Filtered Geosat Altimetry as a Tool for Global Bathymetric Chart Development

Now that the Geosat altimetry and precise orbit information collected during the 1985-1986 Geodetic mission is declassified, the Naval Oceanographic Office is releasing its chart of Geosat altimetry of the world's oceans (Figure 1). This data set has been used at the Naval Oceanographic Office since 1987.

Fig. 1. World Chart of filtered Geosat sea-surface height data. Contour interval is 0.05 m. Blue contours are positive, greens are negative. Notice the difference between seamounts and fracture zones.

Satellite radar altimeters do not measure gravity; they can, however, accurately measure the height of the satellite above the sea surface to the centimeter level. Once a precise orbit for the satellite is calculated, features in the sea surface become apparent. The rises and depressions in the sea surface are caused by local changes in Earth's gravity field. Except for certain ocean dynamic features, these changes in height of the sea surface correlate well with low-frequency gravity and bathymetry.

In 1989 the Naval Oceanographic Office produced its first Geosat product, a worldwide, 5-min, gridded Geosat database gathered from the Geodetic Mission. This database incorporates 7525 revolutions for a total of 49 million data points. The data were manually edited to remove obvious spikes and noise. Grossly misadjusted tracks and segments over polar ice were also removed from the grid. Long wavelength orbit error was reduced by adjusting the Geodetic Mission data to a time average reference network based on the first year of the Earth Repeat Mission.

A high-pass filter was applied to produce a data set of only the highs and lows in the sea surface, free of orbit error. The filter passed data beginning at wavelengths of 231 km, and all data of wavelengths shorter than 130 km were used. In the accompanying chart (Figure 1), seamounts, fracture zones, trenches, and ridges are readily distinguished. Overlays for comparison with the Digital Bathymetric Data Base-5-minute resolution data were prepared. DBDB-5, another Naval Oceanographic Office product, is the only worldwide gridded bathymetry data file, and it is the source of many of the published small-scale bathymetry charts of the world. Comparisons of filtered Geosat and DBDB-5 reveal many uncharted features. A chart and list of locations of these features are provided by Sramek [1992].

Instances where features appear in the DBDB-5 data but not in the Geosat data were also investigated (Figure 2). Mellish Seamount is reported at 34°00'N, 178°15'E [Gazetteer of Undersea Features, 1990; General Bathymetric Chart of the Oceans, 1984]. The Geosat data shows no indication of that seamount, yet all other features in that area are well represented. This feature had been questioned before [Nemoto and Kroenke, 1981]. A review of all available ship surveys in the area also found no evidence of Mellish Seamount.

Fig. 2. Mellish Seamount area, 0.05-m contour interval. Both General Bathymetric Chart of the Oceans and the Gazetteer of Undersea Features show a seamount at 34°N, 178°15'W.

A pseudo bathymetry has also been produced with varying degrees of success. A relationship between the bathymetry and sea-surface height was calculated and then used to scale the sea-surface height to the bathymetry using the DBDB-5 for the low-frequency, general background depths. This was found to be useful in investigating shoal areas and in producing "difference" charts to identify areas where the bathymetry and Geosat data disagreed. One problem encountered is that while the track spacing along the equator averages 5.5 km, gaps of over 11 km are common. This leaves some significant gaps in coverage and creates offsets in the apparent positions of features.

References

Sramek, S., Seamount detection and size estimation using filtered GEOSAT altimetry, Mar. Geod., 15, 151, 1992.

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