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  • Other Sources  (8)
  • 1
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    An ensemble pulsar time (1993)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Millisecond pulsars are galactic objects that exhibit a very stable spinning period. Several tens of these celestial clocks have now been discovered, which opens the possibility that an average time scale may be deduced through a long-term stability algorithm. Such an ensemble average makes it possible to reduce the level of the instabilities originating from the pulsars or from other sources of noise, which are unknown but independent. The basis for such an algorithm is presented and applied to real pulsar data. It is shown that pulsar time could shortly become more stable than the present atomic time, for averaging times of a few years. Pulsar time can also be used as a flywheel to maintain the accuracy of atomic time in case of temporary failure of the primary standards, or to transfer the improved accuracy of future standards back to the present.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center, The 24th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 73-86
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  • 2
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    Precise GPS ephemerides from DMA and NGS tested by time transfer (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: It was shown that the use of the Defense Mapping Agency's (DMA) precise ephemerides brings a significant improvement to the accuracy of GPS time transfer. At present a new set of precise ephemerides produced by the National Geodetic Survey (NGS) has been made available to the timing community. This study demonstrates that both types of precise ephemerides improve long-distance GPS time transfer and remove the effects of Selective Availability (SA) degradation of broadcast ephemerides. The issue of overcoming SA is also discussed in terms of the routine availability of precise ephemerides.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: NASA. Goddard Space Flight Center, Proceedings of the 23rd Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 59-70
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  • 3
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    The need for GPS standardization (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: A desirable and necessary step for improvement of the accuracy of Global Positioning System (GPS) time comparisons is the establishment of common GPS standards. For this reason, the CCDS proposed the creation of a special group of experts with the objective of recommending procedures and models for operational time transfer by GPS common-view method. Since the announcement of the implementation of Selective Availability at the end of last spring, action has become much more urgent and this CCDS Group on GPS Time Transfer Standards has now been set up. It operates under the auspices of the permanent CCDS Working Group on TAI and works in close cooperation with the Sub-Committee on Time of the Civil GPS Service Interface Committee (CGSIC). Taking as an example the implementation of SA during the first week of July 1991, this paper illustrates the need to develop urgently at least two standardized procedures in GPS receiver software: monitoring GPS tracks with a common time scale and retaining broadcast ephemeris parameters throughout the duration of a track. Other matters requiring action are the adoption of common models for atmospheric delay, a common approach to hardware design and agreement about short-term data processing. Several examples of such deficiencies in standardization are presented.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: NASA. Goddard Space Flight Center, Proceedings of the 23rd Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 1-13
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  • 4
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    Accuracy of GPS time transfer verified by closure around the world (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: The precision of time transfer over intercontinental distances by the Global Positioning System common-view method, using measurements of ionospheric delays, precise ephemerides provided by the Defense Mapping Agency (DMA) and a consistent set of antenna coordinates, reaches 3 to 4 ns for a single 13-minute measurement, and decreases to 2 ns when averaging several measurements over the period of one day. It is thought that even this level of precision can be bettered by improving the ionospheric measurements, the ephemerides of satellites, and the antenna coordinates. In the same conditions, an estimation of the accuracy is attained by using three intercontinental links encircling the Earth to establish a closure condition; the three independent links should add to zero. We have computed such a closure condition over a period of 13 months using data recorded at the Paris Observatory, at the Communications Research Laboratory in Tokyo, and at the National Institute for Standards and Technology in Boulder, Colorado. The closure condition is verified to within a few nanoseconds, but a bias, varying with time, can be detected.
    Keywords: PHYSICS (GENERAL)
    Type: NASA. Goddard Space Flight Center, Proceedings of the 23rd Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 331-33
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  • 5
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    Comparison of GLONASS and GPS time transfers between two west European time laboratories and VNIIFTRI (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: The University of Leeds built a Global Positioning System/Global Orbiting Navigation Satellite System (GPS/GLONASS) receiver about five years ago and since then has provided continuous information about GLONASS time and its comparison with GPS time. For the last two years, VNIIFTRI (All Union Institute for Physical, Technical and Radiotechnical Measurements) and some other Soviet time laboratories have used Soviet built GLONASS navigation receivers for time comparisons. Since June 1991, VNIIFTIR has been operating a GPS time receiver on loan from the BIPM (Bureau International des Poids et Mesures). This offered, for the first time, an opportunity for direct comparison of time transfers using GPS and GLONASS. This experiment shows that even with relatively imprecise data recording and processing, in terms of time metrology, GLONASS can provide continental time transfer at a level of several tens of nanoseconds.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: NASA. Goddard Space Flight Center, Proceedings of the 23rd Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 341-35
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  • 6
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    A comparison of GPS broadcast and DMA precise ephemerides (1994)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: We compare the broadcast ephemerides from Global Positioning Satellites (GPS) to the postprocessed ephemerides from the Defense Mapping Agency (DMA). We find significant energy in the spectrum of the residuals at 1 cycle/day and higher multiples. We estimate the time variance of the residuals and show that the short term residuals, from 15 min, exhibit power law processes with greater low frequency perturbations than white phase modulation. We discuss the significance of these results for the performance of the GPS Kalman filter which estimates the broadcast orbits.
    Keywords: AIRCRAFT COMMUNICATIONS AND NAVIGATION
    Type: NASA. Goddard Space Flight Center, The 25th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 293-306
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  • 7
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    The use of precise ephemerides, ionospheric data, and corrected antenna coordinates in a long-distance GPS time transfer (1990)
    In:  Other Sources
    Details
    Publication Date: 2013-08-29
    Description: Over intercontinental distances, the accuracy of The Global Positioning System (GPS) time transfers ranges from 10 to 20 ns. The principal error sources are the broadcast ionospheric model, the broadcast ephemerides and the local antenna coordinates. For the first time, the three major error sources for GPS time transfer can be reduced simultaneously for a particular time link. Ionospheric measurement systems of the National Institute of Standards and Technology (NIST) type are now operating on a regular basis at the National Institute of Standards and Technology in Boulder and at the Paris Observatory in Paris. Broadcast ephemerides are currently recorded for time-transfer tracks between these sites, this being necessary for using precise ephemerides. At last, corrected local GPS antenna coordinates are now introduced in GPS receivers at both sites. Shown here is the improvement in precision for this long-distance time comparison resulting from the reduction of these three error sources.
    Keywords: PHYSICS (GENERAL)
    Type: JPL, The 22nd Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 547-558
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  • 8
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    Limits to the Stability of Pulsar Time (1996)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The regularity of the rotation rate of millisecond pulsars is the underlying hypothesis for using these neutron stars as 'celestial clocks'. Given their remote location in our galaxy and to our lack of precise knowledge on the galactic environment, a number of phenomena effect the apparent rotation rate observed on Earth. This paper reviews these phenomena and estimates the order of magnitude of their effect. It concludes that an ensemble pulsar time based on a number of selected millisecond pulsars should have a fractional frequency stability close to 2 x 10(sup -15) for an averaging time of a few years.
    Keywords: Physics (General)
    Type: 27th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; 387-396; NASA-CP-3334
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