Remote Calibration of a GPS Timing Receiver to UTC(NIST) via the Internet*

The timing pulse from a Global Positioning System (GPS) receiver is typically within 1μs of Coordinated Universal Time (UTC) kept at NIST, called UTC(NIST), even if the receiver and antenna have not been calibrated for delays. However, it is not possible for most laboratories to make an accurate measurement of their GPS receiver’s time offset with respect to UTC(NIST), or to monitor its continuous performance. This paper presents a method for remotely calibrating a GPS timing receiver at the customer’s location with respect to UTC(NIST) using the commonview GPS technique and the Internet. We present data from a remote calibration of a GPS timing receiver located at Sandia National Laboratories in Albuquerque, New Mexico, about 561 km from the NIST Laboratories in Boulder, Colorado. The uncertainty of the calibration is shown to be less than 50 ns. INTRODUCTION There is a small but growing demand in the calibration community for laboratories to maintain an on-time 1 pulse per second (pps) reference with a known uncertainty. This timing reference can be used either as a time interval standard, or to synchronize or calibrate other devices. By far the easiest way to obtain accurate timing pulses is to use a Global Positioning System (GPS) receiver. A typical commercial GPS receiver will produce a timing signal within 1 μs of UTC(NIST), even if it has not been calibrated for delays. However, since measuring the actual time offset to UTC(NIST) is usually not an option, laboratories are often limited to using the number quoted on the manufacturer’s specification sheet as their uncertainty figure. This paper describes a measurement system that can remotely calibrate a GPS timing receiver and determine the actual time offset of the receiver with an uncertainty of < 50 ns. The Internet is used to exchange data and to display the measurement results. GPS TIME The time broadcast by the GPS satellites, called GPS time, is controlled by the United States Naval Observatory (USNO) from their alternate master clock facility collocated with the GPS Master Control Station at Schriever Air Force Base, located near Colorado Springs, Colorado. The on-time pulses broadcast by the satellites are in close agreement with both the UTC(USNO) and UTC(NIST) time standards. Figure 1 shows the average daily time offsets of GPS time −