ENVISAT Radar Altimeter Tracker Bias

Abstract

In the past, errors in the determination of the orbit were dominant in radar altimeter missions, but technical advances have improved the orbit accuracy and hence, other sources of error have become more important. Sea-state bias is now the main source of error and can be divided into three sea-state dependent errors: skewness, electromagnetic bias, and tracker bias. We estimated the magnitude of the third term, by retracking ocean waveforms from ENVISAT RA-2. The retracking algorithm used is based on Maximum Likelihood Estimation. Tracker bias shows a seasonal and geographical dependence related to the distribution of significant wave height (SWH) and time origin differences. We estimated a mean value of 0.13 ± 0.07 %SWH. Temporal and regional dependent errors are introduced when using a linear retracker processing approach.

Keywords:

• altimetry
• ocean retracking
• maximum likelihood estimation
• tracker bias

Acknowledgments

This work has been done under the auspices of the European Space Agency in the frame of the Postdoctoral Research Fellowship Programme. Pierre Femenias and Jerome Benveniste, from ESRIN-ESA, supplied the SGDR records used in this work under a Category-1 Project lead by Graham Quartly (National Oceanography Centre, Southampton). The authors want to thanks María Pilar Milagro-Pérez, from ESRIN-ESA, for all the technical documents she provided and the clarifications about the format of the RA-2 SGDR products. We would also like to thank the EOHelp Team at ESRIN-ESA for the help in the distribution of the SGDR data.