An ERS-2 altimetry reprocessing compatible with ENVISAT for long-term land and ice sheets studies

Satellite altimetry has demonstrated a strong potential for the continuous monitoring of ocean, ice sheets and land surfaces over the last 25 years. While instrument processing and retracking techniques of the radar echoes improved over the years for ocean applications to produce homogeneous products for all satellite altimetry missions, less effort has been devoted to applications over land. European Space Agency (ESA) continues improving the quality of ENVISAT data over all type of surfaces through frequent reprocessing of altimeter corrections. No similar effort was performed for the ERS missions until recently with the release of the ESA ERS REAPER product mostly devoted to ocean applications. In parallel, Centre de Topographie des Océans et de l'Hydrosphère (CTOH) processed the ERS-2 waveforms (WAP data) with a new implementation of the Ice-2 retracker algorithm, adding improvements and corrections to make ERS-2 data compatible and homogenous with the ENVISAT mission for continuity and long-term altimetry data analysis over land and ice sheets. The resulting product is evaluated against ENVISAT data during the tandem phase when they flew 30 min apart on the same orbit during 10 cycles of the two missions from June 2002 to July 2003. Maps of along-track differences averaged over the 10 cycles of the tandem phase and associated RMSE are presented over ocean, both Antarctica and Greenland ice sheets and land along with time series of altimetry backscattering coefficients and altimetry-based water levels. Similar comparisons were performed between ENVISAT and ERS-2 REAPER data. Low bias and RMSE values for altimeter heights and backscattering were found between ENVISAT and ERS-2 over ocean and flat areas over land and ice sheets with generally better results obtained using CTOH data. Comparisons with in situ water stages also show good agreement for Ice-2 and especially Ice-1 retrackers derived water levels (R > 0.95).