Altimetry Measurements Research Articles

Introduction to the report on geodesy

The achievements of the U.S. geodetic community during the years 1974–1978 between the General Assemblies of the IUGG are summarized in this section. During this quadrennium, American geodesy dramatically moved from the era of primarily meeting the traditional needs of surveying and mapping into one of supporting a broad spectrum of geophysical and planetary disciplines. This movement was aided by modern technology epitomized by the increases in Doppler satellite observations, multifrequency distance measurements, lunar and satellite laser ranging, very long base line interferometry (VLBI), satellite altimetry measurements and inertial positioning.By far the most spectacular achievement in the field of geodesy during this period was the global sampling of sea surface topography with precision radar altimeters aboard the National Aeronautics and Space Administration (NASA) GEOS‐3 and Seasat‐1 satellites. The GEOS‐3 altimetry data set, acquired over a 3‐year period (1975–78), significantly improved our knowledge of the global gravity field of the earth, the oceanic geoid and the related mean gravity anomalies. Had the Seasat‐1 not failed after 3 months, an even more detailed knowledge of the oceanic geoid would be available today. Even with this early demise, Seasat‐1 in 3 months collected more altimetry data with greater accuracy than did GEOS‐3 in 3 years.

Geoid anomalies across fracture zones and the thickness of the lithosphere

Recent advances in the measurement and interpretation of geoid height anomalies provide a new way to estimate the thickness of the oceanic lithosphere as a function of crustal age. GEOS-III satellite altimetry measurements show abrupt changes in sea level across fracture zones which separate areas of lithosphere with different ages. These changes have the correct location, amplitude, and wavelength to be caused by the combined gravitational attraction of the relief across the fracture zone and the isostatic support of this relief. Eight profiles of geoid height and bathymetry across the Mendocino fracture zone are inverted to determine the depth of the isostatic compensation, assuming that the compensation occurs in a single layer. These depths are then interpreted with a thermal boundary layer model of lithospheric growth. To explain satisfactorily the geoid measurements, the thermal diffusivity of the upper mantle must be 3.3 × 10 −3 cm 2 s −1 and the thickness of the lithosphere, defined as the depth at which the geotherm reaches 95% of its maximum value, must be9.1km m.y. −1/2 × t 1/2, where t is lithospheric age.

Measurement Error Analysis in Determination of Small-Body Gravity Fields

We consider analytically the use of existing instrumentation in determining asteroid gravity fields from orbiting spacecraft. Asteroids (Eros as an example) are modeled as homogeneous triaxial ellipsoids, with gravitational potential given by a sperical-harmonic expansion. Mass concentrations are modeled as point masses. The character of spacecraft orbits about asteroids is discussed, along with detectibility of gravitational coefficients and of mass concentrations. A Kalman-filtering treatment of the observation process, for Eros as example, shows that using DSN tracking and onboard gravity gradiometry, radar altimetry, and celestial angle measurements, a single orbit yields asteroid mass to 0.03% and coefficients C20 to C44 to 1% accuracies.