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4.5 Main RA-2 Level 2 Algorithms (Non-Ocean)

Question 4.19 : What is Retracking ?

Over topographic surfaces, a radar altimeter on-board tracking system is unable to maintain the echo waveform at the nominal tracking position in the filter bank, due to rapid range variation. This results in an error in the telemetered range known as tracker offset. Retracking is a term used to describe a group of non-linear ground processing estimation techniques which attempt to determine the tracker offset from the telemetered echoes, and thereby estimate the range to the point of closest approach on the surface. Peaky echoes from sea ice cause range tracking jitter, which also results in tracker offsets.

Question 4.20 : What does the Slope Correction do ?

The Altimeter antenna boresite is pointed at Nadir and the antenna has a field of view of about 1.3 degrees (half power). The first part of the reflected echo will come from that part of the surface within the field of view, that is closest to the satellite. Over flat surfaces the closest point on the surface is at Nadir but over sloping terrain that will not be the case. When the echo waveform is retracked the resulting range measurement is a slant-range to a point offset from Nadir. To use this measurement it must be corrected for the slant and re-located to the point of first return offset from Nadir.

The correction is performed by using models of the surface slope which enable prediction of the direction of the point of first return for any given satellite location. Initially 2 models are applied: one for Greenland and one for Antartica.

Question 4.21 : What Is the Delta Doppler Correction ?

The Altimeter uses transmitts a chirped pulse which sweeps a range of radar frequencies. Processing converts from the frequency to the time domain. Any motion of the satellite along the line-of-site with the reflecting surface produces a Doppler shift in the pulse frequency. If uncorrected this translated to an error in the time delay. Over a flat surface this effect can be corrected by considering the vertical component of the satellite velocity. However over sloping surfaces the reflection does not come from the Nadir. An additional computation is required to find the component of the satellites velocity which is along the line of site to the reflecting surface.

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