Suppressing Paired Echoes Caused by Stair-Step Antenna Steering in TOPS SAR Imaging

He He,Jin Jin,Yi Yi,Dong Dong,Zhang Zhang

doi:10.3390/rs11212544

Abstract

The use of electronically steered antennas in the azimuth dimension typically leads to a staircase-like antenna beam steering law in the Terrain Observation by Progressive Scan (TOPS) wide-swath synthetic aperture radar (SAR) data acquisition mode, which will introduce paired echoes in the focused images. This paper proposes a new approach for removing such paired echoes from TOPS SAR images based on the generalization of the ideal optimum filtering concept, which can be implemented easily in the SAR data processing. Modeling the amplitude-modulated azimuth signal shows that the absolute phase of the introduced paired echoes cannot be determined due to the random rotation angle jump time for each target, which will prevent the precise use of optimum filtering. An extended optimum filtering approach, which is originally proposed for suppressing the azimuth ambiguities in SAR images, is reintroduced in this particular case, and a new approximated and generalized form of the deconvolving filtering in the approach is redefined to accommodate the undetermined phase for both the strongest paired distortion peaks and the other peripheral peaks in the distorted impulse response function (IRF). Simulated data from a TOPS SAR mode with staircase-like beam steering are used to verify the improvement in image quality by using the new method.

Highlights

The Terrain Observation by Progressive Scan (TOPS) [1,2] is a new alternative data acquisition mode for synthetic aperture radar (SAR) wide-swath imaging and interferometry [3,4], which can achieve the same wide swath coverage as in ScanSAR mode but drastically reducing its drawbacks
It is well known that ScanSAR images suffer an azimuth variation of signal-to-noise ratio (SNR) and azimuth ambiguities, which is a consequence of integrating different azimuth antenna pattern slices depending on the azimuth target position [5]
All the simulations are based on the multi-mode version of SBRAS (Space-borne Radar Advance Simulator) [21,22]

Summary

Introduction

The Terrain Observation by Progressive Scan (TOPS) [1,2] is a new alternative data acquisition mode for synthetic aperture radar (SAR) wide-swath imaging and interferometry [3,4], which can achieve the same wide swath coverage as in ScanSAR mode but drastically reducing its drawbacks. In orbital SAR systems, limited by the onboard storage ability, electronically-steered antennas only use a limited number of azimuth steering angles, which leads to a staircase-like antenna beam steering law [2]. It will cause an undesired amplitude modulation for every observed target in the raw data. The paired echoes will distort the ideal target impulse response function (IRF) by introducing symmetrically-placed distortion peaks [6] These spurious peaks, may appear in some focused images as “ghost targets”, for scenarios where strong scatterers are embedded in a large scene of low backscatters [2], such as for a wide-swath maritime surveillance usage [7]

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Conclusion