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Active transponder development and specifics of application for spaceborne SAR ground and flight test, calibration and validation


T. A. Lepekhina – Ph.D. (Eng.), JSC «Concern «Vega». E-mail:
V. I. Nikolaev – Deputy Head of Department, JSC «Concern «Vega». E-mail:

The Active Transponder for SAR ground, flight tests and validation as a set of hardware and software resources considered in the paper. The task of transponder designing is to obtain a universal facility both for SAR tests at all its lifetime phases and for periodic calibration and performance measurement during the mission. This approach is to provide test technique portability and result comparability for different engineering stages. Test impact forming is provided by means of an Active Transponder converting SAR sounding signal to a simulated echo signal of a ground scene with given backscatter properties considering survey mode and carrier movement. Three variants of transponder step-by-step designing differing by digital signal processor structure, functions and productivity. The first, based on a digital delay line with a filter, is successfully applied in SAR calibration and testing practice. New design of such a transponder is reasonable for front-end and antenna workout and calibration, these elements being similar for all the three variants but the most responsible for transponder gain stability. The second variant of the transponder allows to simulate not only singular point targets but arbitrary ground scenes as well, the scene being simulated beforehand, and radar signal being recorded during the previous mapping session. Precise synchronization of return signal generator with the received sounding pulse is a problem of principal importance. The third variant implies application of high-performance signal processor to provide real-time converting of received sounding signal to simulated echo for arbitrary scene. Application of two last variants of the transponder should allow to synthesize test impacts for SAR through characteristic measurement, including the radiometric performances, at all its lifetime phases starting from early designing stages and ground tests. A prototype of the second variant of the transponder, without an antenna and a front-end, has been assembled of Agilent® modular devices in Vega Corporation’s laboratory and used as a part of a hardware-in-the-loop workbench for SAR through characteristic measurement procedure workout. The test sample of spaceborne SAR «Condor-E» instrument has been used as the object of testing. Obtained radar image quality corresponds to rated SAR through characteristics. The task of innovative active transponder development for Russian radar calibration and validation sites becomes of great actuality in connection with «Condor-E» successful launch and its mission start.

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