Consideration of Inter-Pulse and Intra-Pulse Satellite Motion in Zero Doppler SAR Processing

Abstract

In recent years, several studies showed for SAR SLC image products that pixel location accuracy at centimeter level is attainable. Prerequisite for such high accuracy is that the entire SAR processing chain from sensor calibration via orbit determination, raw data acquisition and SAR focusing to the point of location measurements in the SAR image complies with tight quality standards. In former papers, we primary addressed SAR image users and consequently, we primary discussed the aspect of earth dynamics, wave propagation and precise location measurements, whereas SAR focusing aspects which are out of the users' influence were omitted. The current contribution, addressed to SAR data providers, shall fill this gap and shall give suggestions w.r.t. the SAR processor design of future SAR missions. Aiming at centimeter accuracy level or even below, processor approximations in SAR focusing - once introduced for the sake of algorithmic manageability and computational effectivity - have to be reassessed. In particular, the so-called Stop-Go-Approximation is questionable because the satellite motion during a single echo event is no longer negligible at the aspired accuracy level. In detail, one can distinguish the inter-pulse satellite motion, from the instant of radar pulse transmission to the instant of echo reception, and the intra-pulse satellite motion during echo transmission and reception. The inter-pulse satellite motion primarily causes a constant position offset of the entire SAR image in azimuth when not compensated for. This average shift is already considered in most of the operational SAR processors of ongoing SAR missions. A consequence of intra-pulse satellite motion is a skew of the focused SAR image relative to ideal Zero Doppler geometry. This effect is due to the fact that echoes from near range targets are received at slightly earlier azimuth time than echoes from far range targets. However, even the satellite motion during chirp pulse duration is not negligible as it introduces coupling terms between range and azimuth in 2-D spectral domain. It is so far a unique feature of the TerraSAR-X mission that operational SAR products are compensated for all these effects. It is most convenient for the user when all satellite motion effects, inter-pulse and intra-pulse, are already considered in SAR focusing so that the geometry of the resulting image exactly corresponds to Zero Doppler geometry. Otherwise, a post-processing of measured target positions was necessary where the uncompensated satellite motion effects are corrected for. However, as some corrections request detailed knowledge about the SAR instrument or about the used SAR focusing algorithms, only a few expert users are able to apply those correctly. Therefore, data providers should disburden their users as far as possible from this task and future SAR products should be designed as simple-to-use as possible and all possible corrections should be performed already in the SAR processor. The presentation quantifies the effects for TerraSAR-X and for Sentinel-1 and shows how they can be treated efficiently in the SAR processor.