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  • 1
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    Tests and comparisons of satellite-derived geoids with Skylab altimeter data (1976)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: During the Skylab 4 mission, the S-193 radar altimeter was operated nearly continuously for a revolution around the world on Jan. 31, 1974. This direct measurement to the sea surface has provided an independent basis for the evaluation of the precision of global geoids computed from satellite-derived earth gravity models. This paper presents comparisons between the Skylab data and several recent gravity models published by Goddard Space Flight Center, the Smithsonian Astrophysical Observatory, and the National Oceanic and Atmospheric Administration. The differences between the altimeter geoid and the satellite geoids were as large as 20 m, rms values ranging from 8 to 10 m. These differences also indicated a systematic long-wavelength variation (about 100 deg) not related to error in the Skylab orbits. Truncation of the models to degree and order 8 did not eliminate the long-wavelength variation, but in every case the rms agreement between the satellite geoids and the altimeter geoid was slightly improved. Orbits computed with the truncated models were found to be inferior to those computed with the complete models.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 81; July 10
    Format: text
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  • 2
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    Skylab earth resources experiment package /EREP/ - Sea surface topography experiment (1976)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: The S-193 Skylab radar altimeter was operated in a round-the-world pass on Jan. 31, 1974. The main purpose of this experiment was to test and 'measure' the variation of the sea surface topography using the Goddard Space Flight Center (GSFC) geoid model as a reference. This model is based upon 430,000 satellite and 25,000 ground gravity observations. Variations of the sea surface on the order of -40 to +60 m were observed along this pass. The 'computed' and 'measured' sea surfaces have an rms agreement on the order of 7 m. This is quite satisfactory, considering that this was the first time the sea surface has been observed directly over a distance of nearly 35,000 km and compared to a computed model. The Skylab orbit for this global pass was computed using the Goddard Earth Model (GEM 6) and S-band radar tracking data, resulting in an orbital height uncertainty of better than 5 m over one orbital period.
    Keywords: OCEANOGRAPHY
    Type: Journal of Spacecraft and Rockets; 13; Apr. 197
    Format: text
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  • 3
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    The Earth Observing System Microwave Limb Sounder (EOS MLS) on the Aura Satellite (2006)
    In:  Other Sources
    Details
    Publication Date: 2018-06-11
    Description: The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, HO2, H2O, O3, HCl, ClO, HOCl, BrO, HNO3, N2O, CO, HCN, CH3CN, volcanic SO2), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administration's Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82 S to 82 N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained.
    Keywords: Earth Resources and Remote Sensing
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); Volume 44; No. 5; 1075-1092
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  • 4
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    Gravity fields of the solar system (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The most frequently used formulations of the gravitational field are discussed and a standard set of models for the gravity fields of the earth, moon, sun, and other massive bodies in the solar system are defined. The formulas are presented in standard forms, some with instructions for conversion. A point-source or inverse-square model, which represents the external potential of a spherically symmetrical mass distribution by a mathematical point mass without physical dimensions, is considered. An oblate spheroid model is presented, accompanied by an introduction to zonal harmonics. This spheroid model is generalized and forms the basis for a number of the spherical harmonic models which were developed for the earth and moon. The triaxial ellipsoid model is also presented. These models and their application to space missions are discussed.
    Keywords: ASTROPHYSICS
    Type: NASA-SP-8117
    Format: application/pdf
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  • 5
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    Global detailed geoid computation and model analysis (1974)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Comparisons and analyses were carried out through the use of detailed gravimetric geoids which we have computed by combining models with a set of 26,000 1 deg x 1 deg mean free air gravity anomalies. The accuracy of the detailed gravimetric geoid computed using the most recent Goddard earth model (GEM-6) in conjunction with the set of 1 deg x 1 deg mean free air gravity anomalies is assessed at + or - 2 meters on the continents of North America, Europe, and Australia, 2 to 5 meters in the Northeast Pacific and North Atlantic areas, and 5 to 10 meters in other areas where surface gravity data are sparse. The R.M.S. differences between this detailed geoid and the detailed geoids computed using the other satellite gravity fields in conjuction with same set of surface data range from 3 to 7 meters.
    Keywords: GEOPHYSICS
    Type: NASA-TM-X-70709 , X-921-74-131
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  • 6
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    Detailed gravimetric geoid confirmation of short wavelength features of sea surface topography detected by the Skylab S-193 altimeter in the Atlantic Ocean (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A detailed gravimetric geoid was computed for the Northwest Atlantic Ocean and Caribbean Sea area in support of the calibration and evaluation of the GEOS-C altimeter. This geoid, computed on a 15 ft. x 15 ft. grid was based upon a combination of surface gravity data with the GSFC GEM-6 satellite derived gravity data. A comparison of this gravimetric geoid with 10 passes of SKYLAB altimeter data is presented. The agreement of the two data types is quite good with the differences generally less than 2 meters. Sea surface manifestations of numerous short wavelength (approximately 100 km) oceanographic features are now indicated in the gravimetric geoid and are also confirmed by the altimetry data.
    Keywords: OCEANOGRAPHY
    Type: NASA-TM-X-70905 , X-921-75-110
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  • 7
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    A detailed gravimetric geoid for the United States (1971)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Development of gravimetric geoid for US from satellite derived gravity data and local gravimetric measurements
    Keywords: GEOPHYSICS
    Type: NASA-TM-X-65691 , X-552-71-219
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  • 8
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    Detailed gravimetric geoid for the GEOS-C altimeter calibration area (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The GEOS-C spacecraft scheduled for launch in late 1974 will carry a radar altimeter for the purpose of measuring sea surface topography. In order to calibrate and evaluate the performance of the altimeter system, ground truth data are required. In this respect a detailed gravimetric geoid has been computed for the GEOS-C altimeter calibration area in the Atlantic Ocean off the East Coast of the U.S. This geoid is based upon a combination of mean free air surface gravity anomalies and the Goddard Space Flight Center GEM-6 satellite-derived spherical harmonic coefficients. Surface gravity anomalies have been used to provide information on the short wave length undulations of the geoid while the satellite-derived coefficients have provided information on the long wave length components. As part of these analyses, GSFC, SAO and OSU satellite-derived gravity models were used in the computations. Although geoid heights based upon the various satellite models differed by as much as 30 meters in the Southern Hemisphere, the differences in this Atlantic Ocean area were less than 4 meters.
    Keywords: GEOPHYSICS
    Type: International Symposium on Applications of Marine Geodesy; Jun 03, 1974 - Jun 05, 1974; Columbus, OH
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  • 9
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    Tests and comparisons of satellite derived geoids with Skylab altimeter data (1975)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The SKYLAB-193 radar altimeter was operated nearly continuously around the world on January 31, 1974. This direct measurement of the sea surface topography provided an independent basis for the evaluation of global geoids computed from satellite derived gravity models. The differences between the altimeter geoid and the satellite geoids were as large as 25 meters with rms values ranging from 8 to 10 meters. These differences also indicated a systematic long wavelength variation (approximately 100 deg) not related to error in the SKYLAB orbits. Truncation of the models to degree and order eight did not eliminate the long wavelength variation, but in every case the rms agreement between satellite and altimeter geoids was improved. Orbits computed with the truncated models were in contrast found to be inferior to those computed using the complete models.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-TM-X-70985 , X-921-75-176 , Ann. Meeting of the Am. Geophysical Union; Jun 16, 1975 - Jun 20, 1975; Washington, D. C.; United States
    Format: application/pdf
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  • 10
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    Detailed gravimetric geoid for the United States. (1972)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A detailed gravimetric geoid was computed for the United States using a combination of satellite-derived spherical harmonic coefficients and 1 by 1 deg mean gravity values from surface gravimetry. Comparisons of this geoid with astrogeodetic geoid data indicate that a precision of plus or minus 2 meters has been obtained. Translations only were used to convert the NAD astrogeodetic geoid heights to geocentric astrogeodetic heights. On the basis of the agreement between the geocentric astrogeodetic geoid heights and the gravimetric geoid heights, no evidence is found for rotation in the North American datum. The value of the zero-order undulation can vary by 10 to 20 meters, depending on which investigator's station positions are used to establish it.
    Keywords: GEOPHYSICS
    Type: International Symposium on The use of artificial satellites for geodesy; Apr 15, 1971 - Apr 17, 1971; Washington, DC
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