Reprocessing of Altimeter Products for ERS (REAPER): consistent reprocessing of ERS orbits
European Space Agency (ESA) Remote Sensing Satellites ERS-1 launched in July 1991 and operated until June 1996 and ERS-2 launched in April 1995 and still operating are used besides remote sensing also for altimetry applications. However, the existing ERS altimetry data available from ESA are not homogeneous due to the changes in processing and algorithms made during the two-mission lifetime. To reprocess all existing ERS-1 and ERS-2 Radar Altimetry (RA) and Microwave Radiometer data for 12-year time period from July 1991 till June 2003 when ERS-2 on-board tape recorder stopped working and to produce a coherent and homogeneous long-term series of altimetry products that is cross calibrated and offers continuity with other RA missions are the purposes of the international project „Reprocessing of Altimeter Products for ERS“ ( REAPER ) executed with the participation of scientists from Mullard Space Science Laboratory (MSSL, United Kingdom), Collecte Localisation Satellites (CLS, France), IsardSAT (Spain), ESA European Space Operations Centre (ESOC) Navigation Support Office (Germany), Delft Institute of Earth Observation and Space Systems (DEOS) at Delft University of Technology (the Netherlands), Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences (Germany) and Altimetrics LLC (USA).
An important part of the project is consistent reprocessing of ERS orbits to provide the best possible homogeneous orbit solution making use of almost 20 years of advances in satellite orbit modeling. The orbits are derived by three orbit groups, namely, DEOS, ESOC, and GFZ ones. The orbit validation and evaluation is performed by Altimetrics LLC. The same standards and models are used for ERS-1 and ERS-2, as those for ENVISAT and to lesser extent for Jason-1/2 allowing homogeneity with these altimetry missions. However, each orbit group uses separate orbit determination software, different parameterization and its own experience in altimetry satellite orbit determination allowing an independent control and inter-comparison of three orbit solutions and creation of a combined one. The new consistent precise orbit solution for ERS-1 and ERS-2 derived in the REAPER project should be available in 2010 showing significant improvement in the orbit accuracy, as compared to the previous ESA D-PAF orbits of ERS-1 and ERS-2. The improvements in satellite orbits will result in the improvements of altimetry products, in particular, in the final surface height measurements.