Status of the geopotential

In the past 4 years the determination of the gravitational field of the earth has taken evolutionary steps. The main emphasis has been in the determination of potential coefficients. The Standard Earth (SE) II solution [ Gaposchkin and Lambeck , 1971] was updated to the SE III solution [ Gaposchkin , 1973] by the addition of new satellite and terrestrial data with improved analytic techniques. The SE III solution gave potential coefficients complete to degree 18, while the SEII solution had coefficients complete to degree 16. Work at the Goddard Space Flight Center progressed through a state of three basic solutions, each basic solution being composed of a set of coefficients based on satellite data alone and on a combination solution with terrestrial gravity data. In this sequence of solutions we had Goddard Earth Model (GEM) 1 and 2 [ Smith et al. , 1973], GEM 3 and 4 [ Lerch et al. , 1972], and GEM 5 and 6 [ Richardson and Lerch , 1974]. The satellite alone solution (GEM 5) is complete to degree 12, while the combination solution is complete to degree 16. In these solutions the satellite data most strongly determine the potential coefficients up to about degree 12 plus resonance terms, while the terrestrial data are most strongly felt in the other coefficients. In the SE and GEM solutions the combination procedures use normal equations formed in terms of potential coefficients from the terrestrial gravity material, which are then added to the normal equations formed from the other data. In contrast to a strict least squares adjustment procedure, Rapp [1973 a ] determined a set of potential coefficients complete to degree 20 based on the method of least squares collocation by using the satellite alone solution of GEM 3 and 5° anomalies [ Rapp , 1972]. A comparison of some combination techniques may be found in a paper by Hopkins [1972].