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First guess

The altimeter provides the sea surface height relative to the reference ellipsoid. It is the sum of the geoid plus the dynamic topography. By averaging altimetric heights over a given period, a Mean Sea Surface ( MSS ) can be estimated.

The MSS is referenced to the Earth ellipsoid.

The MSS is the sum of the geoid plus the mean dynamic topography MDT . Thus, by subtracting the geoid we can obtain the MDT ; this is the so called "direct method" or geodetic MDT

Unfortunately, satellite only geoid models do not resolve as small scales as MSS . The use of the recent geoid data allows resolving scales down to around 100-125 km of resolution but it is still larger than the 10 km resolution of the  MSS . Thus, the difference (Mean Sea Surface - Geoid ) has to be filtered to remove scales not accurately resolved by the geoid model. We use an optimal filter which is based on an objective analysis that takes into account the omission and commission geoid errors. We obtain a medium scale MDT which is called "first guess". 

The two surfaces (Mean Sea Surface - Geoid ) are subtracted, and  provide after optimal filtering, a first guess of the mean dynamic topography at a medium scale. 

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