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ENVISAT Ra-2 Sigma-0 Absolute Calibration, Methods and Results

Annalisa Martini(1) , Nazzareno Pierdicca(2) , Christian Bignami(2) , Bruno Greco(3) , Monica Roca(4) , Harry Jackson(5) , Pierre Femenias(3) , Cristina Celani(6) , and Carolina N. Loddo(1)

(1) Serco S.p.a., Via Sciadonna, 24, 00044 Frascati, Rome, Italy
(2) University of Rome, La Sapienza, via Eudossiana, 18, 00184 Rome, Italy
(3) ESA-ESRIN, Via Galileo Galilei, 00044 Frascati, Rome, Italy
(4) PiLDo Labs, Llacuna 162, 08018 Barcelona, Spain
(5) Serco FM BV., c/o ESA-ESTEC, Keplerlaan 1, 2200AG, Noordwijk, Netherlands
(6) Telespazio s.p.a., via Cannizzaro 71, 00156 Rome, Italy


The nadir backscattering coefficient, sigma-0, measured by radar altimeters is being used for wind-speed retrieval over ocean using empirical methods. For this reason only relative calibration has been traditionally performed between the different missions. In the last few years the need aroused to perform an absolute calibration to be used within physically based models.

Two methods have been used to pursue the goal: an active approach implemented by using a transponder has been followed by ESA as part of the Cal/Val activity and instrument long term monitoring(Roca et al., 2002, Femenias et al., 2004). In parallel, during the Commissioning phase, a passive approach has been tested for the first time, which was implemented by setting the RA-2 instrument in a noise listen mode(Pierdicca et al., 2004).

This paper describes the rational behind the two methods and the results obtained.


M. Roca, H. Jackson, and C. Celani, "RA-2 Sigma-0 Absolute Calibration" EnviSat Validation Workshop Proceedings, Frascati, December 2002.

N. Pierdicca, C. Bignami, L. Pulvirenti, P. Ciotti, F. Marzano, P. Basili, V. Mattioli, S. Bonafoni, P. Ferrazzoli, G. Schiavon, L. Guerriero, “Passive Calibration of the Backscattering Coefficient of the Envisat RA-2”, Final Report, ESTEC Contract N. 15925/02/NL/SF

P. Féménias, A. Martini, M. Roca, N. Pierdicca, C. Bignami, M. Fascetti, M. Mazzetta, “ENVISAT RA-2 SIGMA-0 ABSOLUTE CALIBRATION: PHASE E RESULTS”, Envisat Symposium, Salzburg, September 2004



                 Last modified: 07.10.03