Preliminary Comparison of Two-Way Satellite Time and Frequency Transfer and GPS Common-View Time Transfer During the Intelsat Field Trial

Preliminary Comparison of Two-Way Satellite Time and Frequency Transfer and GPS Common-View Time Transfer During the Intelsat Field Trial
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Total Pages: 13
Release: 1995
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For a decade and a half GPS Common-View time transfer has greatly served the needs of primary timing laboratories for regular intercomparisons of remote atomic clocks. However, GPS as a one-way technique has natural limits and may not meet all challenges of the comparison of the coming new generations of atomic clocks. Two-Way Satellite Time and Frequency Transfer (TWSTFT) is a promising technique which may successfully complement GPS. For two years, regular TWSTFT's have been performed between eight laboratories situated in both Europe and North America, using INTELSAT satellites. This has enabled an extensive direct comparison to be made between these two high performance time-transfer methods. The performance of the TWSTFT and GPS Common-View methods are compared over a number of time-transfer links. These links use a variety of time-transfer hardware and atomic clocks and have baselines of substantially different lengths. The relative merits of the two time-transfer systems are discussed.

Satellite Two-Way Time Transfer: Fundamentals and Recent Progress

Satellite Two-Way Time Transfer: Fundamentals and Recent Progress
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Total Pages: 14
Release: 1989
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Experiments in precise clock comparisons using the two-way time transfer technique via satellite began in 1962. Experience gained from a variety of experiments since that time steadily improved the precision and accuracy of such comparisons. Recent growth in the fixed satellite service, or FSS, has created new opportunities at moderate costs for high-accuracy time transfers using geostationary satellites. We discuss fundamental aspects of two-way timing and show an implementation of a satellite two-way time transfer system which has been used for two-years between USNO, Washington, D.C. and NIST, Boulder, CO. The raw data collection procedure will be discussed. We also outline the rationale for the choice of satellite uplink/downlink frequencies, signal structure, and reduction of data. Short-term noise in the time transfer limits the precision to about 300 ps in a 300 s average. Uncertainty in accuracy is due to uncertainty in the non-reciprocity of the two-way signal path. Accuracy limits due to the atmosphere, earth-satellite rotating system (Sagnac effect), and the equipment are discussed. The goal is to achieve an accuracy level of 1 ns after a suitable calibration of earth-station (differential) equipment delays.

Two-Way Satellite Time Transfer: Overview and Recent Developments

Two-Way Satellite Time Transfer: Overview and Recent Developments
Author: Gerrit De Jong
Publisher:
Total Pages: 17
Release: 1993
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ISBN:

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The experiment using small earth stations for transatlantic two-way satellite time transfer between the USA (USNO, Washington, DC) and Germany (DFVLR), Oberfaffenhofen) has had its 10th anniversary this year. Pseudo Random Noise coded time signals were phase modulated and demodulated at each stations using a modem developed by Professor Hartl and his staff. Recently, during the last two weeks of August 1993, six European time laboratories have used the INTELSAT 307E satellite for line-up tests and preliminary time transfer sessions using the same type of MITREX modem. This opportunity was given by INTELSAT, thanks to the help of Dr. Veenstra. The need for a uniform format for the exchange of data was felt heavily after these sessions. This problem was foreseen and addressed in international working parties. During April 1993 in Task Group 712 of the ITU Radiocommunication Sector (formerly CCIR), a very intense discussion has taken place about what procedures should be recommended for TWS TFT and what items the header and data lines of the resulting data fields should contain. A difficulty is that two different methods of calibration of the earth station delays exist which results different sets of delay data to be exchanged. Further study and discussions are necessary. Also, a meeting of the CCDS Working Group on TWSTFT addressed this in October 1993. The outcome of the discussions and the prospect for future developments will be presented.

Two-Way Time Transfer Through 2.4 GBit/s Optical SDH System

Two-Way Time Transfer Through 2.4 GBit/s Optical SDH System
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Total Pages: 9
Release: 1997
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An experiment to transfer time and frequency over 2.488 Gbits SDH (Synchronous Digital Hierarchy) systems using 175-km commercial optical fibers has been set up by CRL and NTT. We confirm that the frequency stability of the time comparison data is 10(12)/square root of tau at averaging times above 10 s. This equals that of the Cs frequency standard (H5071A) used in this experiment. The time comparison resolution is of the order of 10( -11) s (square root of time variance). The long-term stability of this system is expected to be better than 1 ns. The time comparison results of this experiment agree well the GPS common-view results.