Motion compensation for airborne SAR based on inertial data, RDM and GPS

Abstract

Two new motion compensation methods for airborne SAR are presented. Both methods are based on the evaluation of acceleration data measured by an Attitude and Heading Reference System (AHRS). The motion compensation algorithms were developed for the Experimental SAR System (E-SAR) of DLR which operates in L/C/X-Band. It is installed in a small Dornier Do-228 turboprop aircraft. Due to atmospheric turbulences and instabilities of the aircraft a highly accurate motion compensation becomes necessary to obtain high quality SAR images. The first method is furthermore based on results obtained by the Reflectivity Displacement Method (RDM). The RDM was developed by DLR and evaluates the radar raw data of the SAR sensor. The RDM provides a very accurate estimate of the forward velocity of the sensor which usually cannot be calculated by an inertial system with the required accuracy. Bias and drift components of the inertial data are eliminated by filtering. Residual systematic position errors are negligible for most applications and a high resolution SAR image can be generated. The advantages of this signal processing method are its robustness and its autonomy. The second method is based on the combination of inertial data and GPS position data. The GPS measurements are used to compensate the bias and drift components of the inertial data. Thus the actual position of the sensor can be estimated very accurately which guarantees a geometrically consistent high resolution SAR image. This signal processing method combines the absolute accuracy of GPS position measurements respectively its long term stability with the high bandwidth and data rate of the inertial measurements. In practice this method also turned out to be very robust. >