Raw Signal Processing

The digital part consists of multiple individual processes, firstly the analogue-to-digital converter which digitises the converted RF signal with a frequency of 395 MHz to in-phase (real) and in-quadrature (imaginary) counts. Each individual pulse is then both compressed, using the replica digital chirp stored onboard, and shifted to match the centre of a 20 Hz radar cycle. The replica digital chirp is chosen to mimic the actual transmitted chirp. After compression, the individual waveform is centred within a range window and finally, for LRM processing, the power is extracted (square-law detection). In order to build the power waveform and decrease the level of random noise, all the individual power (squared signal) echoes within a radar cycle (20 Hz) are summed together (multi-looking). 

On ground, the downlinked, uncalibrated 20 Hz LRM waveforms are first calibrated to account for any changes in the real chirp with respect to the onboard digital replica chirp (based on cal-1), and for the TNR (Thermal Noise Response) which is a correction for the receiver's response to input thermal noise (based on cal-2). After these calibrations have been completed, the data are time stamped and geo-located to the bounce time and location on the ocean surface.