Ra-2 Absolute Range Calibration

The EnviSat altimeter, RA-2, is intended to continue an uninterrupted series of measurements of sea-level and ice-sheet elevation started by ERS-1 in 1991. To fully exploit these measurements, an absolute reference in the time series, and a distinction between instrumental artifacts and significant geophysical signals, is necessary. Therefore, the range bias and instrument drift shall be determined with a accuracy of 1 cm and 1 mm/year respectively. Such accuracies can only be achieved by using: • a large number of measurements to reduce random errors; • a diversity of measurement techniques and independent data analysis to reduce susceptibility to systematic errors. Due to the limited temporal sampling of the 35-day repeat orbit of EnviSat, the resulting overall concept is a regional calibration which makes use of the north-western Mediterranean basin as a reference surface, with a number of particular of high-confidence “super-sites”.