Oceanographic applications of precise geodesy and airborne altimetry

The geoid is the gravity equipotential surface that approximates global mean sea-level (msl), and if sufficiently accurate, can provide a reference surface useful for water level measurement systems in oceanography and hydrography. An accurate geoid can be used to connect the ellipsoid referenced positioning capabilities of GPS with sea-level referenced quantities. The US Naval Research Laboratory (NRL) and the Danish National Survey and Cadastre (KMS) have been developing capabilities in airborne gravity and altimetry that can simultaneously improve the geoid, provide precise water level profiles and transfer absolute vertical coordinates to tide-gages and bottom-moored inverted echo-sounder or pressure gage systems. Examples of high-precision geodesy and altimetry in the Gulf of Mexico demonstrate the general possibilities of the technique and its potential application to the Baltic. Recent airborne gravity surveys of the Baltic performed by KMS, when combined with regional historical terrestrial and marine gravity measurements and the new intermediate wavelength gravity data from the CHAMP and GRACE satellite gravity missions allow the production of a precise gravimetric geoid for the area. This msl reference can provide a unified vertical reference frame for water-level measurement systems and future advanced circulation models of water level and currents in the Baltic.