ERS Precision Orbit Determination and Accuracy Verification

C. Shum, J. Ries, J. Bordi, J. Seago, and B. Tapley 

Center for Space Research, The University of Texas at Austin,
Austin, Texas 78712, USA.  Tel: 512-471-5573, Internet:
shum@csr.utexas.edu

Development in the Earth's gravity field and ocean tide 
models, non-conservative force models, terrestrial reference
frame definition, and precision orbit determination
methodologies to employ dual-satellite altimeter crossovers, has
enable the current radial orbit accuracy for ERS-1 and ERS-2 to
approach the 5 cm rms level.  These improved orbit determination 
methodologies enable an improvement in both the constant and the
variable component of the ERS-1 and ERS-2 orbit error, and
provide improved alignment of the absolute sea surface
measurements from ERS within the terrestrial reference frame.
Non-conservative forces are the current limitations for the
ERS-1 orbit accuracy.  The significant contribution by the
global PRARE range and range-rate tracking measurements provides
an improvement in the handling of non-conservative forces for
ERS-2.  In addition to crossover analyses, independently 
determined orbits were used to assess and verify the radial 
accuracy of the orbits.  The orbit accuracy assessment also 
employed a technique which uses TOPEX/POSEIDON measurements 
for the characterization of the geographically
correlated component of the gravity error in ERS-1 and ERS-2
orbits.  

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry