At present, only altimeter systems are capable of measuring Sea Surface Height (SSH), with which information on ocean circulation patterns and sea levels are determined on a global scale.
SSH is the height of the sea surface with respect to the reference ellipsoid. SSH measurement includes dynamic contributions from ocean circulation and variations in the geoid with respect to the reference ellipsoid. Dynamic topography is the variation in SSH with respect to the geoid.
Sea level anomalies (a variable part of SSH) are usually derived by subtracting the Mean Sea Level (MSL) from the SSH. MSL is the local SSH averaged over a long period, whereas Global Mean Sea Level (GMSL) is the MSL averaged across all the oceans of the globe. An increase in GMSL is an indicator of possible global warming.
The Sentinel-3 mission SSH goal accuracy is 10 cm for Near Real-Time (NRT) products and 3-5 cm for Slow Time Critical (STC) products.