Analysis of Monostatic SAR Processing Techniques to Focus Stationary Bistatic Airborne Data

Abstract

Due to their potential applications and flexibility, bistatic configurations have awakened a great deal of interest in the recent past among the SAR research community for both airborne and spaceborne remote sensing. However, such configurations still pose a number of operational problems; notwithstanding the technical challenge of tuning a bistatic SAR system, current efforts are being conducted to derive reliable techniques that allow the scientific and technical exploitation of the data. Focussing stands out as a fundamental issue to facilitate further research. In February 2003, DLR and ONERA carried out successfully [1] one of the very first bistatic airborne campaigns. The E-SAR and RAMSES systems were flown in two basic geometrical configurations, both involving parallel flights at the same speed: one defined by along-track acquisitions and the other by across-track acquisitions. These bistatic configurations share with monostatic SAR the invariance in the along-track dimension, often referred as stationary, which allows and suggests the using of Doppler domain focussing techniques. Different directions can be outlined in the recent literature concerning bistatic SAR focussing. Several recent papers address the problem of obtaining airborne bistatic SAR images using time-domain techniques [2]-[4]. Time-domain techniques are usually computationally expensive, but provide accurate results especially in the case of motion errors. Other focusing techniques neglect such errors and try to overcome the efficiency problem by using Doppler domain techniques [4]-[6]. They benefit from the existing literature in monostatic focussing to provide adequate solutions to some idealised bistatic scenarios. A third approach is presented in [7] which uses purely geometrical considerations to map a bistatic stationary survey into a monostatic one. This paper addresses the efficient processing of airborne bistatic SAR data in the presence of motion errors. For this, the potential to adapt efficient airborne monostatic focussing algorithms to the stationary bistatic case will be analysed. A derivation of the point response in range-Doppler domain is presented; the differences between bistatic and monostatic point responses are outlined. Using these results, bistatic focussing solutions based on range-Doppler (RD) [9] and extended chirp scaling (ECS) [10] algorithms are analysed. Particular implementation problems, like Doppler parameter estimation and bistatic motion compensation [11] strategies are also presented. Finally, the algorithms are tested with real data from the DLR-ONERA bistatic airborne campaign.