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Bistatic/multistatic synthetic aperture radar: Approaching the new era

Science China Press

Bistatic/multistatic SAR has attracted global attention and made remarkable progress recently. Many key techniques have been overcome. Aiming to grasp the developing trend of the bistatic/multistatic SAR, the corresponding special issue has been compiled and published on SCIENCE CHINA Information Sciences, no. 6, 2015, which is devoted to integrate the key techniques together such as bistatic imaging, interferometry, change detection and the experiment with high quality etc.

The modified Stolt map in the frequency domain was introduced in the bistatic imaging algorithm to deal with the echo data from very long integration time and large target area. Based on the ellipsoid map, the retrieval of bistatic DEM was solved for bistatic/multistatic SAR system. As for the experiments, hardware standardization of the bistatic/multi -static SAR was improved, and the transmitters of opportunity has covered the satellite from low-, medium-and high-earth orbits, and the bistatic image is as good as the rival of monostatic SAR. The theoretical improvement and the hardware standardization suggested that the bistatic/multistatic SAR has been mature as an information acquirement tool. Therefore, the new era of bistatic/multistatic SAR would be marked as the integration of the system with the specific application.

As the first try to the developing trend, a latest research investigates the application of differential interferometry based on GNSS as transmitter in deformation measurement technique, which is used to solve the problems of high accuracy, real time and low cost traditional techniques encountered. High accuracy deformation monitoring technology has vital significance in the field of security such as the subsidence of high-speed railway roadbed.

The related paper titled "A novel subsidence monitoring technique based on space-surface bistatic differential interferometry using GNSS as transmitters", has been published in on SCIENCE CHINA Information Sciences?no. 6, 2015. The authors are ZENG Tao, ZHANG Tian, TIAN WeiMing and HU Cheng from Beijing Institute of Technology.

Synthetic Aperture Radar Differential Interferometry (D-InSAR) implements deformation measurement by performing differential interferometry to at least two SAR images obtained in the same region at different times. However, traditional SAR satellite has long revisit period (30 days), limiting the time resolution of deformation monitoring and thus it cannot meet the real time requirement. Different from dedicated SAR satellites, the Global Navigation Satellite System provides navigation service through broadcasting navigation signal globally and all day. As the signal has certain bandwidth, bistatic SAR imaging with GNSS as illuminators has been demonstrated to be feasible. On this basis, researchers abroad have done some jobs on investigation on bistatic differential interferometry SAR using GNSS as illuminators. They realized accuracy of about 1 cm with time resolution of 7 days. Nevertheless, the system still cannot meet the demand of practical application. In the navigation field, the Differential GNSS has the ability of real time, highly accurate deformation monitoring. But to use the DGNSS systems, an expensive DGNSS device has to be deployed in each monitoring spot, limiting its application region.

The research proposes a novel deformation monitoring method based on bistatic differential interferometry GNSS as illuminators. The method combines the DGNSS and D-In-SAR. In the system configuration, low cost transponders are deployed in the region of interest and base station is setup in stable region to receive the signal from the transponders and perform the calculation; in the processing approach aspect, space and time differential are applied to eliminate the air delay, a certain time of coherent integration is performed to increase the SNR, and then signals from multiple satellites are combined to form several differential equations to resolve the three ?dimensional deformation in real time, which realizes real time, low cost and highly accurate deformation monitoring. Experimental results suggest that the real-time accuracy of deformation measurement is better than 1mm.

As the method is low cost and highly accurate, it can be a replacement of DGNSS and a effective compliment of traditional differential interferometry SAR technique, and is promising going to be applied in the real time subsidence monitoring of high speed railway roadbed, making a contribution to the security of nation and people.

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See the article: ZENG Tao, ZHANG Tian, TIAN Wei Ming, HU Cheng. A novel subsidence monitoring technique based on space-surface bistatic differential interferometry using GNSS as transmitters. SCIENCE CHINA Information Sciences, 2015, http://info.scichina.com/sciFe/EN/abstract/abstract517603.shtml

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