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34. Space, time and processing constraints
Space constraint: The interferometer baseline has to satisfy a condition determined by the radar characteristics and the imaging geometry. For ERS-1, the baseline (i.e., the component of the cross-track orbit separation perpendicular to the SAR slant range direction must not be greater than about 600 metres for the practical application of INSAR.
The complete set of ERS-1 restituted orbits that have been processed so far and an INSAR orbit listing has been produced at ESRIN. Using this listing, one can identify those repeated orbit acquisitions for which the orbits satisfy the INSAR baseline criterion.
Time constraint: Ideally, the two SAR images should be acquired simultaneously with a true interferometer. Therefore, in multi-pass INSAR, there should be no changes in surface conditions between the images which could effect the image phase values and lead to anomalous values of the computed terrain height. The effect of surface change is known as temporal de-correlation.
Research teams working with ERS-1 data are reporting good results with generally low values of temporal de-correlation for the 3-day repeat phases. In addition, INSAR has also been successful with ERS-1 data for some areas in the 35-day repeat phase.
Processing constraint: The SAR processor used to generate the imagery from raw satellite data must not introduce artefacts that corrupt the phase quality of the final image.
Currently, SAR processors within the ERS ground segment that have been validated for INSAR applications comprise:
- the Verification Mode Processor (VMP) used for calibration of the instrument, located at ESRIN;
- the precision processor located at the German PAF, from which users can order off-line SAR image products