Geophysical Measurements

The main geophysical parameters to be measured by the SENTINEL-3 topography mission are:

  • Surface Height (SSH)
  • Significant Wave Height (SWH)
  • Wind speed over ocean surface.

These parameters are retrieved from the physical characteristics of the altimeter waveforms received by the SRAL instrument.

Figure 1: Physical Characteristics of the Altimetry Waveforms (Credit: isardSAT)


  • Epoch: this is related to the range. SSH is computed from it.
  • P: power of the useful signal. This power, with respect to the emitted power, gives the backscatter coefficient, sigma0. Sigma0 (together with the SWH) is used to estimate wind speed.
  • Leading edge slope: this can be related to the Significant Wave Height (SWH).
  • Trailing edge slope: this is linked to antenna mis-pointing and Significant Wave Height (SWH).
Var/mode NRT/STC/NTC
Range 1.25 cm 1.67 cm 2.53 cm
SWH 9.7 cm 11.4 cm 16.2 cm
Wind Speed 16 cm.s-1 21 cm.s-1 17 cm.s-1

Table 1 : Precision of the geophysical parameters derived from retracking over open ocean (averaged conditions of SWH = 2m and wind speed = 7 m.s-1).

NRT 1.25 cm 1.67 cm 2.53 cm

Table 2 : Precision of the SARM SSH estimated at crossovers over ocean for the different L2 products timeliness (NRT,STC and NTC). Metrics are impacted by the precision of the altitude as well as the precision of several geophysical corrections (derived from predicted analyses in near real time eg: meteo parameters).

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