On the determination of the deflection of the vertical by satellite altimetry

Abstract

With the advent of satellite altimetry it has become possible to estimate the height of the marine geoid above the ellipsoid to an accuracy of better than 1 m. At a crossover of a satellite's ascending and descending tracks, altimeter data can be used to obtain two linearly independent estimates of the geoid's horizontal gradient. These two estimates determine the value of the deflection of the vertical at the crossover. We have used altimeter data obtained during the SEASAT mission to estimate deflections in an area of the Pacific Ocean in the vicinity of the Izu‐Bonin and Mariana Trenches. The altimeter‐derived deflections generally point outwards from the axis of the trench, reaching maximum amplitudes of up to 33 arc‐sec over the walls of the trench. At a crossover of SEASAT's repeat tracks, altimeter data can be used to determine a number of estimates of the deflection at approximately the same point. The mean root mean square (RMS) difference in the magnitude and direction of the deflection at 13 intersections of SEASAT's repeat tracks are ± 1.9 arc‐sec and ± 14.9°, respectively. We attribute these variations to instrument noise and oceanic variability. Deflections have also been estimated in two other regions of the Pacific Ocean by using altimeter data: one in the region of the Magellan Seamounts and one in the region of the Line Islands. In general, deflections are largest (up to 25 arc‐sec) in the region of the Magellan Seamounts and smallest (up to 10 arc‐sec) in the region of the Line Islands. We attribute these variations to formation of the Magellan Seamounts on older, stronger, oceanic lithosphere than the Line Islands.

Notes

Presently at: Meteorological College, Asahi‐cho 7–4–81, Kashiwa‐shi, Chiba‐ken, Japan 277.

Also at Department of Geological Sciences of Columbia University, New York, NY.

Presently at Yale University, New Haven, CT.