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Calibration of a GPS Receiver Used as a Time Reference
Appendix A
Calibration of a GPS Receiver Used as a Time Reference The US Naval Observatory (USNO) has a master clock facility at Schreiver Air Force Base, near Colorado Springs, Colorado (the same location as GPS’ Master Ground Station). The time standard used is a variant of Coordinated Universal Time, called UTC(USNO). GPS satellites broadcast time—GPS time. GPS receivers can be used as a time reference by laboratories. It is possible to calibrate such a receiver so that it provides an on-time 1 pulse per second (pps) reference with a known uncertainty. There are delays to a signal being processed by a receiver that are caused by the antenna and the receiver itself. If a receiver has not been calibrated to account for these delays then the timing pulse output from the receiver is usually within 1 μs of a UTC standard kept at NIST—called UTC(NIST). Lombardi et al. (n.d.) describe a system for remotely calibrating a GPS timing receiver and determining the time offset of the receiver with an uncertainty of <50 ns. GPS time, UTC(USNO), and UTC(NIST) all differ from one another by small amounts. NIST and USNO have agreed that their times will not differ by more than 100 ns. At the time of the research, the average difference was less than 10 ns. Obviously, GPS time differs little from UTC(USNO), however, a GPS receiver and its antenna can increase the uncertainty of the received time. It is possible to send a receiver and its antenna to NIST, or another laboratory, so that the signal delays that they cause can be estimated. In calibrating, NIST makes use of commonview measurements. Their experience in this technique dates back almost as far as the launch of the first GPS satellite in 1978. Calibration, using common-view measurements, has been undertaken by NIST and other national metrology institutes for many years. Let us explain what this common-view technique is by using an analogy. Imagine that you wish to compare the time on your watch with the time on your friend’s watch, when your friend lives at the other side of your town. You both agree on an event, for example, when a certain church’s bells begin to chime
143
144
Uncertainties in GPS Positioning
S
Clock A
Clock B
Fig. 1 Use of common-view in GPS
on a Sunday morning. When the event occurs, you each write down the time shown on your watch. You then compare the times. Common-view measurements involving GPS are performed by two receivers, located on Earth, receiving the same signal, see Fig. 1. For each receiver, there is a delay in the satellite’s signal reaching the antenna, there is a delay caused by the antenna, and there is a delay caused by the receiver itself. If we perform the calculation (Time on Clock A—Time on Clock B) then the common errors will cancel out. This will leave us with a term (dA − dB ), where dA is the antenna and receiver delays caused at A, dB is the antenna and receiver delays caused at B. Assuming that one of the clocks is accurate, the term (dA − dB ) can be applied to the other clock as a correction. In this work, a measurement system was used. This comprised a circuit board that contained a time interval counter (TIC) and a GPS receiver. The procedure is for the measurement system to be sent to the customer, who connects it to a PC. At the customer’s site, there is the receiver to be tested and the measurement system (another receiver). The 1-pps output from the receiver under test is compared with the 1-pps output from the measurement system. At NIST, there is an identical measurement system which is compared with a 1-pps output reference signal from UTC(NIST). Control of the process of recording and comparing data was done by software. At both the customer’s site and NIST, the software calculated the time difference between pulses every second. In developing the measurement system, several tests were made. First, a 1-pps signal was sent to two identical measurement systems, both located at NIST. The outputs of the measurement systems were slightly different due to noise. Next a 1-pps signal was sent to two proprietary receivers and
Calibration of a GPS Receiver Used as a Time Reference
145
the difference in their 1-pps outputs was noted. The difference in outputs was averaged every 10 min and the experiment lasted for approximately 30 days. Several similar comparisons of pairs of receivers were made. Above, the method for calibrating a customer’s GPS receiver was described. This involved a set up at NIST. A further refinement was made to the procedure. This was the use of an Internet connection between NIST and the customer. When developing this approach, the customer selected was Sandia National Laboratories. The distance between the two sites is about 561 km. Data at Sandia was recorded locally and uploaded to NIST once a day, where it was processed. Unfortunately, different antenna cables were used at the two sites, which caused discrepancies. Ideally, NIST should ship an antenna and cable, which has been calibrated, to the customer. The total receiver delay at each site was, therefore, different. (The total receiver delay includes delays caused by the receiver, the antenna, and the antenna cable.) For example, the antenna delay at NIST was estimated to be 408 ns while that at Sandia was estimated to be 161 ns. The total receiver delay at NIST was estimated to be 180 ns, while that at Sandia was estimated to be 427 ns. This remote calibration of Sandia’s GPS receiver occurred in the 40-day period, October 8 to November 16, 2002. In conclusion, Lombardi, Novick, & Graham (n.d.) performed a remote calibration of a GPS timing receiver.
REFERENCE Lombardi, M., Novick, A., & Graham, R. (n.d.). Remote calibration of a GPS timing receiver to UTC(NIST) via the Internet. In Proceedings of the measurement science conference, Anaheim, CA.
Calibration of a GPS Receiver Used as a Time Reference The US Naval Observatory (USNO) has a master clock facility at Schreiver Air Force Base, near Colorado Springs, Colorado (the same location as GPS’ Master Ground Station). The time standard used is a variant of Coordinated Universal Time, called UTC(USNO). GPS satellites broadcast time—GPS time. GPS receivers can be used as a time reference by laboratories. It is possible to calibrate such a receiver so that it provides an on-time 1 pulse per second (pps) reference with a known uncertainty. There are delays to a signal being processed by a receiver that are caused by the antenna and the receiver itself. If a receiver has not been calibrated to account for these delays then the timing pulse output from the receiver is usually within 1 μs of a UTC standard kept at NIST—called UTC(NIST). Lombardi et al. (n.d.) describe a system for remotely calibrating a GPS timing receiver and determining the time offset of the receiver with an uncertainty of <50 ns. GPS time, UTC(USNO), and UTC(NIST) all differ from one another by small amounts. NIST and USNO have agreed that their times will not differ by more than 100 ns. At the time of the research, the average difference was less than 10 ns. Obviously, GPS time differs little from UTC(USNO), however, a GPS receiver and its antenna can increase the uncertainty of the received time. It is possible to send a receiver and its antenna to NIST, or another laboratory, so that the signal delays that they cause can be estimated. In calibrating, NIST makes use of commonview measurements. Their experience in this technique dates back almost as far as the launch of the first GPS satellite in 1978. Calibration, using common-view measurements, has been undertaken by NIST and other national metrology institutes for many years. Let us explain what this common-view technique is by using an analogy. Imagine that you wish to compare the time on your watch with the time on your friend’s watch, when your friend lives at the other side of your town. You both agree on an event, for example, when a certain church’s bells begin to chime
143
144
Uncertainties in GPS Positioning
S
Clock A
Clock B
Fig. 1 Use of common-view in GPS
on a Sunday morning. When the event occurs, you each write down the time shown on your watch. You then compare the times. Common-view measurements involving GPS are performed by two receivers, located on Earth, receiving the same signal, see Fig. 1. For each receiver, there is a delay in the satellite’s signal reaching the antenna, there is a delay caused by the antenna, and there is a delay caused by the receiver itself. If we perform the calculation (Time on Clock A—Time on Clock B) then the common errors will cancel out. This will leave us with a term (dA − dB ), where dA is the antenna and receiver delays caused at A, dB is the antenna and receiver delays caused at B. Assuming that one of the clocks is accurate, the term (dA − dB ) can be applied to the other clock as a correction. In this work, a measurement system was used. This comprised a circuit board that contained a time interval counter (TIC) and a GPS receiver. The procedure is for the measurement system to be sent to the customer, who connects it to a PC. At the customer’s site, there is the receiver to be tested and the measurement system (another receiver). The 1-pps output from the receiver under test is compared with the 1-pps output from the measurement system. At NIST, there is an identical measurement system which is compared with a 1-pps output reference signal from UTC(NIST). Control of the process of recording and comparing data was done by software. At both the customer’s site and NIST, the software calculated the time difference between pulses every second. In developing the measurement system, several tests were made. First, a 1-pps signal was sent to two identical measurement systems, both located at NIST. The outputs of the measurement systems were slightly different due to noise. Next a 1-pps signal was sent to two proprietary receivers and
Calibration of a GPS Receiver Used as a Time Reference
145
the difference in their 1-pps outputs was noted. The difference in outputs was averaged every 10 min and the experiment lasted for approximately 30 days. Several similar comparisons of pairs of receivers were made. Above, the method for calibrating a customer’s GPS receiver was described. This involved a set up at NIST. A further refinement was made to the procedure. This was the use of an Internet connection between NIST and the customer. When developing this approach, the customer selected was Sandia National Laboratories. The distance between the two sites is about 561 km. Data at Sandia was recorded locally and uploaded to NIST once a day, where it was processed. Unfortunately, different antenna cables were used at the two sites, which caused discrepancies. Ideally, NIST should ship an antenna and cable, which has been calibrated, to the customer. The total receiver delay at each site was, therefore, different. (The total receiver delay includes delays caused by the receiver, the antenna, and the antenna cable.) For example, the antenna delay at NIST was estimated to be 408 ns while that at Sandia was estimated to be 161 ns. The total receiver delay at NIST was estimated to be 180 ns, while that at Sandia was estimated to be 427 ns. This remote calibration of Sandia’s GPS receiver occurred in the 40-day period, October 8 to November 16, 2002. In conclusion, Lombardi, Novick, & Graham (n.d.) performed a remote calibration of a GPS timing receiver.
REFERENCE Lombardi, M., Novick, A., & Graham, R. (n.d.). Remote calibration of a GPS timing receiver to UTC(NIST) via the Internet. In Proceedings of the measurement science conference, Anaheim, CA.