The reference ellipsoid for the MSS is the WGS84 and it’s also the same reference for the geoid  GOCO6S (Kvas, A. et al, 2021, Complete GOCE mission fully reprocessed (Nov 2009-October 2013).

References can be found on the AVISO website  

• elipsoide reférence : MSS CNES_CLS2022 description  

• Geoide reférence : MDT Description 

For more details : 

1. The explanation of the calculations can be found in the QUID for L4 products section II.3 CMEMS-SL-QUID-008-032-068.pdf 

1. The details of the altimeter measurements can be found in QUID CMEMS-SL-QUID-008-032-068.pdf section : II.3 Introduction to the altimeter measurement and also in the website How are defined the different heights used in altimetry? | Duacs