Introduction

Most applications of radar altimetry require accurate determination of the satellite orbit, and predominantly of the radial (vertical) component. Therefore, the satellite accommodates two precise tracking systems: the Precise Range and Range Rate Equipment (PRARE) and a passive Laser Retro-Reflactor (LRR) array. Unfortunately, the space segment of PRARE suffered a catastrophic failure only short after launch and does not provide any useful satellite tracking data.

Since the uncertainty of the satellite's orbital altitude is the largest error source in the determination of the sea surface height, any improvement in the orbit determination facilitates the research of ocean currents. A radial orbit error of less than 20 cm is striven for.

Already since 30 July 1991, only two weeks after launch, Delft University of Technology's Section Space Research and Technology (DUT/SSR&T) is processing global Satellite Laser Ranging (SLR) tracking data to ERS-1, for various purposes:

• The calibration of the ERS-1 radar altimeter required the satellite's altitude to be known at centimetre level, while it was flying over an off-shore platform in the Adriatic Sea, 15 km off the coast of Venice. During the ESA ERS-1 radar altimeter calibration campaign, SSR&T has been involved in the orbit determination, altimeter data smoothing, and the combining of altimetric, environmental and in-situ data into a definite value of the altimeter range bias [Wakker et al., 1991; Scharroo et al., 1991; Francis et al., 1993].
• For the United States' (National Oceanic and Atmospheric Administration (NOAA), SSR&T has been computing ERS-1 orbits on an operational basis since December 1991. These orbits are incorporated in the NOAA ERS-1 Interim Geophysical Data Records (IGDRs) [Cheney et al., 1991], which are used by several ERS-1 altimeter data users throughout Europe and the United States.
• To increase the accuracy of the orbit computations, several off-line experiments have been performed, using better gravity and surface forces models and incorporating also altimeter data as tracking data in the orbit determination process.

Various aspects of the ERS-1 operational orbit determination, orbit improvement and orbit accuracy assessment will be described in the following Sections.