The Global Positioning System,GPS, is widely used for time comparisons between distant laboratories. Over distances of the order of 1000km or less, the system has the capability of 1 to 2ns accuracy. However this requires a relative positioning with errors lower than 30cm. We show that this positioning can be derived from theGPS time comparisons themselves. An example for European laboratories is given.

Improvement of the GPS time comparisons by simultaneous relative positioning of the receiver antennas

The problems with the use of the Global Positioning System (GPS) for time transfer are discussed. The Bureau International des Poids et Mesures (International Bureau of Weights and Mesures, BIPM) has developed a method of differential positioning using the data of time comparisons themselves. The consistency of the coordinates is within 30 cm for distances up to 1000 km. The method was applied to the European laboratories one year ago (1988). The consistency of time comparison improved from about 10 ns to about 2 ns. The principles of this technique and the results of its application to the North American time laboratories are presented. Work on differential positioning by geodetic double-frequency receivers between the US Naval Observatory and the Maryland VLBI (very long baseline interferometry) point are reported. >

Positioning of GPS antennas in time-keeping laboratories of North America

: An overview of past accomplishments is presented that shows the development of satellite time transfer techniques and capabilities that are used today. The development is traced from the concepts and early demonstrations using a single satellite to the global coverage now provided by a constellation of satellites. Predictions of future technology are compared with what has been accomplished over the last 25 years. Co-operative experiments performed jointly by colleagues of the PTTI Meetings were key to the development. These experiments demonstrated the potential of time transfer by satellites having global coverage. Some of the results are reviewed and compared to current capabilities.

Satellite Time Transfer Past and Present

: Historically, precise time has been transferred between two sites by means of a method using portable atomic clocks. The method entails the carrying of an active frequency standard and its associated clock from site A to site B. Personnel from the United States Naval Observatory (USNO), Bendix Field Engineering Corporation (BFEC), Naval Research Laboratory (NRL), and others have made portable clock trips by airplane and surface vehicles for the past 15 years. The accuracy obtained using this method for the transfer of time ranges from a few nanoseconds (on a short surface trip) to hundreds of nanoseconds (on an extended overseas trip). Typically the origin of the portable clock trip is a major time keeping observatory, such as USNO, where the portable clock is initially synchronized as close as possible to the master clock (NC) time at that observatory. Prior to departure a stationary rate is determined between the two clocks. Upon return to the originating observatory, closure is again made with the master clock and a rate of the portable clock against the master clock is measured. These two rates (before and after) are then compared. Assuming no major difference occurs, the time accumulation between the two clocks is estimated and linearly applied to results obtained from each location on the trip. The important thing to note in such a method is that the portable clock must be kept running during the entire trip; that is, transported "hot". Many logistics problems and additional costs result from this necessity.

M. J. Lister

Papers

R-Cats - A New Method for Calibrating Global Positioning System (GPS) Remote Sites