Showing papers by "Frank G. Lemoine published in 1998"

The Development of the Joint NASA GSFC and the National Imagery and Mapping Agency (NIMA) Geopotential Model EGM96

Abstract: The NASA Goddard Space Flight Center (GSFC), the National Imagery and Mapping Agency (NIMA), and The Ohio State University (OSU) have collaborated to develop an improved spherical harmonic model of the Earth's gravitational potential to degree 360. The new model, Earth Gravitational Model 1996 (EGM96), incorporates improved surface gravity data, altimeter-derived gravity anomalies from ERS-1 and from the GEOSAT Geodetic Mission (GM), extensive satellite tracking data-including new data from Satellite Laser Ranging (SLR), the Global Postioning System (GPS), NASA's Tracking and Data Relay Satellite System (TDRSS), the French DORIS system, and the US Navy TRANET Doppler tracking system-as well as direct altimeter ranges from TOPEX/POSEIDON (T/P), ERS-1, and GEOSAT. The final solution blends a low-degree combination model to degree 70, a block-diagonal solution from degree 71 to 359, and a quadrature solution at degree 360. The model was used to compute geoid undulations accurate to better than one meter (with the exception of areas void of dense and accurate surface gravity data) and realize WGS84 as a true three-dimensional reference system. Additional results from the EGM96 solution include models of the dynamic ocean topography to degree 20 from T/P and ERS-1 together, and GEOSAT separately, and improved orbit determination for Earth-orbiting satellites.

New high-resolution model developed for Earth's gravitational field

Abstract: After 3 years of intense work by some two dozen collaborating scientists at three institutions and after scores of evaluation tests, the Earth Gravitational Model 1996 (EGM96) was completed and released to the scientific community in September 1996. This model was developed jointly by the NASA Goddard Space Flight Center (GSFC), the National Imagery and Mapping Agency (NIMA, formerly the Defense Mapping Agency), and The Ohio State University. EGM96 provides a more accurate reference surface for the topography, improves models of the ocean circulation, improves orbit determination for low-orbiting satellites, and contributes to global and regional studies in tectonics and geodynamics. The new spherical harmonic model, is complete to degree 360, corresponding to a global resolution of about 55 km. EGM96 incorporates newly released surface gravity data from around the globe, over three decades of precise satellite tracking data and altimeter measurements of the ocean surface from the TOPEX/POSEIDON, ERS-1 and GEOSAT missions. Figure l a shows a global map of the geoid undulations implied by EGM96, while Figure l b shows the corresponding gravity anomaly field.

Gravity Field Improvement Activities at NASA GSFC

Abstract: Since the completion of the EGM96 geopotential model, additional satellite tracking data has been added to the satellite-only geopotential model solution. The new data include, TRANET Doppler tracking data from the GEOSAT Geodetic Mission, TDRSS tracking of the Gamma Ray Observatory (GRO), the X-Ray Timing Explorer (XTE), the Earth Radiation Budget Satellite (ERBS), as well as additional data from the Extreme Ultraviolet Explorer (EUVE). The new data from the TDRSS tracked satellites make an important contribution to the satellite-only geopotential solution. Comparisons with independent 30′ × 30′ altimeter derived anomalies from the GEOSAT Geodetic Mission provided by NIMA show that the ERBS data contribute by reducing the residual at degree 70 by 0.47 mGal2. The data from XTE are valuable because of their unique inclination of 23°. Data from low inclination satellites is sparse in most satellite-only derived geopotential models, although substantial amounts of tracking data from EUVE (at an inclination of 28.5°) were included in EGM96. The performance of permutations and subsets of the EGM96 model are also shown to highlight different aspects of the model’s performance.

Reduction of crossover errors in the earth gravity model (EGM) 96

Abstract: Using the established procedure, the calibrated covariance matrix of harmonic geopo‐tential coefficients of the new (Earth Gravity Model) EGM96 (to 70 × 70) is projected to single‐and dual‐satellite crossover errors, and their spectral latitude lumped coefficient constituents. These results are compared with previous gravity solutions, such as JGM 2 and JGM 3, to assess the strengths and weaknesses of the new solution. This analysis quantifies the level of improvement over previous solutions, as well as suggests areas where further refinements are required to achieve subdecimeter accuracy over a wide range of satellite missions.

Lunar Basins: New Evidence from Gravity for Impact-Formed Mascons

Abstract: The prominent gravity highs (mascons) associated with uncompensated mass anomalies in lunar mare basins are a dramatic expression of the present-day rigidity of the lunar lithosphere First discovered in Lunar Orbiter tracking data, these about 350-mGal gravity highs have been redetermined from the analysis of Clementine and historical tracking These highs coincide with topographic lows, indicating nonisostatic support One of the rediscoveries of this analysis is the encirclement of the highs by substantial negative anomalies over topographic highs Recent gravity fields are providing the increased resolution necessary to determine the causes of this unique mascon signature The compensation of the basin anomalies remains controversial The mascon highs have long been interpreted as the result of mare loading, subsequent to the decay of residual stresses resulting from the impact Substantially more mare fill is required to produce mascon highs than has been inferred on geological grounds, and the amount of near-surface mass deficit required to produce a gravity most exceeds bounds inferred from terrestrial examples This problem is most acute for the youngest basin, Orientale Recent gravity fields from Lunar Prospector have suggested mascon highs associated with nonmare basins such as Mendel-Rydberg, or minimally filled basins like Humboldtianum, further calling this explanation into question We suggest that the mascon gravity signal is produced by a combination of crustal thickness changes, manifested by central mantle uplift, outward displacement of crust, and downward flexure of the lithosphere under mare loading The mantle uplift is superisostatic, maintained by residual stresses resulting from the process of impact cratering and modification In particular, the process of crater collapse and mantle rebound terminates abruptly, leaving the mantle plug in a non-equilibrium state, surrounded by a ring of thickened crust Viscous relaxation over geological timescales has erased some but not all of the signature of the impact process Mantle uplift inferred from gravity modeling is inversely correlated with age While the oldest basins such as South Pole Aitken are mainly compensated isotatically, the younger basins appear to have been in a state of superisostatic loading prior to mare emplacement If this is true, this places an important constraint on the impact process at basin scales The idea that rebound of the transient crater via acoustic fluidization may freeze substantial stresses imcomplete to this day, may be tested by examining the gravity signatures of major basins on terrestrial bodies The moon provides the clearest resolved examples to date, but uncertainty in gravity knowledge remains problematic