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  • GEOPHYSICS  (21)
  • EARTH RESOURCES AND REMOTE SENSING  (3)
  • SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING  (3)
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  • 1
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    CASA Uno GPS orbit and baseline experiments (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: CASA Uno data from sites distributed in longitude from Australia to Europe have been used to determine orbits of the GPS satellites. The characteristics of the orbits determined from double difference phase have been evaluated through comparisons of two-week solutions with one-week solutions and by comparisons of predicted and estimated orbits. Evidence of unmodeled effects is demonstrated, particularly associated with the orbit planes that experience solar eclipse. The orbit accuracy has been assessed through the repeatability of unconstrained estimated baseline vectors ranging from 245 km to 5400 km. Both the baseline repeatability and the comparison with independent space geodetic methods give results at the level of 1-2 parts in 100,000,000. In addition, the Mojave/Owens Valley (245 km) and Kokee Park/Ft. Davis (5409 km) estimates agree with VLBI and SLR to better than 1 part in 100,000,000.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 643-646
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  • 2
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    UT/CSR analysis of earth rotation from Lageos SLR data (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The 1983-1984 data collected by NASA and stations participating in the Crustal Dynamics Project from satellite laser ranging (SLR) systems are used to generate solutions for the earth polar motion. Solutions obtained using the MERIT Lageos standard data set are compared to operational results based on quick-look data and generated in near real-time, and the capability of Lageos SLR for the determination of earth orientation parameters (EOP) with high temporal resolution is investigated. Finally, the sensitivity of the MERIT campaign results to the number of tracking stations and to changes in the MERIT standard model is evaluated. It is concluded that the departures from the IAU/IUGG MERIT standards do not significantly change the solution and that solutions accurate at the 2 milliarcsec level can be maintained with a network of fewer than 10 appropriately selected stations.
    Keywords: GEOPHYSICS
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  • 3
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    Simulation and analysis of a geopotential research mission (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A computer simulation was performed for a Geopotential Research Mission (GRM) to enable study of the gravitational sensitivity of the range/rate measurement between two satellites and to provide a set of simulated measurements to assist in the evaluation of techniques developed for the determination of the gravity field. The simulation, identified as SGRM 8511, was conducted with two satellites in near circular, frozen orbits at 160 km altitude and separated by 300 km. High precision numerical integration of the polar orbits was used with a gravitational field complete to degree and order 180 coefficients and to degree 300 in orders 0 to 10. The set of simulated data for a mission duration of about 32 days was generated on a Cray X-MP computer. The characteristics of the simulation and the nature of the results are described.
    Keywords: GEOPHYSICS
    Type: NASA-CR-176789 , NAS 1.26:176789
    Format: application/pdf
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  • 4
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    Simulation and analysis of a geopotential research mission (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Computer simulations were performed for a Geopotential Research Mission (GRM) to enable the study of the gravitational sensitivity of the range rate measurements between the two satellites and to provide a set of simulated measurements to assist in the evaluation of techniques developed for the determination of the gravity field. The simulations were conducted with two satellites in near circular, frozen orbits at 160 km altitudes separated by 300 km. High precision numerical integration of the polar orbits were used with a gravitational field complete to degree and order 360. The set of simulated data for a mission duration of about 32 days was generated on a Cray X-MP computer. The results presented cover the most recent simulation, S8703, and includes a summary of the numerical integration of the simulated trajectories, a summary of the requirements to compute nominal reference trajectories to meet the initial orbit determination requirements for the recovery of the geopotential, an analysis of the nature of the one way integrated Doppler measurements associated with the simulation, and a discussion of the data set to be made available.
    Keywords: GEOPHYSICS
    Type: NASA-CR-180696 , NAS 1.26:180696
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  • 5
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    Observed temporal variations in the Earth's gravity field from 16-year Starlette orbit analysis (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Satellite laser ranging data to Starlette, collected during the period from 1975 to 1990, are analyzed to determine yearly values of the second degree annual (Sa) and semiannual (Ssa) tides, simultaneously with average values of other low degree and order tide parameters. The yearly fluctuations in the values for Sa and Ssa are associated with changes in the Earth's second degree zonal harmonic caused by meteorological excitation. The Starlette-determined mean values for the amplitude of the annual and semiannual variations in J2 are 32.3 x 10 exp -11 and 19.5 x 10 exp -11, respectively; while the rms about the mean values are 4.1 x 10 exp -11 and 6.3 x 10 exp -11, respectively. The annual delta-J2 is in good agreement with the value obtained from the combined effects of air mass redistribution without the oceanic inverted-barometer effects (non-IB) and hydrological change. Approximately 90 percent of the observed annual variation from Starlette is attributed to the meteorological mass redistribution occurring on the Earth's surface.
    Keywords: GEOPHYSICS
    Type: In: From Mars to Greenland: Charting gravity with space and airborne instruments - Fields, tides, methods, results (A93-55951 24-46); p. 83-91.
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  • 6
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    Secular variation of earth's gravitational harmonic J2 coefficient from Lageos and nontidal acceleration of earth rotation (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Analysis of 5.5 years of Lageos satellite range data reveal significant residual nodal signatures: an acceleration and annual and semiannual periods. These signatures primarily reflect variations in the zonal gravitational harmonic J2 coefficient and hence the polar moment of inertia. The implied decrease of J2 = -3 x 10 to the -11th/yr is consistent with both historical observations of the nontidal acceleration of the earth's rotation and models of viscous rebound of the solid earth from the decrease in load due to the last deglaciation.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 303; June 30
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  • 7
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    Comparison of earth rotation as inferred from radio interferometric, laser ranging and astrometric observations (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Results of measurements of the earth's rotation vector for a 400-day period from late September 1980 to December 1981, for which date from VLBI, satellite laser ranging (SLR), and lunar laser ranging (LLR) were available, are compared. The acquisition of the data and their evaluation are described. VLBI, SLR, and classical astrometric determinations of the X-parameter required to describe the location of the rotation pole on the earth's surface are shown, and VLBI, LLR, and classical astrometric determinations of the angle of rotation about this pole (UT1) are presented. The results indicate that VLBI and SLR, at their present stages of development, yield standard errors under 20 cm in the determinations of X, about twofold smaller than obtained from classical measurements, and that VLBI and LLR yield determination of UT1 with standard errors less than 40 cm, somewhat smaller than that of the corresponding determinations from classical observations. Methods for improving these types of intercomparisons are suggested.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 302; April 7
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  • 8
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    Relative performance of algorithms for autonomous satellite orbit determination (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Limited word size in contemporary microprocessors causes numerical problems in autonomous satellite navigation applications. Numerical error introduced in navigation computations performed on small wordlength machines can cause divergence of sequential estimation algorithms. To insure filter reliability, square root algorithms have been adopted in many applications. The optimal navigation algorithm requires a careful match of the estimation algorithm, dynamic model, and numerical integrator. In this investigation, the relationship of several square root filters and numerical integration methods is evaluated to determine their relative performance for satellite navigation applications. The numerical simulations are conducted using the Phase I GPS constellation to determine the orbit of a LANDSAT-D type satellite. The primary comparison is based on computation time and relative estimation accuracy.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: Journal of the Astronautical Sciences; 29; Jan
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  • 9
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    Evaluation of the Seasat altimeter time tag bias (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Two methods were used in analyses of Seasat altimeter data, aimed at the corroboration of an inherent altimeter microprocessor delay compensation value of -79.4 msec, which benefited from a global data distribution in the oceanic areas: (1) the crossover method, using altimeter data differenced at points where the Seasat ground track intersected with itself, and (2) the direct use of the altimeter data. Because the former method is independent of errors in the geoid model, it is considered the more reliable. For all crossover method results, the adopted value of -79.4 msec is within the bounds of the standard deviation associated with the estimates, of which -78.1 + or - 2.0 msec is considered the best representative.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Journal of Geophysical Research; 87; Apr. 30
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  • 10
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    Autonomous satellite navigation methods using the Global Positioning Satellite System (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: This investigation considers the problem of autonomous satellite navigation using the NAVSTAR Global Positioning System (GPS). The major topics covered include the design, implementation, and validation of onboard navigation filter algorithms by means of computer simulations. The primary errors that the navigation filter design must minimize are computational effects and modeling inaccuracies due to limited capability of the onboard computer. The minimization of the effect of these errors is attained by applying the sequential extended Kalman filter using a factored covariance implementation with Q-matrix or dynamical model compensations. Peformance evaluation of the navigation filter design is carried out using both the CDC Cyber 170/750 computer and the PDP-11/60 computer. The results are obtained assuming the Phase I GPS constellation, consisting of six satellites, and a Landsat-D type spacecraft as the model for the user satellite orbit.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
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