Modern needs for marine positioning in ocean current measurements include precise location and speed of surface ships and aircraft, location of free floating and anchored acoustic devices, satellite altimetry, and mapping of high altitude or orbital scanning radiometer imagery. Absolute location requirements range from 1 km to 10 km, depending on the application, but relative positioning not to exceed ± 3 m in 3 km, or 1:103, is necessary for several applications in the deep sea. Surface vessel drift velocities with a tl cm sec−1error maximum are required, and satellite altimetry slope measurements of 2.5 × 10−8is the minimum to achieve the same geostrophic velocity accuracy at latitudes of 10° or less. Location of picture elements from satellite measurements must be within one picture element diameter; picture elements range from 10 km for imagery from geostationary operational spacecraft to 25 m from an experimental synthetic aperature radar.