Atmospheric correction method for interferometric synthetic array radar systems operating at long range

• US 5,726,656 A
• Filed: 12/19/1996
• Issued: 03/10/1998
• Est. Priority Date: 12/19/1996
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Electronics Corporation (Rtx Corporation)
• Inventors: Frankot, Robert T.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US08/769,568
• Time in Patent Office: 446 Days
• Field of Search: 342/25, 342/26, 342/156, 342/174, 342/195, 342/196, 342/194, 342/203
• US Class Current: 342/25.00C
• CPC Class Codes: G01S 13/9023 combined with interferometric techniques
• CPC Class Codes: G01S 7/4004 of parts of a radar system

Abstract

A method of computing and correcting phase errors due to atmospheric turbulence in a dual-antenna multiple-pass synthetic array radar (SAR) interferometer system. The method improves topographic mapping accuracy at very long ranges. The method computes an atmospheric correction from the residual phase difference between a two-pass interferometer output (containing atmospheric phase errors) and a dual-antenna single-pass interferometer output (for which atmospheric phase errors cancel because the two channels are collected simultaneously). The residual phase measured in a single resolution element is then filtered by averaging over an area of many resolution elements, typically on the order of several thousand resolution elements. This exploits the long correlation distance of the atmospheric phase errors to reconstruct the dominant low-frequency part of the error spectrum. In addition, unwrapping the phase of the output of the complex Wiener filter produces an atmospheric turbulence measurement map.

Claims

1. A method for use with an interferometric synthetic aperture radar system having two antennas separated by a predetermined separation distance which generates data comprising complex image pairs representative of an image scene, and wherein the method computes and corrects phase errors due to atmospheric turbulence, said method comprising the steps of:

generating a first pair of images having a short baseline using the synthetic aperture radar system;

generating a second pair of images having a long baseline using the synthetic aperture radar system;

combining the respective first and second pairs of images to produce first and second summed complex interferograms;

processing the first complex interferogram sum to unwrap its phase, wherein the phase of each complex pixel is computed and the absolute phase is reconstructed, up to a single additive constant and to produce an unwrapped phase image;

processing the real-valued unwrapped phase image and a scalar value given by the ratio of the grazing angle differences between the long baseline images and that of the short baseline images to scale the unwrapped phase image by the baseline ratio;

processing the scaled reference phase image and the second complex interferogram sum to remove the phase reference from the second complex interferogram sum to produce a complex image whose phase is that of the second complex interferogram sum minus the reference phase image;

processing the complex image using a complex Wiener filter to produce a low-pass filtered complex interferogram;

processing the complex image and the low-pass filtered complex interferogram to remove the phase of the Wiener-filtered interferogram from the complex image and provide an atmospherically corrected complex image whose phase is the phase of the complex image minus the phase of the low-pass filtered complex image, which complex image;

processing the atmospherically corrected complex image to produce a real-valued atmospherically corrected image having unwrapped phase; and

adding the real-valued atmospherically corrected image and the real-valued reference phase image provided by the baseline ratio scaling function to produce an image that is proportional to terrain elevation.

2. The method of claim 1 wherein the complex Wiener filter comprises a filter based on average radar measurements of atmospheric propagation spectra.

3. The method of claim 1 wherein the complex Wiener filter is created by fitting a spectral model to the long baseline interferogram after subtracting the phase of the short baseline interferograms.

4. The method of claim 1 further comprising the step of:

processing the low-pass filtered complex interferogram to unwrap its phase and produce an atmospheric turbulence measurement map.

5. A method for use with an interferometric synthetic aperture radar system having two antennas separated by a predetermined separation distance which generates data comprising complex image pairs representative of an image scene, and wherein the method computes and corrects phase errors due to atmospheric turbulence, said method comprising the steps of:

generating a first pair of images having a short baseline using the synthetic aperture radar system;

generating a second pair of images having a long baseline using the synthetic aperture radar system;

combining the respective first and second pairs of images to produce first and second summed complex interferograms;

processing the first complex interferogram sum to unwrap its phase, wherein the phase of each complex pixel is computed and the absolute phase is reconstructed, up to a single additive constant and to produce an unwrapped phase image;

processing the real-valued unwrapped phase image and a scalar value given by the ratio of the grazing angle differences between the long baseline images and that of the short baseline images to scale the unwrapped phase image by the baseline ratio;

processing the scaled reference phase image and the second complex interferogram sum to remove the phase reference from the second complex interferogram sum to produce a complex image whose phase is that of the second complex interferogram sum minus the reference phase image;

processing the complex image using a complex Wiener filter to produce a low-pass filtered complex interferogram; and

processing the low-pass filtered complex interferogram to unwrap its phase and produce an atmospheric turbulence measurement map.