Natural SciencesEarth SciencesOcean EngineeringInstrumentationMetrologyRemote SensingCalibrationRadar CalibrationRadar Clutter SuppressionRadar InstrumentationRadar Signal ProcessingSatellite InstrumentationAbsolute CalibrationBackscatter CoefficientClutter SuppressionData ProcessingRadar AltimetersRelative CalibrationSea State DependenceWave Period Measurement
Measurement of the vertical-incidence backscatter coefficient, sigma-0, by radar altimeters has been used for the determination of wind-speed over the ocean. The models used are empirical and so it has been sufficient to perform relative calibration between missions. These are traced back to GEOS-3 resulting in a minimum uncertainty in the absolute sigma-0, for all altimeters, of more than 1 dB. Recent applications of the altimeter sigma-0, such as physically based models of sea-state bias and wave period, require an absolute sigma-0 measurement. A dedicated activity has been developed at ESA for the EnviSat altimeter, RA-2, Sigma-0 absolute calibration. The measurement technique makes use of a newly-developed transponder, using a delay-line for clutter suppression. The altimeter operates in a preset mode, and it also acquire individual echoes over the transponder. Special data-processing techniques are also being developed. We describe the principle, measurement and data processing techniques and the results obtained during the EnviSat Commissioning phase. 1.