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  • 1
    Publication Date: 2019-01-25
    Description: Knowledge of the gravitational field, in combination with surface topography, provides one of the principal means of inferring the internal structure of a planetary body. The highest resolution gravitational field for Mars published thus far was derived from Doppler tracking data from the Mariner 9 and Viking 1 and 2 spacecraft and is complete to degree and order 18 corresponding to a half wavelength resolution of approximately 600 km. This field, which is characterized by a spatial resolution that is slightly better than that of the highest resolution (16x16) topographic model, has been utilized extensively in analyses of the state of stress and isostatic compensation of the Martian lithosphere. However, the resolution and quality of current gravity and topographic fields are such that the origin and evolution of even the major physiographic features on Mars, such as the hemispheric dichotomy and Tharsis rise, are not well understood. We have re-analyzed the Viking and Mariner data sets and have derived a new gravitational field, which we designated GMM-1 (Goddard Mars Model-1). This model is complete to spherical harmonic degree and order 50 with a corresponding (half wavelength) spatial resolution of 200-300 km where the data permit. In contrast to previous models, GMM-1 was solved to as high degree and order as necessary to nearly exhaust the attenuated gravitational signal contained in the tracking data.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z; p 1317-1318
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  • 2
    Publication Date: 2018-06-08
    Keywords: Geophysics
    Type: 2002 Fall Meeting of the American Geophysical Union; San Francisco, CA; United States
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  • 3
    Publication Date: 2011-08-24
    Description: An improved model of the Earth's gravitational field is developed from a combination of conventional satellite tracking, satellite altimeter measurements, and surface gravimetric data (GEM-T3). This model gives improved performance for the computation of satellite orbital effects as well as a superior representation of the geoid from that achieved in any previous Goddard Earth Model. The GEM-T3 model uses altimeter data directly to define the orbits, geoid, and dynamic height fields. Altimeter data acquired during the GEOS-3 (1975-1976), SEASAT (1978), and GEOSAT (1986-1987) missions were used to compute GEM-T3. In order to accommodate the non-gravitational signal mapped by these altimeters, spherical harmonic models of the dynamic height of the ocean surface were recovered for each mission simultaneously with the gravitational field. The tracking data utilized in the solution includes more than 1300 arcs of data encompassing 31 different satellites. The observational data base is highly dependent on SLR, but also includes TRANET Doppler, optical, S-Band average range-rate and satellite-to-satellite tracking acquired between ATS-6 and GEOS-3. The GEM-T3 model has undergone extensive error calibration.
    Keywords: GEOPHYSICS
    Type: In: From Mars to Greenland: Charting gravity with space and airborne instruments - Fields, tides, methods, results (A93-55951 24-46); p. 29-44.
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  • 4
    Publication Date: 2011-08-24
    Description: Each of the components required for the computation of precise orbits for the TOPEX/Poseidon (T/P) spacecraft - gravity field modeling, nonconservative force modeling, and satellite tracking technologies - is examined. The research conducted in the Space Geodesy Branch at Goddard Space Flight Center in preparation for meeting the 13-cm radial orbit accuracy requirement for the T/P mission is outlined. New developments in modeling the earth's gravitational field and modeling the complex nonconservative forces acting on T/P are highlighted. The T/P error budget is reviewed, and a prelaunch assessment of the predicted orbit determination accuracies is summarized.
    Keywords: ASTRODYNAMICS
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); 31; 2; p. 333-354.
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  • 5
    Publication Date: 2011-08-24
    Description: Monthly values of the J2 and J3 earth gravitational coefficients were estimated using LAGEOS satellite laser ranging data collected between 1980 and 1989. Monthly variations in gravitational coefficients caused by atmospheric mass redistribution were calculated using measurements of variations in surface atmospheric pressure. Results for correlation studies of the two time series are presented. The LAGEOS and atmospheric J2 time series agree well and it appears that variations in J2 can be attributed to the redistribution of atmospheric mass. Atmospheric and LAGEOS estimates for J3 show poorer agreement, J3 estimates appear to be very sensitive to unmodeled forces acting on the satellite. Results indicate that the LAGEOS data can be used to detect small variations in the gravitational field.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 7; p. 595-598.
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  • 6
    Publication Date: 2013-08-31
    Description: Understanding the origin and evolution of major photographic features on Mars, such as the hemispheric dichotomy and Tharsis rise, will require improved resolution of that planet's gravitational and topographic fields. The highest resolution gravity model for Mars published to date was derived from Doppler tracking data from the Mariner 9 and Viking 1 and 2 spacecraft, and is of 18th degree and order. That field has a maximum spatial resolution of approx. 600 km, which is comparable to that of the best topographic model. The resolution of previous gravity models was limited not by data density, but rather by the computational resources available at the time. Because this restriction is no longer a limitation, the Viking and Mariner data sets were reanalyzed and a gravitational field was derived complete to the 40th degree and order with a corresponding maximum spatial resolution of 300 km where the data permit.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 85-86
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  • 7
    Publication Date: 2013-08-31
    Description: If the geoid and the satellite position are known accurately, satellite altimetry can be used to determine the geostrophic velocity of the surface ocean currents. The purpose of this investigation is to simultaneously estimate the sea surface topography, zeta, the model for the gravity field, and the satellite orbit. Satellite tracking data from fourteen satellites were used; along with Seasat and Geosat altimeter data as well as surface gravity data for the solution. The estimated model of zeta compares well at long wavelengths with the hydrographic model of zeta. Covariance studies show that the geoid is separable from zeta up to degree 9, at which point geoid error becomes comparable to the signal of zeta.
    Keywords: GEOPHYSICS
    Type: Ohio State Univ., Progress in the Determination of the Earth's Gravity Field; p 142-145
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  • 8
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An analysis of Satellite Laser Ranging (SLR) data to the LAGEOS satellite has yielded improved estimates of the horizontal motion for a subset of 34 tracking sites within the global tracking network. The analysis, called SL8.3, utilized data acquired between 1980 January and 1993 June by the global network composed of 71 sites. The solution design provides for the simultaneous estimation of site positions and their velocities within a pre-defined kinematic frame. The solution is statistically rigorous and retains the full correlation information content. Least-squares estimates of relative poles of rotation, which are used to model the motion of one plate relative to another, were made based on the SLR estimated velocities for sites known to be well away from deformation zones. The resulting SLR-based relative rotation poles differ slightly from those of NUVEL-1, but in general, indicate that the magnitude of the SLR implied velocities is slower than those implied by NUVEL-1, consistent with the 4–5 per cent slowing in relative spherical rates noted in earlier comparisons. Spherical rates between sites in western North America support models of extension in the Basin and Range Province and the rotation of the Sierra Nevada microplate. An analysis of the spherical rates crossing the North Atlantic shows that SL8.3 estimated extension between North America-Eurasia sites is generally smaller than those implied by NUVEL-1: meanwhile SL8.3 rates between North America-Africa sites are in better agreement with NUVEL-1, although they are not so well determined. The maintenance and ongoing monitoring of global SLR site kinematics provides a well-defined global reference which will aid in combination global kinematic solutions where information from other technologies are merged (e.g. Very Long Baseline Interferometry and Global Positioning System) and in providing the context for densification studies of regional kinematics derived from terrestrial and Global Positioning System observations.
    Type of Medium: Electronic Resource
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  • 9
    Publication Date: 2013-12-20
    Type: paper
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  • 10
    Publication Date: 2006-06-13
    Description: The TOPEX/POSEIDON altimeter measurements will be the first global observations of the sea surface with accuracy sufficient to make quantitative determinations of the ocean's general circulation and its variations. These measurements are an important step to understanding global change in the ocean and its impact on the climate. Our investigation will focus on the examination of features in the sea surface elevation at the largest spatial and temporal scales. TOPEX/POSEIDON altimeter measurements will be used in conjunction with observations from past satellite-altimeter missions, such as NASA's GEOS-3 and Seasat, the U.S. Navy's Geosat and SALT, and the European Remote Sensing satellite in order to address the following issues: (1) Improve models of the marine geoid, especially at wavelengths needed to understand the basin-scale ocean dynamic topograpy. (2) Measure directly from the altimeter data the expression of the mean global ocean circulation in the sea surface at the largest scales through a simultaneous solution for gravity, orbital, and oceanographic parameters. (3) Examine the sea surface measurements for changes in global ocean mass or volume, interannual variations in the basin-scale ocean circulation, and annual changes in the heating and cooling of the upper ocean.
    Keywords: OCEANOGRAPHY
    Type: JPL, TOPEX(Poseidon Science Investigations Plan; p 76-81
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