IF Calibration strategy
One of the components of the Envisat RA-2 sensor is an Intermediate Frequency (IF) Pass Band Filter. In theory the spectrum of the impulse response of such a filter should be a rectangle, but in reality this is not the case.
In order to compensate for the distortion on the RA2 waveforms due to this component, the measurement of the filter impulse response is needed.
To retrieve the shape of the IF Filter impulse response spectrum, the RA2 is commanded to operate in the so-called IF Calibration mode.
The different IF Calibration strategies applied during the whole ENVISAT mission are summarized in the table below.
In-flight Internal Calibration
The RA-2 Range and Sigma0 measurements are corrected to take into account the internal path delay and attenuation respectively. This is done by measuring those two variables in relation to the internal Point Target Response. The two correction factors are calculated during the L1b processing and directly applied. The Time Delay and the Sigma0 calibration factors were continuously monitored.
To be noted that the monitoring of correction factors on S-Band is stopped from cycle 65 onwards due to the loss of the S-band transmission power, which occurred on 17th January 2008.
?0 Absolute Calibration
The transponder calibration is based on the comparison between the theoretical power at the output of the altimeter instrument when it captures the impulse emitted by the transponder, and the actual power detected by the instrument as produced by the RA-2 level 1b (L1B) processing software.
The ?0 absolute calibration of the RA-2 is performed using a reference target given by a transponder that has been developed at ESA-ESTEC. This was exploited during the 6 month Commissioning phase to generate early calibration results. In order to consolidate the calibration results and to monitor the RA-2 calibration of ?° during the Envisat lifetime, continual monitoring was accomplished by operating the transponder for as many Envisat overpasses as possible.