Algorithms

The first function of the Level-2 processing chain is to apply different re-tracking algorithms to Level-1 waveforms to calculate the final altimeter range, backscatter coefficient, wind speed over ocean and SWH. There are different types of re-tracking algorithms according to the type of waveforms re-tracked (ocean, ice, sea-ice).

The second function of the Level-2 processing chain is to compute and apply all geophysical corrections to the measurements. Examples of geophysical correction algorithms are the tides or the reference surface used (e.g. the geoid).

There are three types of algorithms in the Level-2 processing chain:

• algorithms computing time-derived geophysical/environmental parameters
• algorithms computing retracking and computing physical parameters
• algorithms performing altimeter/radiometer geophysical processing.

Products

A Level-2 SRAL/MWR product contains three netCDF files:

• one "reduced" (//red//) data file, containing a subset of the main 1 Hz Ku-band parameters
• one "standard" (//std//) data file containing the standard 1 Hz and 20 Hz Ku and C-band parameters
• one "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 (at least in LRM mode). Each of these three files contains one or more data sets from among the following:
• 20 Hz LRM/SAR_Ku data set: Set of 20 Hz Ku-band parameters issued from SRAL tracking measurements performed in LRM and SAR modes
• 20 Hz LRM/SAR_C data set: Set of 20 Hz C-band parameters issued from SRAL tracking measurements performed in LRM and SAR modes
• 1 Hz LRM/SAR_Ku/C data set: Set of 1 Hz Ku/C-band parameters issued from SRAL tracking measurements performed in LRM and SAR modes. 

The Sentinel-3 Surface Topography Mission (STM) Level -2 products are split between Land & Marine products. This split is related to the respective Sentinel-3 responsibilities shared between the European Space Agency (ESA) for the Land products production and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) responsible for the Marine products production. However, as far as today the content of the Level-2 products is similar, the following description is thus applicable to Land & Marine level-2 products.