Level1 LRM processing chain  Sentinel3 Altimetry Technical Guide  Sentinel Online
See S3 SRAL Algorithms Technical Baseline Document for further information about S3 Altimetry algorithms.
The main algorithms of the Level1 LRM chain are:

Determine surface type
This algorithm computes the surface type ("open ocean or semienclosed seas", "enclosed seas or lakes", "continental ice" or "land") determining the position of a "landsea mask" Auxiliary Data File nearest to the geolocated measurement. The latitude and longitude resolution of this landsea mask is 2 minutes.

Compute tracker ranges corrected for USO frequency drift
This algorithm computes the USO correction from an Auxiliary Data File called "USO file" and this correction is applied to the tracker range. The "USO file" provides the real USO frequency drift measured onboard wrt the USO frequency nominal value. This algorithm also computes the tracker range rate converted into distance versus time.

Compute tracker ranges corrected for internal path correction
This algorithm computes the internal path correction from an Auxiliary Data File called "CAL1 LTM file" and this correction is applied to the tracker range. The "CAL1 LTM file" provides the internal path delay measured onboard thanks to the CAL1 calibration mode, which measures the difference of travel between the transmission and the reference lines within the altimeter. This algorithm also computes and applies the instrumental delay correction measured onground, due to the distance between the duplexer and the antenna reference point.

Compute Doppler correction
This algorithm computes and applied the Doppler correction to the tracker ranges. The Doppler correction is a function of the emitted frequency, the pulse emitted duration, the orbit altitude rate, the emitted bandwidth and the sign of the slope of the transmitted chirp.

Correct the AGC for instrumental errors
This algorithm computes the Automatic Gain Control (AGC) instrumental correction and applies this correction to the AGC. The AGC instrumental correction is computed taking into account the real gain value applied onboard and stored as a matrix table on an Auxiliary Data File called "characterisation file".

Compute sigma0 scaling factor
This algorithm computes the sigma0 scaling factor that is used at Level2 to determine the backscatter coefficients from the retracked amplitudes. The sigma0 scaling factor accounts for all power attenuations and gains, which has an impact on the signal received onboard.

Correct the waveforms.
Onboard, there is a calibration mode called CAL2 that is able to compute the Gain Profile Range Window (GPRW) that provide the information of the attenuation of the samples of the waveforms available at Level0 products due to several instrumental effects (e.g. intermediate frequency filters gain response). This algorithm corrects these Level0 waveforms by the GPRW instrumental effects.