The European Space Agency (ESA) Remote Sensing Satellites ERS-1 (launched in July 1991 and operated until March 2000) and ERS-2 (launched in April 1995 and operated until July 2011) 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 long mission lifetimes. 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), Altimetrics LLC (USA) and Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences (Germany).

An important part of the project was the 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 further improvement of ERS-1 and ERS-2 orbits was achieved in 2009-2010 within this project. The orbits were derived by three orbit groups, namely, DEOS, ESOC, and GFZ ones. The orbit validation and evaluation was performed by Altimetrics LLC. The same standards and models were used for ERS-1 and ERS-2, as those for Envisat and to lesser extent for Jason-1/2 allowing homogeneous analysis with these altimetry missions. However, each orbit group used 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 also the creation of a combined one. New improved models, such as the EIGEN-GL04S time varying gravity field model, the IAU2000A precession-nutation model (IERS Conventions, 2003), the Mendes-Pavlis model for troposphere correction following the IERS 2003 update, the Lenze-Thirring and De Sitter (geodetic precession) effects and some other models were used. The orbits were derived in the  ITRF2005 reference frame. The new consistent precise orbit solutions for ERS-1 and ERS-2 derived in the REAPER project are available via anonymous ftp at ftp://dgn6.esoc.esa.int/reaper/. The combined orbit solutions show the best quality among four solutions and indicate 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 resulted in the improvements of altimetry products, in particular, in the final surface height measurements. A detailed description of these orbits is given in the following paper.

Rudenko, S., Otten, M., Visser, P., Scharroo, R., Schoene, T., Esselborn, S. New improved orbit solutions for the ERS-1 and ERS-2 satellites. Advances in Space Research, 49, 8, 1229-1244, doi:10.1016/j.asr.2012.01.021, 2012.

It is planned to derive in 2013 ERS-2 orbits also for the time interval from July 2003 until the end of the mission in July 2011 using the same models, as those used in the REAPER project.