A refined gravity model from Lageos (GEM‐L2)

Abstract

A refined gravity field model, Goddard Earth Model GEM‐L2, has been derived using the Lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion. This field also contributes to an improved understanding of long wavelength features, such as the sea slope across broad ocean basins, through its significant improvement of the long wavelength geoid (through degree and order 4). The geoid for these terms has an accuracy estimated at ± 8 cm in GEM‐L2. GEM‐L2, as in all recent Goddard Earth Models, relies heavily on the precise near‐Earth satellite laser ranging data, in this case provided by NASA's Crustal Dynamics Program. Two and a half years of Lageos laser data acquired from over 20 well‐distributed stations were combined with the existing data from the best satellite‐derived model, GEM‐9, to develop the new Lageos model. Testing shows that the Lageos gravity field error at long wavelengths is less than half that for GEM‐9. Independent tests using well determined longitude accelerations of 24‐hour satellites have verified the improved accuracy of the new model. A comparison of global laser "base" stations from independent data sets of alternating 15 day data segments over the 2 1/2 years of Lageos show total inter‐station positioning to ± 1.8 cm when using this new field. The same comparison using the 1979 versus the 1980 Lageos data yields ± 5.2 cm; this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets. Five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than .5 msec have been derived in the solution. The adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in GEM‐L2.

Dynamically supported geoid highs over hotspots: Observation and theory

“…Table 2 -Correlation coefficients at harmonic degree 6 between upper mantle shear velocity models (Tanimoto, 1986;Woodhouse and Dziewonski, 1984) and the observed geoid, the slab residual geoid, and the hotspot distribution. (Lerch et al, 1983) referred to the hydrostatic figure of the Earth (Nakiboglu, 1982). In ( Observed and residual geopotentials, "yM/R (fraction of geopotential at surface), (7 is the gravitational constant, M the mass of the Earth, and R the Earth's radius.)…”

“…The Earth's long-wavelength geoid (shown in Figure la referred to the hydrostatic figure) has been well determined from observations of satellite orbits (Kaula, 1963;Lerch et al, 1983), but its interpretation has remained somewhat enigmatic because of its lack of resemblance to surface features such as continents and midocean ridges. However, by filtering out the lowest harmonics (degrees 2-3) which dominate the geoid spectrum (see Figure 2), it is obvious that many of the "intermediate" wavelength geoid highs are located over active subduction zones ( Figure Ib).…”

A dynamic model of Venus's gravity field

“…Figure 1 illustrates the correlation between the longi wavelength components of gravity and topography as a function of spherical barmonic degree for Venus. For comparison, the correlation between the Earth's gravity field (Lerch et al, 1982), referred to its hydrostatic figure (Nakiboglu, 1982), and "equivalent rock" topography (NOAA, 1980) is also shown. For degrees 3 through 18, the correlation is positive and statistically significant for Venus.…”

Novel considerations about the error budget of the LAGEOS-based tests of frame-dragging with GRACE geopotential models

“…As another example, Milani et al [83, p. 17] evaluated the uncertainty in J 2 just by taking the difference between the estimated values for it from two global gravity field models, i.e. GEM-L2 [64] and GEM 9 [65]: while the uncertainty of the GEM 9 value was s J 2 ¼ 1 Â 10 −9 , GEM-L2 yielded s J 2 ¼ 4 Â 10 −10 . The same approach was followed by Lerch et al [66] with the model GEM-L2 [64], as correctly recognized by Ciufolini [7, p. 1713] himself.…”