A Pilot Experiment for GPS Link Calibration Between TL-MSL

J-L. Wang, Y-J. Huang, H-T. Lin, C-S. Liao, S-Y. Lin, T. Armstrong, S-Y. Huang

Abstract:	GPS time and frequency transfer is one of the most effective ways for the comparison of remote clocks, and the comparison results are very important for the calculation of TAI (International Atomic Time) and UTC (Coordinated Universal Time). Moreover, the comparison results of these time links need to be calibrated and periodically evaluated to ensure their accuracy and long term stability. A major issue in calculating the total uncertainty budget of the UTC is the calibration. Presently, there are two methods of calibration of time links, receiver calibration and link calibration. A number of studies have shown that TWSTFT (Two-Way Satellite Time and Frequency Transfer) adopting link calibration can be done with a calibration uncertainty (uB) of 1 ns. However, GPS time transfer using receiver-calibration strategy has the uB of 5 ns. It is attractive that, for link calibration method, measuring of any cable delay is unnecessary. When conducting link calibration, we do not measure delays of any inter-connecting cables between receiver, antenna, and input reference port. It is the reason that uncertainty can be reduced by avoiding error due to different types of cable delay measurement [2]. Since 2012, TL, the National standard time and frequency Laboratory of Telecommunication Laboratories (TL) in Taiwan, has supported coordination of Asia Pacific Metrology Programme (APMP) Technical Committee for Time and Frequency (TC-TF) project and hosted GPS receiver calibration exercise for TC-Initiative project in 2012. In this paper, we demonstrate GPS receiver calibration campaign based on link calibration concept between TL and Measurement Standards Laboratory (MSL) in New Zealand. Our purpose is to conduct link calibration between TL’s and MSL’s GNSS receivers so as to measure UTC(TL) - UTC(MSL). Some GNSS receivers including the local, remote and traveling receivers have been used for collecting data. At TL, we use an Ashtech Z-XII3T receiver to measure un-calibrated UTC(TL) - GPST, and this receiver is marked TWTF. At MSL, we use a Topcon Euro-80 receiver to measure un-calibrated UTC(MSL) - GPST, and this receiver is marked MSL. Moreover, we use a Septentrio PolaRx PRO 4 receiver as the traveling receiver (TR). The calibration concepts used in the experiment are receiver calibration and link calibration, respectively. For the receiver calibration, TL makes use of the differential calibration developed by BIPM which is a common approach to calibrate GPS facilities in time laboratories contributing to TAI. The GPS equipment under calibration located at MSL is compared with respect to a reference equipment, TWTF. In the differential calibration, TWTF uses a common clock and near zero baseline setup along with TR. We can calculate “the total delay of the time link [1, 2]”, or called “common clock difference (CCD) [3]” between TWTF and TR. And then the new internal value must be corrected by the value of common clock difference result. For conducting link calibration, TR is circulated between TL and MSL. At the remote site MSL, MSL1 should be compare with TR for some days by connecting both to a common clock. The total delay of the time link (or CCD) between MSL1 and TR can be calculated. We can calculate the total delay of the time link (or CCD) between MSL1 and TR. After the link calibration between TL and MSL is finished, the total delay of the time link between TWTF and MSL1 is obtained. This total delay is the calibration result which is used to directly measure UTC(TL) and UTC(MSL). Therefore, any systematic error between local and remote receivers, the internal and antenna cable delays of the traveling receiver can be cancelled out by correcting this total delay. This manner can potentially reduce the calibration uncertainty uB because the measurement of cable delay can result in different values depending on different methods [2]. The total uncertainty estimation of the link calibration value is the root-mean-square value of each uncertainty term. These terms are composed of uncertainty of the common clock difference, 1PPS delay from the local UTC, instability of receivers and antenna, multipath errors, and closure measurement value. The calibration results were obtained by using C/A, P3 and PPP comparison techniques. It shows that link calibration and uncertainty value for TWTF-MSL and TWTF-MSL1 GPS links using C/A code are 265.019± 2.96 ns and 196.183± 1.13 ns, respectively. The link calibration and uncertainty value for TWTF-MSL1 GPS links using P3 code is 192.693±1.47 ns. The link calibration and uncertainty value for TWTF-MSL1 GPS links using P3 code is 192.641±1.02 ns. In this paper, we demonstrate the GPS link calibration campaign with long baseline link beyond 10,000 km in Asia-Pacific region. This pilot experiment is implemented successfully between TL and MSL by using a Septentrio PolaRx PRO 4 receiver. The evaluated uncertainty of link calibration value indicates that the accuracy of GPS time link calibration can achieve better than 2 ns (P3) and 1ns (PPP) in long baseline link. It also shows that GPS receiver calibration using time link calibration rather than traditional receiver calibration method can greatly decrease calibration uncertainty (uB) which is the dominant term of total uncertainty. In order to verify more links result in Asia-Pacific region, TL will plan to host calibration campaign again with APMP members who are interested in this experiment in the future.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3474 - 3479
Cite this article:	Wang, J-L., Huang, Y-J., Lin, H-T., Liao, C-S., Lin, S-Y., Armstrong, T., Huang, S-Y., "A Pilot Experiment for GPS Link Calibration Between TL-MSL," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3474-3479.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In