phase difference of the carrier signal received at Global Positioning System satellite signal receiving antennas thus performing a differential phase measurement. The result of the differential phase measurement is supplied to a computer for identifying an ambiguous boresight position using the phase differences measured by the phase comparator. The position of the GPS satellites is known with respect to the geo-location of the radar antenna. Thus, the boresight angle is derived from the phase difference of the carrier signal from the GPS satellite being received and fixed, mechanical alignment information between the GPS satellite receiving antennas and the boresight. The relationship between the GPS antennas and boresight is mechanically fixed, and stored during calibration/manufacture of the radar antenna. The ambiguity in the received phase position is resolved by taking differential phase reading from a second GPS satellite signal supplied by a second satellite, or a third, or a fourth.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure describes a method and apparatus for collecting antenna position measurements using phase measurements from a plurality of GPS sensors mechanically connected to structurally stiff locations on a large radar antenna, and combining the plurality of target motion measurements to improve overall antenna orientation accuracy as compared to previous approaches.

FIG. 1shows an antenna101of the prior art where the azimuth orientation of the antenna is measured by a rotating angle transducer103capable of reporting the antenna azimuth angle position. From geometric considerations, if the uncertainty, or error of the reported azimuth angle from transducer103is θ, then the azimuth error at range R is:
Azimuth Error=R sin θ

Thus, mechanical angle transducers having an accuracy in the order of ½ degree are too coarse, or inaccurate to provide a meaningful indication of antenna azimuth position in radar systems requiring azimuth (cross range) accuracy in the order of 2 ft. Even a ten fold improvement in measuring θ by an angle transducer103would not be satisfactory in such a scenario: Given these high errors and associated limitations in deriving accurate images, it is desired to implement a viable method of measuring antenna orientation around an axis, applicable to both azimuth and elevation in the case of relatively large antennas having an energy receiving area in the order of 20 meters by 20 meters.

In one embodiment of the present teachings, shown inFIG. 2, radar antenna202has four separate GPS (satellite signal) receiving antennas204,206,208,210mechanically connected (welded) to the corners of radar antenna202. It is assumed radar antenna202structure is relatively mechanically stiff, having a natural frequency under 10 hz. The mechanical stiffness insures that the antenna boresight initial alignment with the mechanical reference points represented by GPS receiving antennas204,206,208,210remains constant during antenna motion whether said motion is induced from intentional antenna rotation or due to wind forces, rain, snow or ice. For example, radar antenna202is 20 meters by 20 meters in size, having a parabolic cross-section.

GPS receiving antennas204,206,208and210monitor not the exact position of the receiver itself derived from pulse code as is customary, but rather the relative carrier phase of the. GPS signal from a constellation of satellites being received at their particular locations on radar antenna202. Thus, an accurate estimate of antenna two dimensional inertial orientation is derived using a measurement of relative GPS spatial phase. The relative carrier phase of the GPS signal from a plurality of geo-orbiting satellites, part of the GPS constellation, is measured between a plurality of GPS antennas, such as GPS receiving antennas204,206,208and210. Phase bias errors and phase ambiguities are eliminated with measurements from multiple satellites, part of the GPS constellation of satellites.

For example, at typical GPS operating frequencies, thermal noise and associated position error of each GPS antenna is given by

Where SNR is signal to noise ratio, and NPDI is number of pulses integrated.

The corresponding error is 0.076 milliradians.

The phase center error, defined as the GPS phase center relative to the nearest optical calibration point of radar antenna202is
λ/40=5 mm

where 5 mm corresponds to an angle error of 0.262 milliradians.

Phase linking with a common local oscillator between GPS receiving antennas204,206,208,210and associate receiver(s) is not required.

FIG. 3further details the configuration of the embodiment ofFIG. 2. Here the optical boresight corresponding to the peak of the antenna gain pattern of antenna202forms and angle α with the carrier signal form the GPS satellite. Antenna (optical) boresight is initially calibrated in azimuth by comparing the position of the antenna radiation/reception maximum, i.e. the peak of the antenna pattern, against the phase differences from GPS antennas204and208positioned at or near the outer periphery of antenna202. Phase comparator301compares the incoming carrier signal received by GPS antenna204and GPS antenna208to generate a measured phase. The measured phase is initially calibrated against the antenna boresight position and stored in a table. As antenna202moves, and the reading from phase comparator301is obtained, the new reading is input into the table obtained during the calibration process, and antenna azimuth position read out.

FIG. 2, and3show four GPS antennas generally equidistantly positioned around the periphery of antenna202as an example of how antenna boresight information can be derived from GPS carrier signals. However, a simpler embodiment is shown inFIG. 4. Here, only three GPS antennas,404,406, and408are equidistantly positioned along the periphery of a circular antenna402. Phase difference measurements are taken between GPS antenna404and406to obtain an azimuth reading. Elevation measurements are obtained by comparing the phase received at GPS antenna404with the phase at GPS antenna408. A calibration adjustment adjusts the phase read out for the position of GPS antenna404away from the vertical axis perpendicular to the antenna boresight. To simplify further, if only azimuth boresight position is required, only two GPS antennas, such as GPS antennas404and406, will generate the required azimuth position. It is desired to space GPS antennas404and406as much as possible thereby generating a large phase difference in the received GPS signal carrier for the angular motion of antenna402.

Taken together,FIGS. 2,3and4show a radar antenna202having a reflector for receiving and transmitting radar pulses and an antenna boresight. Antenna202has a maximum gain along the boresight. The reflector forms a curved surface, where the curved surface is generally perpendicular to the boresight at the point of intersection between the boresight and the reflector. The reflector has a periphery, either circular or rectangular. Antenna202comprises three or more Global Positioning System satellite signal receiving antennas such as204,206,208and210inFIG. 2or404,406and408inFIG. 4. The three or more Global Positioning System satellite signal receiving antennas are positioned on the reflector.

The three or more Global Positioning System satellite signal receiving antennas (204,206,208,210) are connected pairwise to a phase comparator for comparing a plurality of first phase differences induced by a first Global Positioning satellite signal received concurrently between the three or more Global Positioning System satellite signal receiving antennas.FIG. 3shows two such Global Positioning System satellite signal receiving antennas,204and208connected to phase comparator301. The signal from the GPS satellite is the carrier, not the actual coded information. It is the carrier that provides the resolution necessary to perform the differential phase measurement.

Phase comparator301supplies the phase difference of the signal received at Global Positioning System satellite signal receiving antennas204and208to computer303for identifying an ambiguous boresight position using said first phase differences measured by said phase comparator. Since the position of the GPS satellites is known with respect to the geo-location of antenna202, the angle α can be derived from the phase difference of the carrier signal from the GPS satellite being received.

Typically, as shown inFIG. 2andFIG. 4two of said three or more Global Positioning System satellite signal receiving antennas are straddling the boresight of the antenna. Also, in one embodiment, the three or more Global Positioning System satellite signal receiving antennas are positioned equidistant from the boresight in a plane perpendicular to the boresight.

For maximum angle sensitivity, it is desired to obtain maximum phase change for the least amount of angular displacement by antenna202, or antenna402, thus the three or more Global Positioning System satellite signal receiving antennas are positioned along the periphery of the reflector, or a boom extending from the antenna, but rigidly connected to it.

The phase comparator output for each pair of antennas, such as204and208, is more efficiently processed if the three or more Global Positioning System satellite signal receiving antennas are positioned equidistantly along the periphery of said reflector. Using the same basis, two of the three or more Global Positioning System satellite signal receiving antennas (such as204and208) are aligned with a horizontal plane, where the horizontal plane includes the boresight facilitating radar antenna202azimuth angular position measurements. Similarly, positioning two of the three or more Global Positioning System satellite signal receiving antennas, such as208and210, aligned with a vertical plane, where the vertical plane includes the boresight facilitates radar antenna202elevation angular position measurements.

The concepts above expand upon GPS operation detailed inGPS: Theory and Practice, by Hoffman Wellenhof, B. H. Lichtenegger and J Collins 1994, 3rd ed, New York: Springer Verlag publishers, incorporated herein in its entirety by reference. Further details of GPS operation for this disclosure are also found inUnderstanding GPS: Principles and ApplicationsElliott D. Kaplan ed., 1996, Boston: Artech House Publishers, incorporated herein in its entirety by reference. Position measurements using phase comparison of GPS signals is further detailed inGPS Satellite Surveying2nd ed, by Leick Alfred, 1995, New York; Wiley and Sons, incorporated herein in its entirety by reference.

The method used is further detailed inFIG. 5. Computer303computes boresight position based on the inputs from the phase comparator301. The initial step is to perform a calibration during the manufacturing of radar antenna202or402. During the calibration, Compute Table of phase differences as a function of time and satellite position501is recorded. This creates a table, or function, within computer303identifying phase differences to be expected as a function of antenna boresight position with respect to the GPS signal received from the constellation of GPS satellites. A second table, or function, computed in Compute table of Ambiguity function and solution503resolves the ambiguity inherent in performing actual measurements by taking a second differential phase measurement using a second, a third, or a fourth GPS satellite.

In accordance withFIG. 5, after having performed the initial calibration, the method for identifying a boresight position of a radar antenna having a reflector for receiving and transmitting radar pulses and a radar antenna boresight, said radar antenna having a maximum gain along said boresight, said reflector forming a curved plane, said curved plane generally perpendicular to said boresight at the point of intersection between said boresight and said reflector, said reflector having a periphery, comprises the following steps.

1) Positioning three or more Global Positioning System satellite signal receiving antennas (e.g.204,206,208,210) on the reflector along its periphery;

2) Connecting the three or more Global Positioning System satellite signal receiving antennas to a phase comparator (such as phase comparator301) for detecting a plurality of phase differences derived from a first Global Positioning satellite signal received among said three or more Global Positioning System satellite signal receiving antennas;

3) Identifying an ambiguous boresight position with respect to the first Global Positioning satellite from said first phase difference measured by the phase comparator by using Read Phase difference505to read the phase difference between GPS antennas of interest, i.e. antenna pairs204and208for an azimuth position, or the pairs208and210for elevation. Using this phase difference, obtained from the phase comparator, such as phase comparator301, computer303uses table501in Use Table to extract boresight position507to extract an ambiguous boresight position. On performing a second phase measurement using another GPS satellite, the ambiguity of the ambiguous boresight position is resolved to generate an unambiguous boresight position using information in table503. Report Boresight position511reports the unambiguous position for subsequent use in the processing of images.

All references cited in this document are incorporated herein in their entirety by reference.

Although presented in exemplary fashion employing specific embodiments, the disclosed structures are not intended to be so limited. For example, although the antenna orientation accuracy improvement herein is described in the context of a radar antenna, the disclosure is also applicable for sonar, or other large antennas where the orientation of the large antenna is independently determined by a plurality of phase sensors mechanically linked to the antenna. The phase sensors supply phase measurements to a phase comparator, thus deriving antenna azimuth and elevation orientation from receiving a reference signal from a source having a known location at a particular time. The concepts herein are fully applicable to the Galileo GPS system proposed for implementation in the near future.

Those skilled in the art will also appreciate that numerous changes and modifications could be made to the embodiment described herein without departing in any way from the invention.