Cross-calibration of multi-mission altimeter and TRMM PR sigma0 over natural land targets
Susan Bramer(1) and Philippa A.M. Berry(1)
De Montfort University,
Faculty of Computer Sciences and Engineering,
Leicester LE1 9BH,
In order to utilise fully the record of sigma0 measurements from the series of altimeter missions flown over the past 15 years, the data have to be cross-calibrated to remove individual biases, and checked for measurement stability over time. The technique generally employed is to cross-calibrate over the earth’s oceans, which requires both spatially and temporally coincident measurements. An alternative approach is to utilise the earth’s land surfaces. Prior work has already shown that natural land calibration sites provide a temporally stable region for the monitoring of altimeter sigma0 over the mission lifetime, and for cross calibration between instruments.
Using a series of natural land targets previously identified as suitable calibration zones for ERS-1 and ERS-2, precise calibration models have been derived by analysing the waveform data using a rule-based expert system. This work has now been extended to permit a full analysis of multi-mission sigma0 including data from Envisat, Topex and Jason-1.
This paper presents the results from analysis of multi-mission sigma0 over both primary and secondary calibration zones. The work includes an analysis of the behaviour of TOPEX sigma0 over the mission lifetime, made possible by the inclusion of Jason-1 data in the calibration models.
In addition to measurement and monitoring of nadir-pointing altimeter sigma0, the application of this technique to other instruments is illustrated by the inclusion of sigma0 data from the Tropical Rainfall Monitoring Mission Precipitation Radar.
The further extension of this technique to multi-frequency sigma0 is discussed, including the development of detailed models at a range of frequencies. One goal of this work is to enable calibration and monitoring of instruments such as the forthcoming EarthCare Cloud Profiling Radar over natural land targets.