Helko Breit

Demonstration of current measurements from space by along-track SAR interferometry with SRTM data

We present one of the first studies in which interferometric synthetic aperture radar (InSAR) data from the Shuttle Radar Topography Mission (SRTM) are analyzed with regard to the detectability of ocean surface current variations. The InSAR system of SRTM was designed for high-resolution topographic mapping, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. For technical reasons, there was an additional along-track antenna separation of 7 m, which results in a time lag of about 0.5 ms between the acquisitions of images by the two antennas. In theory, this time lag causes additional phase differences, which are proportional to the line-of-sight velocity of moving targets and can thus be exploited, to some extent, for measuring oceanic currents. Indeed, some SRTM images acquired over water exhibit clear signatures of typical flow patterns. We show an example of an X-band phase image of the Dutch Waddenzee and discuss the plausibility of interpreting it in terms of sea surface height or current variations or the effect of waves. We find that only currents can be responsible for phase variations of the observed magnitude on spatial scales of a few 100 meters. We convert the data into a surface current field, which is found to be consistent with the theoretical current field at the time of the SRTM overflight according to a current atlas. Based on this encouraging result and theoretical findings, we discuss the general potential of SAR interferometry from space for oceanic applications.

A high precision workstation-based chirp scaling SAR processor

In order to meet the challenges of new spaceborne multi-mode SAR sensors and missions, a multi-sensor chirp scaling SAR processor is currently under development at the German Remote Sensing Data Center DFD. High precision data processing is ensured by floating point data representation, frequent processing parameter update as well as the underlying phase preserving chirp scaling algorithm enhanced to deal with arbitrary Doppler centroid variations over range. The availability of high performance general purpose computers enables the use of standard programming languages and hardware, even if a high throughput is required for operational applications. Multithreading software techniques lead to scalable processing systems running efficiently on different platforms ranging from single CPU workstations to multiprocessor systems. This paper describes the algorithms, the design and the implementation concept of this highly flexible SAR processor. Detailed image quality and throughput results based on simulated as well as real SAR data are presented.

A description of the data-driven SAR data workflow in the TerraSAR-X Payload Ground Segment

TerraSAR-X is a national German satellite providing a high-resolution X-Band SAR instrumentation featuring StripMap, ScanSAR and SpotLight imaging modes in various polarizations. The experimental full polarization mode is carried out using a dual receive antenna capability. An active lifetime of five years is anticipated, the launch is expected in 2006. The TerraSAR-X mission serves both commercial and scientific needs. It is implemented in the frame of a public-private partnership between EADS Astrium GmbH (providing the TerraSAR-X Space Segment) and the German Aerospace Center DLR (providing the TerraSAR-X Ground Segment). The commercial product exploitation rights are with Infoterra GmbH, the scientific ones remain at DLR. The Infoterra business concept foresees the inclusion of a Direct Access Service through which TerraSAR-X data may be directly received by stations either for the sole purpose to produce geo-information products for their own and private applications or to generate and distribute commercial products. Essential tasks of the TerraSAR-X Payload Ground Segment developed (and later operated) by DLR are the SAR payload data reception, their archiving and processing, and the distribution of the generated SAR products to users. This paper summarizes the TerraSAR-X Payload Ground Segment design. It describes the SAR data workflow ranging from the payload data reception at the Neustrelitz Ground Station to the long-term data archiving in the multi-mission Product Library at the German Remote Sensing Data Center. Supplementary data as provided by the TerraSAR-X Mission Operation Segment and used during the data-driven generation of the SAR level 0 products are described together with their dissemination concept. The adaptability of the chosen approach for the operation of a Direct Access Station is addressed

Higher resolution X-SAR images

A new class of X-SAR products-images with a geometrical resolution of 12.5 m by 12.5 m are introduced. A comparison with the standard 25 m by 25 m products is given. Despite of the enlarged amount of speckle these images show obviously a better quality if geometric and not radiometric resolution is on demand. The new class of products supports recognition of man made structure and edge detection.

First results of X-SAR interferometry

Repeat-pass interferometry data have been acquired during the first and second SIR-C/X-SAR missions in April and October 1994. This paper presents the first results from X-SAR interferometry on four different sites. The temporal separations were one day and six months. At two sites the coherence requirements were met, resulting in high quality interferograms. A digital elevation model has been derived. The limitations of the X-SAR interferometry are discussed.