Calibration of ERS-2, TOPEX/Poseidon and Jason-1 Microwave Radiometers using GPS and Cold Ocean Brightness Temperatures
Stuart Edwards(1) and Philip Moore(1)
Sea-level change studies from altimetric satellites are reliant on range stability of the sea-surface heights computed from the orbital positioning and geophysically corrected data. One such correction, namely the wet tropospheric delay induced by the highly variable atmospheric water vapour content, is provided by radiometers onboard ERS-2, TOPEX/Poseidon and its follow on mission Jason-1. In this study the long-term stability of ERS-2 and TOPEX radiometers are investigated together with radiometer performance to date of Jason-1. Each of the three microwave radiometers is investigated with observed drift in the brightness temperatures approximated by reference to the coldest temperatures over the oceans. For example the TOPEX radiometer investigations show that the dominant drift is about 0.2 K/year in the 18 GHz channel over the first 7-8 years but stabilising and even decreasing slightly thereafter. In contrast, the 21 GHz and 37 GHz channels are comparatively stable. Utilising correction formulae a modified wet tropospheric range is inferred from ‘small-change’ analysis of the radiometric correction given on the altimetric Geophysical Data Records. This investigation undertakes the validation of the accuracy of this formulism by independent comparison against GPS derived wet tropospheric delays inferred at 14 coastal IGS stations with near continuous data from Sept. 1992 through to the present day. Early comparison between GPS results for ERS-2 and TOPEX shows the ERS-2 radiometer measuring ~14mm long. Additionally for TOPEX, the altimetric range stability is revisited by comparison against time series from the global network of tide gauges.
Keywords GPS, altimetry, radiometers, calibration/validation, sea level