Estimation of Multi-Channel SAR Instrument Errors using a Hybrid Calibration System Model

To fulfill the requirements for near future radar missions, using a space-borne Synthetic Aperture Radar (SAR), two main parameters are a high azimuth resolution and a wide swath. SAR instruments with multiple channels coupled with digital beam-forming techniques make it possible to achieve these goals. The high azimuth resolution is ensured by the Multiple Azimuth Channels (MACs) technique, while in elevation SCan-On-REceive (SCORE) is employed to improve the Signal-to-Noise Ratio (SNR) and suppress range ambiguities. It is imperative to know the differences between the channels and correct them by calibrating the system. In this paper we propose a hybrid calibration scheme consisting of an internal instrument calibration method and a data-based calibration method. For the internal calibration, a single tone calibration signal is sequentially coupled to the echo signal path. Frequency dependencies of the system can be accounted for by altering the frequency of the calibration signal. The coupling is done at Transmit Receive Module (TRM) level and the signal is evaluated in the Digital Beam-forming Unit (DBU). It allows for an uninterrupted transmit or receive operation while performing the calibration. The data-based calibration accounts for influences of the whole system, while the internal calibration only considers parts included in the loop, e.g. radiators are excluded. To estimate the errors in such a hybrid calibration system and aid in the system design a mathematical model and a simulation tool are developed. The tool also enables the estimation of residual errors in the system. We show results for the internal calibration where the calibration signal lies outside the SAR echo signal spectrum and where the frequency is sequentially altered. We then introduce an improved method for the internal calibration concept to estimate the current drift and predict values