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Team: SAR Processing

SAR processors are designed as so-called end-to-end systems which generate highly precise calibrated products for scientists and commercial users from the bit data stream coming from SAR instruments.

Illustration of interferometric SAR processing based on interferometric repeat-pass data acquisition by TerraSAR-X of the Salar di Arizaro dry lake in Bolivia.

As an example, the figure depicts the essential steps of the interferometric processing procedure as developed for the TanDEM-X mission. By focusing the holographically recorded radar echo, a sharp image can be obtained of the radar reflectivity of the earth’s surface. In addition to signal strength, shown as grey values in the upper left part of the figure, these images also contain very precise distance information in the form of phase angles. From the difference in phases between two SAR images which have been recorded from slightly different perspectives an interferogram is produced, shown in the central part of the figure as a pattern of colored stripes. After resolving phase ambiguities and knowing the exact recording geometry, it can be used to generate a digital elevation model of the earth’s surface, as indicated in the lower right colored portion of the figure.

Development phases of a SAR processor

The development of an operational SAR processor normally requires several years and includes the following steps:

• Defining system specifications derived from user requirements, taking into account the instrument parameters and the orbit characteristics
• Generating realistic test data with the help of simulation tools, especially for innovative receiving modes like Spotlight or TOPS
• Designing modular and robust software
• Efficiently coding the software on parallel computers
• Further developing the software, and version management during the satellite’s 5-10 year operational phase.

Current developments are the TerraSAR Multi-Mode SAR Processor TMSP and the Integrated TanDEM-X Processor ITP.

TerraSAR Multi-Mode SAR Processor (TMSP)

This tool processes all SAR data from the TerraSAR-X mission. It is used in the DLR ground segment as well as in the receiving stations of external partners. The special characteristics of the system are:

• Highly optimized multimode capability and an accurately phasing focusing algorithm for processing data rcorded in the SAR Stripmap, Spotlight and ScanSAR receiving modes
• Calculation of processing parameters and correction functions for the antenna illumination with the help of a terrain model
• Optional subtraction of thermal noise for all reception modes
• Processing the experimental aperture-switching and along-track interferometry modes
• Availability of the system for different hardware architectures and operating systems.

Integrated TanDEM-X Processor (ITP)

The first configuration of two SAR satellites and the bistatic interferometric recording principle of the TanDEM-X mission pose new challenges for SAR data processing. The ITP processing systems combines the aspects of bistatic SAR processing, interferometric processing, and, finally, the generation of digital elevation models from the derived phase information. The special characteristics of the system are:

• Analysis of differential deviations of the oscillators of both satellites and compensation for the temporal and phase drifts in the SAR signals which they cause
• Highly precise, centimeter-scale co-registration of both SAR images
• Innovative and robust methodologies for resolving phase ambiguities (“phase unwrapping”)
• Efficient calculation and geocoding of the elevation data contained in the interferometric phases.