SENTINEL-3 Altimetry Overview

The main application of the Sentinel-3 topography mission is the study of ocean topography including mean sea level, wave height, wind speed over the surface, sea-ice, ocean currents, Kelvin and Rossby waves, eddies and tides.

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

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

The goal of the Sentinel-3 mission is to provide operational measurements for 20 years using robust and stable altimetry technology already proven in previous ESA missions. The minimum baseline altimeter performance required shall be the same as Envisat and CRYOSAT altimeters.

The Sentinel-3 orbit (altitude, inclination and coverage) is almost the same as Envisat, allowing continuation of the ERS/Envisat time series.

In the Sentinel-3 SRAL mission, there are two main modes of operation:

  • High Resolution Mode commonly called Synthetic Aperture Radar (SAR) or delay Doppler altimetry (DDA)
  • Low Resolution Mode (LRM)

The default (or principal) mode for operation of SRAL is at High Resolution Mode, but LRM provides a backup and enables for consistency with earlier missions.


Like many recent altimeters, Sentinel 3 operates at two frequencies (Ju and C band) in order to derive an ionospheric correction. A Level 2 SRAL/MWR complete product contains three data files:
  • a "reduced" (Red) data file, containing a subset of the main 1 Hz Ku band parameters
  • a "standard" (Std) data file containing the standard 1 Hz and 20 Hz Ku and C-band parameters
  • an "enhanced" (Enh) data file containing the standard 1 Hz and 20 Hz Ku and C-band parameters, the waveforms and the associated parameters necessary to reprocess the data.

The SRAL/MWR Level-2 products are generated in Standard Archive Format for Europe (SAFE) format. All the information relevant to the product is gathered into a single package. Inside this package, the specific objects containing measurement data are encoded in netCDF format.

There are different levels of data latency related to the availability of the auxiliary or ancillary data:

  • Near Real-Time (NRT): delivered less than 3 hours after data acquisition
  • Slow Time Critical (STC): delivered within 48 hours after data acquisition
  • Non-Time Critical (NTC): delivered within typically 1 month after data acquisition.

There are different data products associated with the three levels of processing of altimeter data:

  • Level-0 (L0) is the raw telemetered data
  • Level-1 (L1) is the Level-0 data corrected for instrumental effects
  • Level-2 (L2) is the Level-1 data corrected for geophysical effects

Level-1 and level-2 products are available to users. Level-0 products are not available to users and are considered only as inputs to Level-1 processing.

The Sentinel-3 SRAL geophysical error estimates for the altimeter are shown in Table 1.

Table 1: Major error terms for altimeter height estimation. All data in this table are derived using a 1 Hz integration time and a 2 m Hs. Reference: ESA
Parameter - (cm) NRT Threshold NRT Goal STC Threshold STC Goal NTC Threshold NTC Goal
Range Noise 3.0 2.0 3.0 2.0 3.0 2.0
Sea State bias 5.0 2.0 3.0 2.0 2.0 1.4
Ionosphere 3.0 2.0 1.4 1.0 1.0 0.7
Dry Troposphere 3.0 2.0 1.4 1.0 1.4 1.0
Wet Troposphere 4.0 3.0 2.0 1.4 1.0 1.0
RSS Range Error 8.3 6.0 5.0 3.5 4.1 2.9
Radial Orbit Error 10.0 8.0 4.0 3.0 3.0 2.0
RSS SSH Error 13.0 10.0 6.5 4.6 5.1 3.5


The ESA S3 Altimetry toolbox is used to visualise and operate Level-1 and Level-2 products.

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