Optimal Baseline Design and Error Compensation for Bistatic Spaceborne InSAR
Institute of Electronics,
Chinese Academy of Sciences,
Bistatic SAR systems as opposed to monostatic SAR offer some degrees of in choosing transmitter (illuminator) and (passive) receiver separately. Bistatic SAR attain more and more interest over the last years as they are seen as a potential means of countering vulnerability to electronic countermeasure, especially in directional responsive jamming, and avoiding physical attack to the radar platform, aided by its electronic emissions. In addition, the bistatic setup with the target environment opens up several interesting possibilities. For instance, using bistatic interferometric SAR with large bistatic angles it is likely to increase the signature from stealth targets. Other interesting aspects of bistatic SAR are the reduced dynamic range of challenging imaging environments such as urban ones. In this paper, the bistatic spaceborne interferometric SAR (BS-InSAR) was discussed in detail. In general, BS-InSAR uses a separated transmitter and receiver flying on different platforms, can be explored to implement three possibilities of the coherent combination of SAR images, which are Digital Elevation Model (DEM), ocean current imaging and improving resolution in range and azimuth. In general InSAR, the baseline is an important parameter and it is crucial in InSAR system design, data processing and error analysis. For BS-InSAR, the baseline design is more complicated than that for general InSAR.
In this paper, spatial baseline in BS-InSAR and requirements of the optimal baseline for BS-InSAR coherent processing were analyzed in many aspects. An equation to determine the optimal spatial baseline was derived. The influence of the rotation of the BS-InSAR on the baseline was analyzed and the corrected formula of the baseline was obtained. Finally, one method to correct the baseline error caused by the relative moving between the transmitter and receiver are obtained. Simulation results show that the desired results can be obtained under certain conditions. The organization of this paper is as follows:
In section 1, the geometry and basic principle on initial baseline estimation of BS-InSAR were discussed. It is shown that conventional method of InSAR DEM generation may not be used here, because of the features of bistatic SAR, large squint angle and spatial baseline error of BS-InSAR system.
In section 2, an equation to determine the optimal spatial baseline was obtained. It is pointed that, in the condition of acceptable DEM error, there must be an optimal baseline, which makes DEM precision highest. The optimal baseline makes BS-InSAR perform in the best status and accurate optimal estimation would largely increase the precision of terrain height measurement.
In section 3, the corrected formula of the baseline was derived. For general InSAR, its baseline will not change or change less in the whole imaging time. However, the satellite’s attitude, the earth rotation and the bistatic SAR rotation will affect the baseline in BS-InSAR. In this section, the influence of the rotation of the BS-InSAR on the baseline was analyzed. The corrected formula of the baseline was derived. By computer simulation, the errors between corrected and uncorrected height measurement were compared. The analysis results verify the correction of the formula and the necessity to correct the baseline.
In section 4, one method to compensate the baseline error caused by the relative moving between the transmitter and receiver of BS-InSAR were obtained. In BS-InSAR, due to the unstability of the two radar platforms, the parameters of baseline will change during the flying and it should be estimated differently. The common way to calculate the baseline is based on the DEM. Unfortunately, the DEM is often difficult to obtain. In this section we proposed one method based on the geometry of orbits and moving compensation to compensate the baseline error caused by the relative moving between the two radar platform of BS-InSAR system.
Finally, in section 5 various examples were presented and some difficulties and future works in BS-InSAR were discussed. On the whole, the baseline design and error compensation for BS-InSAR were studied in detail and some useful results were obtained in this paper, which can provide theory and design for BS-InSAR system. Some theoretic analysis and computer simulation results have shown that the desired results can be obtained under certain conditions.
Wenqin Wang was born in Sichuan province, P. R. China, in 1979. He received the B.S. degree from the University of Shandong in 2002 in electrical engineering and the M.S. degree from the University of Electronic Science & Technology of China (UESTC) in Mar. 2005, majoring in signal and information processing. Presently, he is pursuing his Ph.D. degree in the Institute of Electronics, Chinese Academy of Sciences. His main interests are associated with SAR system and signal processing, microwave imaging technology and modern signal processing. He has yet published over 30 papers in some technical journals and some conferences as the first author.
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