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  • 1
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    Deriving a geocentric reference frame for satellite positioning and navigation (1988)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: With the advent of Earth-orbiting geodetic satellites, nongeocentric datums or reference frames have become things of the past. Accurate geocentric three-dimensional positioning is now possible and is of great importance for various geodetic and oceanographic applications. While relative positioning accuracy of a few centimeters has become a reality using very long baseline interferometry (VLBI), the uncertainty in the offset of the adopted coordinate system origin from the geocenter is still believed to be on the order of 1 meter. Satellite laser ranging (SLR), however, is capable of determining this offset to better than 10 cm, but this is possible only after years of measurements. Global Positioning System (GPS) measurements provide a powerful tool for an accurate determination of this origin offset. Two strategies are discussed. The first strategy utilizes the precise relative positions that were predetermined by VLBI to fix the frame orientation and the absolute scaling, while the offset from the geocenter is determined from GPS measurements. Three different cases are presented under this strategy. The reference frame thus adopted will be consistent with the VLBI coordinate system. The second strategy establishes a reference frame by holding only the longitude of one of the tracking sites fixed. The absolute scaling is determined by the adopted gravitational constant (GM) of the Earth; and the latitude is inferred from the time signature of the Earth rotation in the GPS measurements. The coordinate system thus defined will be a geocentric Earth-fixed coordinate system.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: The Telecommunications and Data Acquisition Report; p 1-13
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Use of global positioning system measurements to determine geocentric coordinates and variations in Earth orientation (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Geocentric tracking station coordinates and short-period Earth-orientation variations can be measured with Global Positioning System (GPS) measurements. Unless calibrated, geocentric coordinate errors and changes in Earth orientation can lead to significant deep-space tracking errors. Ground-based GPS estimates of daily and subdaily changes in Earth orientation presently show centimeter-level precision. Comparison between GPS-estimated Earth-rotation variations, which are the differences between Universal Time 1 and Universal Coordinated Time (UT1-UTC), and those calculated from ocean tide models suggests that observed subdaily variations in Earth rotation are dominated by oceanic tidal effects. Preliminary GPS estimates for the geocenter location (from a 3-week experiment) agree with independent satellite laser-ranging estimates to better than 10 cm. Covariance analysis predicts that temporal resolution of GPS estimates for Earth orientation and geocenter improves significantly when data collected from low Earth-orbiting satellites as well as from ground sites are combined. The low Earth GPS tracking data enhance the accuracy and resolution for measuring high-frequency global geodynamical signals over time scales of less than 1 day.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: The Telecommunications and Data Acquisition Report; p 21-33
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Doubly-Differenced Measurements for Orbit Determinations (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Report describes accurate method of determining orbit of low-altitude Earth satellite. Method insensitive to clock errors and provides continuous determination of orbit. Report examines effects of employing fewer ground stations, applying different solution strategies, and introducing fictitious thrust parameters to minimize geopotential modeling error - one of major error sources.
    Keywords: ELECTRONIC SYSTEMS
    Type: NPO-16111 , NASA Tech Briefs (ISSN 0145-319X); 9; 1; P. 71
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Estimating Effects of Flicker Noise in Clock Signals (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Two techniques mathematically estimate effects of flicker noise in pulse trains used as clock signals.
    Keywords: ELECTRONIC SYSTEMS
    Type: NPO-15525 , NASA Tech Briefs (ISSN 0145-319X); 8; 2; P. 188
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    GPS-based certification for the microwave landing system (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: An MLS (microwave landing system) certification system based on the Global Positioning System (GPS) is described. To determine the position history of the flight inspection aircraft during runway approach, signals from the GPS satellites, together with on-board radar altimetry, are used. It is shown that the aircraft position relative to a fixed point on the runway at threshold can be determined to about 30 cm vertically and 1 m horizontally. A requirement of the system is that the GPS receivers be placed on each flight inspection aircraft and at selected ground sites. The effects of different error sources on the determination of aircraft instantaneous position and its dynamics are analyzed.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: PLANS ''84 - Position Location and Navigation Symposium; Nov 26, 1984 - Nov 29, 1984; San Diego, CA
    Format: text
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    NASA TECHNICAL REPORTS
  • 6
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    Global Timing With Low- and High-Orbiting Satellites (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Report summarizes method for synchronizing clocks at intercontinental distances employing satellites of Global Positioning System (GPS) in high Earth orbit and transit satellite in orbit at relatively low altitude of about 1,300 km. When fully implemented, method expected to supply precise time measurements for world-wide communication and navigation.
    Keywords: ELECTRONIC SYSTEMS
    Type: NPO-16407 , NASA Tech Briefs (ISSN 0145-319X); 10; 5; P. 57
    Format: text
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    NASA TECHNICAL REPORTS
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