Altimetry : Observing Ocean Variability from Space

D I R E C T measurement of large-scale ocean currents with current meter arrays is difficult and costly on basin scales. Since large-scale currents are very nearly in geostrophic balance, their velocity can be calculated from the pressure gradient on an equigeopotential surface. The surface geostrophic current therefore can be calculated from the deviation of sea level from the equigeopotential at the ocean surface (the marine geoid). Measuring sea level from space by satellite altimetry thus offers a unique opportunity for determining the global surface geostrophic ocean circulation and its variability. Coupled with knowledge of the geoid and the ocean density field, satellite altimetry provides the only feasible approach for determining absolute geostrophic currents in the global ocean. Repeated altimetric observations of sea level at the same locations can resolve the variability of surface geostrophic currents, without the requirement for an accurate geoid model. In this paper, we briefly introduce the technique of satellite altimetry, highlight progress made in the past, assess the challenges that lie ahead, and discuss future plans. The Technique A satellite altimeter is a radar that precisely measures the range from the radar antenna to the ocean surface. A schematic summary of altimeter measurements and the corrections that must be applied to obtain dynamic sea surface elevation is shown in Fig. 1 (p. 6). The altimeter transmits a short pulse of microwave radiation with known power toward the sea surface at satellite nadir (the point directly beneath the satellite). This pulse interacts with the sea surface, and part of the incident radiation reflects back to the satellite, giving the range from the 2-way travel time of the pulse. Near-surface wind speed and significant wave height also can be determined from the power and shape of the returned signal. If the orbit height H of the satellite relative to a reference ellipsoid is known from precision orbit