Once the data is telemetered to the ground stations, it is unpacked, formatted and then packed again for the Ground Processing facilities. These ground processing facilites are responsible of the treatment of the satellite data up to L2 (see Section 4. Product Levels).
Apart from the datation, which is assigning a given time value to each pulse or burst, the main ground processing steps are:
- Beam forming
During the on-board processing, the waveforms have been affected by the instrument gains and attenuations and they may have suffered phase variations as well. All these factors have to be accounted on-ground before the Delay-Doppler processing starts.
Once the waveforms have been corrected by all the on-board factors, the beam forming starts.
The beam forming consists in dividing the Doppler bandwidth of the pulses of each burst in different Doppler cells, defined by the surface sampling that has been performed previously (see Section 2. Resolution cells). This means that the pulses have been split according to Doppler frequencies. And the remaining waveforms are called beams, each of them being the backscattered contribution of each corresponding resolution cell.
The figure below depicts the beam forming process, with all the Doppler beams obtained from the transmitted burst of pulses.
Doppler beams obtained from a burst of pulses. Credit: S.Dinardo ESA
After the Doppler beams have been created, stacks are formed. A stack is the collection of all the beams that have illuminated the same Doppler cell.
Multiple contributions from different bursts conforming one stack. Credit: S.Dinardo ESA
After the collection, the whole stack is averaged. Note that, before averaging, the stack has been aligned and the beams have been converted to power waveforms. The average should be performed in the along track (beams) dimension in order to get the L1B waveform. If we perform the average on the range dimension we obtain the power distribution along track (top right).
Basically, a retracker is a model (it can be both analytical and/or empyrical) that emulates the L1B waveforms (or the stacks) and intends to fit them in order to extract the geophysical parameters from them. In order to do so, it is of high importance that the retracker is aligned with all the L1B processing steps.
In the picture above, a L1B waveform is shown together with the isardSAT ocean retracker. The outputs of a retracker can be:
Furthermore, wind velocity can also be extracted.