643 



d. Findings 



The findings relative to geomagnetics are: 



• Data from the Navy's magnetic surveys could be used to 

 constrain the age of the seafloor, and, combined with satelUte 

 radar altinietry measurements, make it possible to refine 

 plate reconstruction rotation parameters, including finite 

 rotations associated with the long-term evolution of the 

 Pacific and Atlantic oceans. 



* The availability of high resolution magnetic surveys would 

 help scientists to evaluate various proposed models of plate 

 tectonics. 



seafloor topography. Thus, sateUiie altimeter data provide 

 important reconnaissance information over vast areas of largely 

 uncharted seafloor, such as the Southern Ocean and Antarctic 

 Margins. 



Figure 9 shows a sample color shaded relief image (at left) of 

 sea surface gravity anomalies over the Southwest Indian Ridge 

 south of South Africa derived from declassified Geo.sat/Geodetic 

 Mission (GM) and other altimeter data. The right-hand image 

 in Figure 9 is a color shaded relief image of seafloor topography 

 predicted from the data in the left-hand image along with 

 available shipboard depth soundings. Both images are 

 illuminated from the southeast. 



• The good temporal and spatial resolution of theNavy surface- 

 level magnetic data could better define the source of the 

 ambiguities m intermediate wavelength data from satellites. 



• These detailed, well-navigated, and weU-cahbrated three 

 dimensional measurements should provide a unique database 

 for understanding the details in the evolution of Earth's main 

 magnetic field. 



3. GEOSAT ALTIMETRY 



a. Data Description 



Over the past decade, satellite alrimetry has had a considerable 

 impact in the earth sciences. Spacecraft use pulse-limited 

 radars, along with very accurate orbits, to measure the 

 topography of ocean, ice, and land surfaces. Over the ocean, 

 the radar pulse interacts over a wide footprint (3 km) effectively 

 averaging out the surface waves so that small variations (3-5 

 cm) in sea surface topography can be mapped. Over rougher 

 surfaces such as ice and land, the precision of the topography 

 measurements degrades to several meters. These data have 

 created new fields of research in geodesy, glaciology, marine 

 geology, geophysics, and physical oceanography. 



The small bumps and dips in the ocean surface (0.03-10 m) 

 follow the equipotential surface of Earth's gravity field, or 

 geoid, so satellite altimeter measurements of sea surface 

 topography can be used to recover marine gravity anomalies. 

 At short wavelengths (<200 km), the gravity field mimics the 



Satellite data quality (1-2 mga! on a 10-15 km wavelength) is 

 currently considered to be better than the standard regional type 

 of survey that was run in the previous 25 year^ by oceanographers 

 and oil explorers ( I mgal of acceleration is about lO'timesthe 

 pull of gravity). However, the quality is not as good as the new 

 'nigh-resolution data that are being collected in the oceans 

 today, or as good as the new gradiometer data obtained from the 

 Navy system at sea. The new high-resolution data and the 

 unclassified gradiometer data have an accuracy of about 0. 1 

 mgal on a 500 to 1 000 meter wavelength thanlcs to the availability 

 of high-quality GPS data. 



Individual Geosat profiles have a precision of about 6 mgal and 

 along-track resolution of 30-50 km. The close spacing of the 

 profiles (4 km) supports a cross-track resolution that is better 

 than the along-track resolution. This redundancy enables one 

 to construct a two-dimensional gravity field having equally 

 good resolution in all directions (25 km full-wavelength). 



b. Accessibility 



The Geosat altimeter was launched by the U.S. Navy in March 

 1985 and provided data until October 1989. Its primary 

 geodetic mission was to map the marine gravity field at a high 

 spatial resolution on a global basis. Because of their military 

 value at the time, most of the data collected during the first 18 

 months of die mission remain classified, although data from die 

 Southern Ocean (south of 30''S) were declassified in 1992. 

 Some limited result* based on the still-classified data have been 

 published by those having access. 



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