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  • Other Sources  (27)
  • 1
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    The geoid: Definition and determination (1975)
    In:  CASI
    Details
    Publication Date: 2006-01-12
    Description: The principles and problems relative to the determination of the geoid are outlined. Factors discussed include: gravity data requirements for a precise geoid; mean sea level; and satellite altimetry. It is indicated that geoid undulations can be determined on a global basis to plus or minus 3 m. Application of geoid information to oceanography and the determination of sea surface topography considered.
    Keywords: GEOPHYSICS
    Type: Proc. of the Geodesy/Solid Earth and Ocean; 69-77
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  • 2
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    Terrestrial gravity data and comparisons with satellite data (1987)
    In:  Other Sources
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    Publication Date: 2013-08-29
    Description: Figures that demonstrate the state of terrestrial gravity coverage, and comparisons between satellite derived gravity field and terrestrial gravity data are presented. It is shown that only a few areas of the world have information accurate enough for geodesy and geophysics. A gravity field mapping space mission is recommended.
    Keywords: GEOPHYSICS
    Type: ESA, Proceedings of an ESA-NASA Workshop on a Joint Solid Earth Program; p 27-30
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  • 3
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    Accuracy of the determination of mean anomalies and mean geoid undulations from a satellite gravity field mapping mission (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Improved knowledge of the Earth's gravity field was obtained from new and improved satellite measurements such as satellite to satellite tracking and gradiometry. This improvement was examined by estimating the accuracy of the determination of mean anomalies and mean undulations in various size blocks based on an assumed mission. In this report the accuracy is considered through a commission error due to measurement noise propagation and a truncation error due to unobservable higher degree terms in the geopotential. To do this the spectrum of the measurement was related to the spectrum of the disturbing potential of the Earth's gravity field. Equations were derived for a low-low (radial or horizontal separation) mission and a gradiometer mission. For a low-low mission of six month's duration, at an altitude of 160 km, with a data noise of plus or minus 1 micrometers sec for a four second integration time, we would expect to determine 1 deg x 1 deg mean anomalies to an accuracy of plus or minus 2.3 mgals and 1 deg x 1 deg mean geoid undulations to plus or minus 4.3 cm. A very fast Fortran program is available to study various mission configurations and block sizes.
    Keywords: GEOPHYSICS
    Type: REPT-307
    Format: application/pdf
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  • 4
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    A comparison of altimeter and gravimetric geoids in the Tonga Trench and Indian Ocean areas (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Geoids computed from GEOS-3 altimeter data are compared with gravimetric geoids computed by various techniques for 30 x 30 deg areas in the Tonga Trench and the Indian Ocean. The gravimetric geoids were calculated using the standard Stokes integration with the Molodenskii truncation procedure, the modified Stokes integration suggested by Ostach (1970) and Meissl (1971) with modified Molodenskii truncation functions, and three sets of potential coefficients including one complete to degree 180. It is found that the modified Stokes procedure with a cap size of 10 deg provides better results when used with a combined altimeter terrestrial anomaly field data set. Excellent agreement at the plus or minus 1 m level is obtained between the altimeter and gravimetric geoid using the combined data set, with the modified Stokes procedure having a greater accuracy. Coefficients derived from the 180 x 180 solution are found to be of an accuracy comparable to that of the modified Stokes method, however to require six times less computational effort.
    Keywords: GEOPHYSICS
    Type: Bulletin Geodesique; 54; 2, 19; 1980
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  • 5
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    Ellipsoidal corrections for geoid undulation computations using gravity anomalies in a cap (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Ellipsoidal correction terms have been derived for geoid undulation computations when the Stokes equation using gravity anomalies in a cap is combined with potential coefficient information. The correction terms are long wavelength and depend on the cap size in which its gravity anomalies are given. Using the regular Stokes equation, the maximum correction for a cap size of 20 deg is -33 cm, which reduces to -27 cm when the Stokes function is modified by subtracting the value of the Stokes function at the cap radius. Ellipsoidal correction terms were also derived for the well-known Marsh/Chang geoids. When no gravity was used, the correction could reach 101 cm, while for a cap size of 20 deg the maximum correction was -45 cm. Global correction maps are given for a number of different cases. For work requiring accurate geoid computations these correction terms should be applied.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 86; Nov. 10
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  • 6
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    Ellipsoidal corrections for geoid undulation computations (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The computation of accurate geoid undulations is usually done combining potential coefficient information and terrestrial gravity data in a cap surrounding the computation point. In doing this a spherical approximation is made that can cause the errors that were investigated. The equations dealing with ellipsoidal corrections developed by Lelgemann and by Moritz were used to develop a computational procedure considering the ellipsoid as a reference surface. Terms in the resulting expression for the geoid undulation are identified as ellipsoidal correction terms. These equations were developed for the case where the Stokes function is used, and for the case where the modified Stokes function is used. For a cap of 20 deg the correction can reach -33 cm.
    Keywords: GEOPHYSICS
    Type: NASA-CR-164440 , REPT-308
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  • 7
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    GEOS 3 data processing for the recovery of geoid undulations and gravity anomalies (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The paper discusses the analysis of GEOS 3 altimeter data for the determination of geoid heights and point and mean gravity anomalies. Methods are presented for determining the mean anomalies and mean undulations from the GEOS 3 altimeter data available by the end of September 1977 without having a complete set of precise orbits. The editing of the data is extensive to remove questionable data, although no filtering of the data is carried out. An adjustment process is carried out to eliminate orbit error and altimeter bias. Representative point anomaly values are computed to investigate anomaly behavior across the Bonin Trench and over the Patton seamounts.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 84; July 30
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  • 8
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    The determination of gravity anomalies from geoid heights using the inverse Stokes' formula, Fourier transforms, and least squares collocation (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A numerical method for the determination of gravity anomalies from geoid heights is described using the inverse Stokes formula. This discrete form of the inverse Stokes formula applies a numerical integration over the azimuth and an integration over a cubic interpolatory spline function which approximates the step function obtained from the numerical integration. The main disadvantage of the procedure is the lack of a reliable error measure. The method was applied on geoid heights derived from GEOS-3 altimeter measurements in the calibration area of the GEOS-3 satellite.
    Keywords: GEOPHYSICS
    Type: NASA-CR-141442
    Format: application/pdf
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  • 9
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    Mean gravity anomalies and sea surface heights derived from GEOS-3 altimeter data (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Approximately 2000 GEOS-3 altimeter arcs were analyzed to improve knowledge of the geoid and gravity field. An adjustment procedure was used to fit the sea surface heights (geoid undulations) in an adjustment process that incorporated cross-over constraints. The error model used for the fit was a one or two parameter model which was designed to remove altimeter bias and orbit error. The undulations on the adjusted arcs were used to produce geoid maps in 20 regions. The adjusted data was used to derive 301 5 degree equal area anomalies and 9995 1 x 1 degree anomalies in areas where the altimeter data was most dense, using least squares collocation techniques. Also emphasized was the ability of the altimeter data to imply rapid anomaly changes of up to 240 mgals in adjacent 1 x 1 degree blocks.
    Keywords: GEOPHYSICS
    Type: NASA-CR-156843
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  • 10
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    Procedures and results related to the direct determination of gravity anomalies from satellite and terrestrial gravity data (1974)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The equations needed for the incorporation of gravity anomalies as unknown parameters in an orbit determination program are described. These equations were implemented in the Geodyn computer program which was used to process optical satellite observations. The arc dependent parameter unknowns, 184 unknown 15 deg and coordinates of 7 tracking stations were considered. Up to 39 arcs (5 to 7 days) involving 10 different satellites, were processed. An anomaly solution from the satellite data and a combination solution with 15 deg terrestrial anomalies were made. The limited data samples indicate that the method works. The 15 deg anomalies from various solutions and the potential coefficients implied by the different solutions are reported.
    Keywords: GEOPHYSICS
    Type: NASA-CR-140064 , REPT-211
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