Self-calibrating interferometric synthetic aperture radar altimeter
View Patent ↗A synthetic aperture radar system uses RF bandwidth and Doppler beam sharpening principles to develop fine altitude and along-track resolutions. To achieve accurate cross-track position measurements the system and method exploit a combination of modes based on a novel antenna pattern combination. The unique arrangement of the antenna patterns allows the radar to process terrain elevation measurements in three independent modes, namely, time-delay response (TDR), amplitude monopulse (AM) and phase monopulse (PM). The additional modes address the interfering scatter problem and the calibration issues required for practical and cost effective operation. The approach also maximizes the number of terrain measurements made per look, thereby reducing the impact of errors and noise through averaging and “voting” (i.e., the comparison of measurements and discarding of “outliers”).
1. A method of remotely sensing terrain elevation, comprising the steps of:
sampling returns from radar scatterers associated with the terrain along multiple lanes using first and second channels that overlap to form a cross-shaped antenna pattern; and
analyzing the returns to determine the elevation according to one or more or the following modalities:
phase monopulse (PM),
amplitude monopulse (AM), and
time delay response (TDR).
2. The method of claim 1 , wherein the PM mode is based upon interferometric Doppler beam-sharpening.
3. The method of claim 1 ,wherein the AM mode includes the step of misaligning the overlapping antenna patterns to create an amplitude ratio variation as a function of cross-track position in the sampling lanes.
4. The method of claim 1 , wherein the TDR mode includes the step of using different channels to measure the time delay of scatterers within a common lane.
5. The method of claim 1 , further including the step of increasing coherent integration time so as to maintain constant along-track resolution on the ground us the distance to the terrain increases.
6. In an interferometric synthetic aperture radar (IFSAR) system utilizing dual receive antennas, the improvement comprising:
overlapping the antenna patterns to form a cross-shaped footprint enabling simultaneous terrain elevation measurement using one or more of the following modalities:
phase monopulse (PM),
amplitude monopulse (AM), and
time delay response (TDR).
7. A method of remotely sensing terrain elevation, comprising the steps of:
sampling returns from radar scatterers associated with the terrain along multiple lanes A, B, and C using first and second channels that form a cross-shaped antenna pattern that substantially overlaps in lane B; and
analyzing the returns to determine the elevation according to one or more or the following modalities:
phase monopulse (PM) using interferometric Doppler beam-sharpening,
amplitude monopulse (AM) using an amplitude ratio variation as a function of cross-track position in the sampling lanes, and
time delay response (TDR) using the first and second channels to determine the time delay of a scatterer in one of the lanes.
8. The method of claim 7 , wherein the time delay is determined by both the speed of flight and the cross-track position of the scatterer tracked using TDR.
9. The method of claim 8 , further including the step of using timing measurements obtained for the multiple lanes in conjunction with an external reference reference DTED to obtain a more accurate speed of flight.
10. The method of claim 9 , wherein the external reference includes independently derived digital terrain elevation data (DTED).
11. The method of claim 9 , wherein the external reference includes global positioning satellite (GPS) coordinates.
12. The method of claim 9 , further including the step of increasing coherent integration time so as to maintain constant along-track resolution on the ground as the distance to the terrain increases.
13. A multi-mode altimeter system for use on a platform that moves relative to a terrain of interest, the system comprising:
a multi-channel antenna operative to generate an overlapping, cross-shape pattern defining a first channel and a second channel; and
a Doppler processor using selective timing and filtering to perform elevation measurements along three sampling lanes A, B and C on the terrain covered by the cross-shaped channel pattern in accordance with time-delay response (TDR), amplitude monopulse (AM) and phase monopulse (PM) modes or operation.
14. The altimeter system of claim 13 , wherein the PM mode of operation utilizes interferometric Doppler beam-sharpening.
15. The altimeter system of claim 13 , wherein the AM mode of operation is implemented through a misalignment in the overlapping antenna patterns to create an amplitude radio variation as a function of cross-track position in each lane.
16. The altimeter system of claim 13 , wherein the TDR mode of operation is implemented by analyzing the terrain in multiple channels within the same lane.