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

TLDR: In this article, a portable clock trip is described, where the portable clock is initially synchronized as close as possible to the master clock (NC) time at that observatory, and closure is again made with the original master clock and a rate of the Portable Clock against the master Clock is measured.

Abstract: : 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.