De Zan Francesco
Present position: Research Assistant at the German Aerospace Center (DLR)
|Thesis title:||Optimizing SAR Interferometry for Decorrelating Scatterers|
A number of spaceborne Synthetic Aperture Radar (SAR) systems are being designed or are in the process of becoming operational right now. Many of them will feature a revisit time much shorter than typical systems of the past, either adopting wideswath as a baseline mode or through the use of satellite constellations. New possibilities and challenges will emerge, in particular for interferometric applications.
This work contains an analysis of short-time coherence for C-band SAR interferometry that highlights the presence of a number of scatterers with a relatively high coherence but a short lifetime. These targets cannot be treated with algorithms that assume a long lifetime like Permanent Scatterers (PS) but should not be discarded either. A new Maximum-Likelihood (ML) estimator is proposed, based on the sample covariance of the data, which is able to optimally exploit targets with arbitrary coherence patterns.
Later, in order to utilize to the fullest the temporal evolution of the data, the complete problem of phase unwrapping for SAR datasets is investigated.
An unwrapping scheme is proposed that allows to unwrap preferentially interferometric couples with short time span (temporal separation of the two takes) and its relation to the full three-dimensional unwrapping is clarified.
Real data results show that it is possible to recover displacement time series that depart significantly from the constant velocity model, which is the standard way to solve the phase ambiguity in the temporal direction for PS algorithms.
A SAR spaceborne system with short revisit time is thus shown to yield clear benefits for interferometry. However real systems aimed at frequent revisit are usually operated in ScanSAR mode to achieve a wide swath coverage, and this brings some disadvantages for interferometry. Either many looks have to be acquired over the same target or we are left with undesirable quality variations along the track indicated as scalloping (Bamler and Eineder , Monti Guarnieri and Guccione ).
A scalloping-free wide-swath mode has been invented, tested by the German Aerospace Center, and it is proved to operate as predicted. In the appendix of this thesis, I describe this innovative variant of ScanSAR systems, where the radar antenna is turned in the azimuth direction in order to equalize the along-tack image quality. This system has opposite features with respect to a SPOT-light SAR so it was named TOPS. It will enable a ScanSAR product with characteristics much similar to those of a conventional strip-map SAR (single-look, uniform along-track quality), removing some limitations to the use of wideswath as a baseline mode for future systems.