Bathymetry from space: Rationale and requirements for a new, high-resolution altimetric mission

Abstract

Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Applications as diverse as tsunami hazard assessment, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable bathymetric maps to be available on demand. Fundamental Earth science questions, such as what controls seafloor shape and how seafloor shape influences global climate, also cannot be answered without bathymetric maps having globally uniform detail. Current bathymetric charts are inadequate for many of these applications because only a small fraction of the seafloor has been surveyed. Modern multibeam echosounders provide the best resolution, but it would take more than 200 ship-years and billions of dollars to complete the job. The seafloor topography can be charted globally, in five years, and at a cost under $ 100 M. A radar altimeter mounted on an orbiting spacecraft can measure slight variations in ocean surface height, which reflect variations in the pull of gravity caused by seafloor topography. A new satellite altimeter mission, optimized to map the deep ocean bathymetry and gravity field, will provide a global map of the world's deep oceans at a resolution of 6–9 km. This resolution threshold is critical for a large number of basic science and practical applications, including: • determining the effects of bathymetry and seafloor roughness on ocean circulation, mixing, climate, and biological communities, habitats, and mobility; • understanding the geologic processes responsible for ocean floor features unexplained by simple plate tectonics, such as abyssal hills, seamounts, microplates, and propagating rifts; • improving tsunami hazard forecast accuracy by mapping the deep-ocean topography that steers tsunami wave energy; • mapping the marine gravity field to improve inertial navigation and provide homogeneous coverage of continental margins; • providing bathymetric maps for numerous other practical applications, including reconnaissance for submarine cable and pipeline routes, improving tide models, and assessing potential territorial claims to the seabed under the United Nations Convention on the Law of the Sea. Because ocean bathymetry is a fundamental measurement of our planet, there is a broad spectrum of interest from government, the research community, industry, and the general public. Mission requirements. The resolution of the altimetry technique is limited by physical law, not instrument capability. Everything that can be mapped from space can be achieved now, and there is no gain in waiting for technological advances. Mission requirements for Bathymetry from Space are much less stringent and less costly than typical physical oceanography 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 μrad (1 mm km −1). The main mission requirements are: – improved range precision (a factor of two or more improvement in altimeter range precision with respect to current altimeters is needed to reduce the noise due to ocean waves); – fine cross-track spacing and long mission duration (a ground track spacing of 6 km or less is required. A six-year mission would reduce the error by another factor of two); – moderate inclination (existing satellite altimeters have relatively high orbital inclinations, thus their resolution of east–west components of ocean slope is poor at low latitudes. The new mission should have an orbital inclination close to 60° or 120° so as to resolve north–south and east–west components almost equally while still covering nearly all the world's ocean area); – near-shore tracking (for applications near coastlines, the ability of the instrument to track the ocean surface close to shore, and acquire the surface soon after leaving land, is desirable). To cite this article: D.T. Sandwell et al., C. R. Geoscience 338 (2006).