Absolute Calibration of the ERS-2 Altimeter using UK Tide Gauges

This paper describes the use of UK tide gauges, the sea level data from which have been located in the same geocentric reference frame as altimeter data from ERS2 using Global Positioning System (GPS) receivers, to provide an ongoing absolute calibration of the altimeter bias. The method is an extension of earlier work for TOPEX/Poseidon. However, the present analysis extends to the ERS-2 altimeter, which flies on a sun-synchronous orbit. The S₂ tide error of ERS-2 will be aliased to a permanent local bias, so it cannot be detided by the harmonic method which was used in TOPEX/Poseidon calibration. The calibration sites were chose at the TOPEX/Poseidon and ERS-2 dual-satellite crossover points, in order to use TOPEX altimeter data to extract the tidal parameters. These tidal parameters were applied to ERS-2 altimeter data to reduce the affect of tide differences between data from the ERS-2 calibration sites and tidal gauges. The timeaveraged ERS-2 biases are derived over 24 crossover points in 6 tidal gauges, and the mean is 44.0cm with a 6.6cm RMS. When using first 1000 days TOPEX/Poseidon data which is not overlay with ERS-2 data, the time-averaged biases is only a few millimetres different at each site. It suggest that other sun-synchronous satellites (with or without an overlap with TOPEX/Poseidon) can be similarly evaluated.