Transponder calibration of the ENVISAT RA-2 altimeter sigma naught

As far as Ku band is concerned, the backscatter coefficient (sigma naught) of the radar altimeter (RA-2) aboard the ENVISAT satellite has been calibrated using a transponder (TPD) developed at ESA/ESTEC. The transponder has been exploited during the 6 month Commissioning Phase to generate early calibration results. In order to consolidate this calibration results and to check the altimeter during the satellite lifetime, a continuous monitoring was performed by operating the transponder as much as possible. The first calibration campaign dates back to February 24^th, 2004. The last campaign occurred on October 5^th, 2010. Since then the change of the ENVISAT orbit hampered the prosecution of the activity. This paper aims to review the entire effort for calibrating the RA-2 sigma naught measurements, which lasted for almost seven year. It presents the final outcomes of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability during almost the entire ENIVISAT lifetime.