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4.3 Main RA-2 Level 1b algorithms

Question 4.15 : How is the time window delay calculated?

The window delay (in time) is the first measurement of the range performed during the Level 1b RA2 processing. It is referred to the antenna phase centre, and therefore, the delays of the Tx and Rx paths, characterised during testing on ground, are taken into account.

In fact, during the L1b processing, the on board Rx distance (obtained by combining Rx coarse and fine components, and compensating for the PRI ambiguity rank and for the on board tracking filter rate of change) is corrected for instrument induced errors using both ground (Tx and Rx paths) and flight PTR calibration parameters. Finally, the retrieved on board time delay measurements are corrected for the Doppler effect, before making it available in the L1b products.

These measurements are used at Level 2 to calculate the final retracked range.

Question 4.16 : How is the sigma0 scaling factor calculated?

The power scaling factor for sigma0 evaluation is obtained at Level 1b from the radar equation, taking into account the following components:

  • on board AGC, from the AGC coarse and fine values, corrected for instrument errors using both ground (Tx and Rx paths) and flight PTR calibration parameters,
  • antenna gain and overall gain of the receiving chain
  • correction of illuminated area for Earth curvature

The scaling factor thus obtained, is used at Level 2 to compute, from the amplitude of the retracked waveform, the backscattering coefficient of the echo.

Question 4.17 : How is the datation calculated?

The UTC datation is the exact time when the on-board averaged waveform is reflected from the surface, and hence, when the middle of the 100 pulses averaged waveform is at the surface.

It's of vital importance to have a precise and coherent measure of the time, since the altimeter range is derived from the measurement of the time between the transmission of the pulse and its reception.

The datation is derived through correlation of the RA-2 ICU clock counter OBDH datation with the satellite on-board reference binary time which is in turn correlated to a UTC reference time.

The following components are also taken into account in the final L1b expression of the records datation:

  • the location of the mid point of the averaged waveform,
  • estimated time for the first received echo of the current data block, since the signal that is actually triggering the request for the datation is the pulse transmitted just before the first returned echo contained in the current data block
  • the propagation time delay

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry