Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Although the history of spaceborne altimeters goes back to the early seventies, the absolute calibration of the backscattering coefficient
has never been deeply investigated. This information has been primarily used to infer the wind speed via an empirical model, and
the intercalibration among different satellite altimeters has revealed to be suitable for this purpose, being the wind retrieval based on an
empirical relationship. As far as Ku band system is concerned, the sigma naught absolute calibration of the Envisat altimeter (RA-2) has
been performed using an active reference target provided by a transponder. This has been exploited during the 6-month Commissioning
phase to generate early calibration results. In order to monitor the RA-2 backscatter calibration during the Envisat lifetime, a continuous
calibration effort has been carried out by operating the transponder as much as possible. This paper aims to review the entire effort for
calibrating the RA-2 sigma naught measurements, which lasted for almost seven years. It presents in detail the adopted methodology and
the final outcome of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability
during almost the entire Envisat lifetime. Specifically, it is concluded that the RA-2 backscatter measurements were quite stable,
even if a bias of about 1 dB should be considered with respect to the actually released product. Some small changes in the bias as function
of time can be identified during most of the Envisat lifetime, consisting in a slight increase in the first two years, followed by a more stable
period and a final drop observed at the end of 2009, until the conclusion of the calibration activity (corresponding to the change in Envisat
orbit).