Ocean retracking

An accurate global geoid, forms a vital component of GPS systems and is vital for a range of scientific and commercial applications.
See International Geoid service
The primary information source over the ocean, particularly for short wavelength variations in the gravity field, is satellite radar altimetry.

Over the open ocean, many waveforms conform to the Brown model and are well retracked by the Space Agencies.

A significant percentage fail and are rejected. As the coast is approached, this proportion increases rapidly and at 5km from the coast, more than 50% of the echoes are not Brown model in shape. By retracking all echoes using the Berry expert system (BES), heights are successfully retrieved right up to the coast. The global plot shows the ERS1 Geodetic Mission dataset, plotting echoes from calm water, which fail the Brown model fit and are retracked by one of 12 other retracking algorithms in the BES.

To look at the enhanced height retrieval, the results are compared with the RADS database over the Malay archipelago. The first plot shows the calm water echoes, and the second plot shows the enhanced data retrieval from the BES for all retrackers c.f. RADS.

To illustrate the enhanced geoid retrieval, the next two two plots show geoid heights derived from RADS data off the Greenland coast compared with the geoid results from the BES.

The ERS1 Geodetic Mission was retracked using the BES in its entirety, and the results were used to create the global marine gravity field DNSC08 Gravity and for the coastal and cryospheric ocean geoid in EGM08.