RA-2 radar altimeter: instrument operation concept and system performance

Abstract

The European Space Agency (ESA) has awared a contract to DAIMLER-BENZ Aerospace, Dornier to fly on ENVISAT-1 mission the Radar Altimeter RA-2 developed by Alenia Spazio SpA. RA-2 is a nadir pointing pulse limited radar altimeter operating at two frequencies: 13.575 Ghz (Ku-band) and 3.2 Ghz (S-band). Its key feature is represented by a new on board signal processing algorithm (Model Free Tracker-MFT) able to autonomously detect, acquire, lock-on and track the earliest part of the radar echoes at Ku band from ocean, ice and land surfaces without interruption, irrespective of sudden changes in surface characteristics and elevation. The radar echo samples collected at Ku-band are processed by the MFT algorithm to adjust the radar resolution and receiver gain to values which allow the proper sampling of the earliest part of the radar echo. Four resolutions are available corresponding to the transmitted pulse bandwidth of 320, 80, 20 MHz and CW pulse. The radar echo samples at S-band are collected at fixed resolution corresponding to a transmitted pulse bandwidth of 160 Mhz. They are processed on ground to remove from the height measurement performed at Ku-band the bias error introduced by the propagation through the ionosphere. Point Target Responce (PTR) measurements are also collected at every transmission without interrupting the tracking to monitor the variation of the parameters which have been calibrated on ground. To assess validity of the design, a series of simulation runs over a variety of simulated test scenarios, representing typical operative conditions as tracking over mountains, ice regions, wet lands, have been performed.