The main purpose of this Full Bit Rate (FBR) dataset is to allow independent investigation of the synthetic aperture processor (beam-formation and direction, slant range correction and multi-looking). Data is raw time-ordered, and for SAR and SARIn modes, it is provided prior to the application of the synthetic aperture, interferometric or accumulation processing.
To support such investigations, the data will have instrument and geophysical corrections appended, together with a precise orbit. Not all of these corrections will be applied however. In conventional pulse-limited altimetry, as part of the radar pulse-compression technique, echoes are fast-Fourier transformed into the 'spectral domain' prior to multi-looking.
In the LRM, the SIRAL echoes are incoherently multi-looked on-board the satellite prior to altimetry (as with, for example, ERS satellite radar altimeters). In this mode, the FBR data consist of the multi-looked echoes at a rate of ~20 Hz.
In the SAR and SARIn modes consists of the individual, complex (I and Q) echoes. In SARIn mode, there are two such echoes, one for each antenna of the interferometer. In these modes, data are sent to ground in the 'time domain' prior to the on-board FFT. Some corrections (such as those for the IF ant-alias filter) can only be performed once this transformation is performed. The user will have to apply these corrections.
For more information on the format specification of Level 1 (FBR), please download the CryoSat L1b Product Specification document.
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