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  • 1
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    A new gravitational model for the Earth from satellite tracking data: GEM-T1 (1988)
    American Geophysical Union
    Details
    Publication Date: 1988-01-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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    PAPER CURRENT
  • 2
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    Surface elevation contours of greenland and Antarctic Ice Sheets (1983)
    American Geophysical Union
    Details
    Publication Date: 1983-01-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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    PAPER CURRENT
  • 3
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    Regional mean sea surfaces based on GEOS-3 and SEASAT altimeter data (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Altimetric sea surfaces provide a basis for detailed analyses of the earth's gravity, crustal structure, and the oceanic surface circulation. Long-term mean surfaces have been computed for the Bering Sea, Northwest Atlantic Ocean, and Gulf of Mexico based on a combination of the entire SEASAT (three-month) and GEOS-3 (3.5-year) altimeter data sets. The number of available passes ranged from 558 in the gulf to 1396 in the Atlantic. The large amount of data in these areas, coupled with the incresed constraint provided by the combination of data from two orbital inclinations, has permited the accurate removal of the effects of radial ephemeris error through crossing arc adjustments. The precision of these regional mean sea surfaces is approximately 15 cm, with horizontal resolutions approaching 25 km.
    Keywords: OCEANOGRAPHY
    Type: Marine Geodesy (ISSN 0149-0419); 8; 1-4,
    Format: text
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  • 4
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    The Seasat altimeter mean sea surface model (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Gridding techniques are used to combine an 18-day set of Seasat altimeter data and two precisely-computed Seasat ephemerides, in order to arrive at global contour maps of the mean sea surface topography. The altimeter data have an rms agreement of 111 cm with the SS3 mean sea surface computed by means of the PGS-S3 ephemerides, and of 70 cm with the SS4 mean surface derived from the PGS-S4 ephemerides. While comparisons with the GEM 10B 1 x 1 deg gravimetric geoid have yielded rms differences of 2.8 m, those with a global mean sea surface derived from GEOS 3 altimeter data show rms differences of 1.3 m and 1.1 m for the cases of the SS3 and SS4 surfaces, respectively. An SS4 mean sea surface topograph is featured among the study findings presented. Further improvements in the representation of mean sea surface topography are expected with the development of more accurate gravity models for orbit computation.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research; 87; Apr. 30
    Format: text
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  • 5
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    Analysis and retracking of continental ice sheet radar altimeter waveforms (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The Seasat-1 radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of Seasat-1 altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 88; Feb. 28
    Format: text
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  • 6
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    Surface elevation contours of Greenland and Antarctic ice sheets (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Preliminary results from Seasat radar altimetry over Antarctica north of 72 deg S and Greenland south of 72 deg N are presented. Surface elevations of the ice sheets, obtained from computer retracking of the radar altimeter waveforms, are contoured at 50-m intervals for Greenland and at 100-m intervals for Antarctica. Elevation differences at orbital crossover points are analyzed to obtain a precision of 1.9 m; this figure is partly determined by radial errors of approximately 1.0 m in orbital determination and partly by noise due to ice surface irregularities. Adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small, relatively flat region reduces the rms difference to 0.25 m, which is indicative of the optimum precision obtainable over the ice sheets. However, the precision degrades as the slope of the surface or amplitude of the undulations increases, yielding an overall precision of + or - 1.6 m.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 88; Feb. 28
    Format: text
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  • 7
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    Global mean sea surface computation using GEOS 3 altimeter data (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: A mean sea surface map has been determined for the global ocean areas between +62 deg and -62 deg latitude using GEOS 3 altimeter data. A grid of laser reference orbits computed using the GEM 10B gravity model has been used to orient the altimeter data in a center of mass coordinate system. The density of the altimeter tracks has enabled the computation of the sea surface heights above the reference ellipsoid on 1 deg x 1 deg grid in most of the oceanic areas. In the northwest Atlantic the dense coverage has enabled computations on a 0.25 deg x 0.25 deg grid. Comparisons of the global surface with an independently computed mean sea surface based upon SEASAT altimeter data indicate an rms agreement of a little over a meter. Comparisons of the regional solution in the northwest Atlantic with SEASAT profiles indicate a precision of a few decimeters in this surface. An analysis of the global crossover differences has indicated the possibility of a timing error in the altimeter data. Timing bias values of within 0.5 msec of 9.2 msec for 1975 and within 0.6 msec of 18.7 msec for 1976 have been recovered from an analysis of the altimeter data.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research; 87; Dec. 10
    Format: text
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  • 8
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    An improved model of the Earth's gravitational field: GEM-T1 (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Goddard Earth Model T1 (GEM-T1), which was developed from an analysis of direct satellite tracking observations, is the first in a new series of such models. GEM-T1 is complete to degree and order 36. It was developed using consistent reference parameters and extensive earth and ocean tidal models. It was simultaneously solved for gravitational and tidal terms, earth orientation parameters, and the orbital parameters of 580 individual satellite arcs. The solution used only satellite tracking data acquired on 17 different satellites and is predominantly based upon the precise laser data taken by third generation systems. In all, 800,000 observations were used. A major improvement in field accuracy was obtained. For marine geodetic applications, long wavelength geoidal modeling is twice as good as in earlier satellite-only GEM models. Orbit determination accuracy has also been substantially advanced over a wide range of satellites that have been tested.
    Keywords: GEOPHYSICS
    Type: NASA-TM-4019 , REPT-87B0451 , NAS 1.15:4019
    Format: application/pdf
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  • 9
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    Precision orbit determination software validation experiment (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: This paper presents the results of an experiment which was designed to ascertain the level of agreement between GEODYN and UTOPIA, two completely independent computer programs used for precision orbit determination, and to identify the sources which limit the agreement. For a limited set of models and a seven-day data set arc length, the altitude components of the ephemeris obtained by the two programs agree at the sub-centimeter level throughout the arc.
    Keywords: ASTRODYNAMICS
    Type: Journal of the Astronautical Sciences; 28; Oct
    Format: text
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  • 10
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    Satellite-satellite tracking for estimating geopotential coefficients. (1972)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Results of simulation studies of tracking of low-altitude satellites by a high-altitude satellite using range measurements between satellites. The high-altitude satellite is tracked by ground-based range trackers. The low-altitude satellites are tracked only by the high-altitude satellites. Orbital elements for one tracking satellite and four tracked low-altitude satellites are estimated, along with a set of low degree and order geopotential coefficients. Estimation of geopotential coefficients of low degree and order is shown to be considerably more accurate using satellite-to-satellite tracking than using ground-based tracking. The use of data from one high-altitude satellite tracking four low-altitude satellites in a range of inclinations is shown to produce geopotential coefficient estimates with smaller errors than presently exist in individual coefficients or would be obtained using ground-based tracking of the same satellites.
    Keywords: GEOPHYSICS
    Type: International Symposium on The use of artificial satellites for geodesy; Apr 15, 1971 - Apr 17, 1971; Washington, DC
    Format: text
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