The main mode of operation of RA2 is the TRACKING mode. During this mode all instrument's units are powered-on and operative. The main task is the tracking of echo waveform which is executed without interruption.
The second mode of operation is the IF CALIBRATION mode.
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 RA-2 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 RA-2 is commanded to operate in the so-called IF Calibration mode. This consists of the measurement of noise power spectra by setting the instrument in listen-only mode (no power transmitted) and putting the receiving window delay to a value such that no echo from the surface could be received.
The different IF Calibration strategies applied during the whole Envisat mission are summarized in the table here below. In the table detailed IF masks acquisition for both ascending (A) and descending (D) overpasses over Himalaya and Rocky Mountains.