Bathymetry from Space: White paper in support of a high-resolution, ocean altimeter mission

Perhaps the most important ocean science observation in the last two decades was provided by the Geosat radar altimeter during its 18-month geodetic mission (1985-86). The declassification of these data in July of 1995 set off a flurry of activity in basic research, industrial research/development, and public interest (Appendix A). While these data fill a huge gap in our understanding of the ocean basins, they also triggered a thirst for more. This paper reviews: i) the need for improved ocean bathymetry and gravity, ii) the fundamental physical limitations for recovering seafloor topography from measurements of ocean surface slope, and iii) the mission requirements to achieve significant improvements in accuracy and spatial resolution. While most areas of ocean science use bathymetric information, we focus on those applications where a new altimeter mission would provide the greatest benefit. These include: • resolving the fine-scale (~15 km wavelength) tectonic structure of the deep ocean floor in areas that have not been surveyed by ships (e.g., abyssal hills, microplates, propagating rifts, seamounts, meteorite impacts, . . .); • measuring the roughness spectra (15-100 km wavelength) of the seafloor on a global basis to better constrain models of tidal dissipation, vertical mixing, and mesoscale circulation of the oceans; • and resolving the fine-scale (~15 km wavelength) gravity field of the continental margins for basic research and petroleum exploration. Mission requirements for Bathymetry from Space are much less stringent and less costly than physical oceanography-type missions. Long-term sea-surface height accuracy is not needed; the fundamental measurement is the slope of the ocean surface to an accuracy of ~1 microradian. This can be achieved without application of the usual environmental corrections. The main requirements are improved altimeter range precision and dense coverage (< 7-km cross-track spacing) of the oceans for 6 years in order to reduce the noise from ocean waves, coastal tides, and mesoscale ocean variability. A low inclination orbit (50-65 ̊) is best for recovery of the low-latitude gravity field since the E-W slopes are poorly constrained by the Geosat and ERS altimeters. Existing and planned repeat-orbit altimeters will not achieve these objectives. Moreover, the satellite gravity missions, CHAMP, GRACE, and GOCE will recover sea surface slope at wavelengths greater than about 200 km but because of upward continuation, they cannot recover the shorter wavelengths. The primary science objective could be achieved with a relatively cheap mission. US petroleum exploration companies are keenly interested in these data, especially in coastal areas, and are willing to offer support for this mission. Scripps Institution of Oceanography, La Jolla, CA, 92093-0225; Laboratory for Satellite Altimetry, NOAA, Silver Spring Maryland, 20910-3282; CIRES and Dept. of Physics, Univ. of Colorado, Boulder, CO 80309-0216; Conoco Inc., 600 North Dairy Ashford, Houston, TX, 77252-2197; Dept. of Geology, Tulane Univ., New Orleanes, LA 70118.