Land Calibration and Monitoring of Envisat Ra-2 Sigma0

ASTRACT A global evaluation of RA sigma0 has been carried out over all non-ice covered land surfaces for which altimeter data are available, using ERS-1 Geodetic Mission data, together with ERS-1 and ERS-2 35 day data. This global survey has identified several large land areas over which the surface characteristics and aridity permit cross-calibration of altimeter sigma0 to be made to an accuracy of +/0.1dB on individual measurements. The methodology used to characterise each site involves modelling the area using an expert system to select and interpret individual altimeter waveforms, and then combine these data to create a surface of sigma0 variation across the entire region. Somewhat unexpectedly, these optimal land calibration targets are found to contain appreciable topographic relief. Additionally, sigma0 is not constant spatially within the target regions, varying over a range of approximately 5 dB. However, the spatial variation is coherent and can be modelled. Crucially, within these areas sigma0 does not exhibit variation with time over the combined lifetimes of ERS-1 and ERS-2 (with the exception of extremely rare and dramatic environmental events which are immediately evident in affected sigma0 values). It is therefore proposed that these huge natural targets be used to monitor the performance in terms of stability of ENVISAT RA-2 sigma0 over the entire mission lifetime. This paper presents the results of the global analysis of altimeter sigma0 and demonstrates the consistency of crosscalibration of altimeter sigma0 over the identified sites using ERS-1 and ERS-2 ice and ocean mode data. A case study using simulated small offsets is also presented, to illustrate the potential of this technique for ENVISAT monitoring.