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A new satellite altimeter mission to map the deep ocean bathymetry and gravity field five times better than existing global maps is being considered. This mission, which would be 16 times faster and cheaper than mapping the sea floor with conventional multibeam systems, would be used to probe the internal structure of the continental margins, and estimate sea floor topography and roughness spectra for geological, oceanographic, and climatological purposes (see boxed sidebar). Of course, the highest-resolution bathymetry maps come from shipboard systems, but so far, only 10% of the sea floor has been surveyed, and it will take 125 ship-years to map the deep oceans at a cost of about one billion dollars.

The accuracy of current altimeter-derived maps is limited by ranging noise and short mission duration. A new altimeter with improved technology and a non-repeating orbit could provide accuracy to 1 mGal within 6 years (http://fermi.jhuapl.edu/abyss). In the deep ocean, where there is little sediment cover, gravity and topography are highly correlated, so that gravity can be used to predict topography, but there is a fundamental resolution limit of π times the mean ocean depth, which is ∼12 km full-wavelength, due to upward continuation. On the shallow continental margins, the gravity field reveals variations in sediment and crustal structure.