The EnviSat RA-2 Instrument Design and Tracking Performance

The EnviSat satellite embarks an innovative radar altimeter, the RA-2 which represents a new generation of radar altimeters compared to previous instruments such as the European Remote Sensing Satellite (ERS) altimeters and The Ocean Topography Experiment/Poseidon. This is due to its integration of many new features. In particular, the RA-2 is the first satellite altimeter to incorporate autonomous resolution selection and a model-free tracker. The behavior of the instrument in orbit is controlled by onboard algorithms which determine the instrument logic, and a list of onboard parameters which control when switches in resolution occur. This logic and the choice of these parameters have a strong influence on the tracking performance. This paper describes the operation of the RA-2 onboard tracker and the resolution selection logic. We provide a detailed description of the algorithm and its configuration parameters after optimization during the EnviSat Commissioning Phase. Finally, we show the tracking performance, in particular compared to the ones of the ERS-2 altimeter, over different types of surfaces.