Littoral Geodesy and Airborne Measurement
of Dynamic Sea Surface Topography

Code 7421, Principal Investigator: Dr. John Brozena

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Figure 1		Figure 2		Figure 3		Figure 4
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Littoral warfare is a concern for the Navy at present.  Whereas water current direction and speed can be accurately predicted in deep water regions by present ocean circulation models, these models break down in shallow water environments.  Current speed and direction are related to bathymetry and local conditions that are not well measured.  Most importantly, there is no absolute mean sea level (MSL) but only locally determined reference surfaces from tidal gauges.  Sea-surface height anomaly (deviation from MSL) is a primary data source assimilated into ocean circulation/flow models.  The lack of such a reference surface to calculate these anomalies makes shallow water modeling difficult.  Satellite altimetry data that have contributed so much to the understanding of deep water circulation are absent or inaccurate in the shallow water regions because the altimeter must be at least one “footprint-width” away from land before its measurements are valid.

Deep water basins have presented fewer problems for modeling.  MSL reference surface is determined from long-period averages of satellite altimetry measurements and steric leveling. 

The geoid is a surface of equal gravitational potential that corresponds to MSL. An accurate gravimetric geoid can provide a reference surface for determining sea surface height anomalies, important information that indicates the location of steady-state and temporary oceanographic currents.  Marine geoids to date have not been sufficiently accurate for oceanographic use.  Code 7420 brings its expertise in airborne measurement of gravity and sea surface topography to address the above problems.  Our demonstrated accuracy in Arctic gravimetric surveys (< 2 mGal rms error) is sufficient to generate a geoid accurate enough for these applications.  Such a gravimetric geoid can be then differenced from sea surface topography, as measured by radar altimeters aboard the aircraft to yield the sea surface height anomalies.

Code 7420 has been addressing these issues in conjunction with the North Gulf Littoral Initiative conducted along the northern margin of Gulf of Mexico.  A gridded survey was planned over a region of poor coverage of gravity measurements (Figure 1) as a test of the improvement offered by airborne gravity measurement.  Figure 2 shows the airborne measurement over this region and Figure 3 shows the sea surface topography.  Figure 4 demonstrates the sea surface height anomalies calculated by subtracting the improved geoid (made from existing plus airborne gravity data) from the sea surface topography measured by the aircraft.

Marine Geoscience Division
Marine Physics (Code7420)
Geophysical and Geodetic Systems Section (Code7421)