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# Wide Angle SAR Imaging

ALGORITHMS FOR SYNTHETIC APERTURE RADAR IMAGERY XI, (2004): 164-175

Abstract

We consider imaging strategies for synthetic aperture radar data collections that span a wide angular aperture. Most traditional radar imaging techniques are predicated on the assumption of isotropic point scattering mechanisms, which does not hold for wide apertures. We investigate point scattering center images for narrowband, wide angl...More

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Introduction

- Wide-angle synthetic aperture radar (SAR) refers to any synthesized aperture whose angular extent, ∆φ, exceeds the sector required for equal resolution in range and cross-range:

∆φ > 2 sin−1 (BW/(2fc)) (1)

where BW and fc are the bandwidth and center frequency of the radar. - For X-band systems, which operate near fc = 10 GHz, Table 1 lists the aperture sizes to give equal downrange and crossrange resolution for several radar bandwidths.
- UAVs can in many applications fly closer to the scene of interest, and can traverse a wider angle aperture in a given amount of time compared to a platform with a greater standoff distance.
- In which a distant standoff platform acts as the transmitter and one or more UAVs act as closer-in receivers, can provide greater angular coverage.
- As the authors discuss in Section 3, phase coherence of these subapertures may not be necessary for effective wide-angle imaging

Highlights

- Wide-angle synthetic aperture radar (SAR) refers to any synthesized aperture whose angular extent, ∆φ, exceeds the sector required for equal resolution in range and cross-range:

∆φ > 2 sin−1 (BW/(2fc)) (1)

where BW and fc are the bandwidth and center frequency of the radar - Unmanned air vehicle can in many applications fly closer to the scene of interest, and can traverse a wider angle aperture in a given amount of time compared to a platform with a greater standoff distance
- In Section 2 we present some results on wide angle imaging image responses for ideal point scattering centers and for scattering centers whose response persists over a subset of the measurement aperture
- We have considered aspects of wide angle synthetic aperture radar imaging
- We have proposed a composite nonlinear combination of subaperture images as an alternative to coherent imaging over a wide aperture; the subaperture combination has the interpretation of a generalized likelihood ratio test (GLRT) for matched filtering of scattering responses with unknown peak azimuth response direction
- Wide-angle composite synthetic aperture radar images are obtained by noncoherent combination of narrower-aperture subimages

Results

- Scattering persistence for various peak response azimuths; for this aperture, the downrange bandwidth given in equation (8) is about 30% higher than the redar bandwidth.

Conclusion

- The authors have considered aspects of wide angle synthetic aperture radar imaging. Traditional SAR imaging techniques often assume rectangular or near-rectangular frequency-domain data and are based on a point scattering assumption; neither holds true in the wide angle imaging case.
- The authors have proposed a composite nonlinear combination of subaperture images as an alternative to coherent imaging over a wide aperture; the subaperture combination has the interpretation of a generalized likelihood ratio test (GLRT) for matched filtering of scattering responses with unknown peak azimuth response direction.
- Using this approach, wide-angle composite SAR images are obtained by noncoherent combination of narrower-aperture subimages.
- The authors briefly explored the use of regularized inverse imaging solutions, and showed by example that substantial resolution enhancement is possible using narrowband, wide angle data

- Table1: Nominal aperture size for equal downrange and crossrange resolution at X-band with fc = 10 GHz

Reference

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