, pp.35-40 http://dx.doi.org/10.14257/ijseia.2014.8.4.04 Clock Synchronization of Pseudolite Using Time Transfer Technique Based on GPS Code Measurement Soyoung Hwang and Donghui Yu* Department of Multimedia Engineering Catholic University of Pusan, South Korea {soyoung, dhyu}@cup.ac.kr Abstract Pseudolite is a contraction of the term pseudo-satellite, used to refer to something that is not a satellite which performs a function commonly in the domain of satellites. A pseudolite is installed on the ground, which sends the same waves as those from a GPS satellite to enable positioning in locations where it is difficult to receive the waves from GPS satellite, such as between tall buildings, underground, and indoors. The pseudo-range measurement accuracy of pseudolite depends on the performance of clock synchronization between pseudolite and GPS satellites. This paper proposes clock synchronization algorithm for pseudolite. It is a revision and improvement of the time comparison technique based on GPS code transfer in order to determine the UTC. Keywords: clock synchronization, GPS, pseudolite 1. Introduction Pseudolite is a contraction of the term pseudo-satellite, used to refer to something that is not a satellite which performs a function commonly in the domain of satellites. Pseudolites are most often small transceivers that are used to create a local, ground-based GPS alternative[1]. A pseudolite is installed on the ground, which sends the same waves as those from a GPS satellite to enable positioning in locations where it is difficult to receive the waves from GPS satellite, such as between tall buildings, underground, and indoors. The pseudo-range measurement accuracy of pseudolite depends on the performance of clock synchronization between pseudolite and GPS(Global Positioning System) satellites. This paper proposes clock synchronization algorithm for pseudolite. The proposed algorithm adjusts clock of pseudolite by producing clock error between GPS satellites and pseudolite. It is a revision and improvement of the time comparison technique based on GPS code transfer in order to determine the UTC(Universal Time Coordinated). The rest of this paper is organized as follows. Section 2 describes traditional clock synchronization scheme such as utilizing 1PPS of GPS, IGS product and time comparison methods. In section 3, GPS time transfer technique is discussed. Section 3 proposes the clock synchronization algorithm for pseudolite. Finally, we conclude this paper in section 5. 2. Traditional Clock Synchronization Methods In this section, we survey traditional clock synchronization methods such as utilizing 1PPS of GPS, IGS product and time comparison methods. First of all, there are utilizing 1PPS of GPS and utilizing IGS product. ISSN: 1738-9984 IJSEIA Copyright c 2014 SERSC
Utilizing 1PPS of GPS: This method utilizes 1PPS(Pulse Per Second) signal of GPS receivers to synchronize clock of target system. It constructs the architecture of DP-PLL(Digital Processing Phase Locked Loop). The 1PPS signal maintains an accuracy of 100 ns or better depending on the performance of GPS receivers. Utilizing IGS product: This method utilizes IGS(International GNSS service) product such as precise orbit of GPS satellites, precise clock error of GPS satellites and measurements of carrier phase. It synchronizes clock of the target system by post processing of IGS product. It has better accuracy than 1PPS of GPS. Then, there are TWSTT and GPS common-view as time comparison methods. TWSTT(Two-way Satellite Time Transfer): Two-way time transfer between two timing standards may be accomplished by having each of two time standards send a 1PPS signal to the other time standard over a communications circuit. It is a point-to-point communications link. The communications circuit used is not important and may be made through any wide-band circuits, such as coaxial cable, fiber-optic cable, microwave transmission, television, laser light transmission, or communications satellites, to name a few. The transmission medium introduces delays, but this delay must be nearly reciprocal, i.e. the delay is the same in both directions, thus cancelling out. Each lab measures the time interval between the transmission of its local 1PPS and the time it receives the remotely generated 1PPS signal, typically using a time interval counter. The true time offsets of the two time standards can be measured very precisely and accurately. By taking data over a period of time, the long-term behavior that will affect the accuracy and stability may be characterized. The operational usefulness of the clocks is improved as well as the confidence in related decision making. The day-to-day stability of two-way time transfers can nearly reach the performance of the best reference clocks [2]. It provides data about clock error with an accuracy of 1 ns or better. GPS common-view: This is a subsidiary method of TWSTT. The time difference between two clocks may be determined by simultaneously comparing each clock to a common reference signal that may be received at both sites. As long as both end stations receive the same satellite signal at the same time, the accuracy of the signal source is not important. The nature of the received signal is not important, although widely available timing and navigation systems such as GPS or LORAN are convenient[3]. The accuracy of GPS common-view is typically 1-10ns. 3. Time Transfer Technique Based on GPS Code Measurement This section describes time transfer technique based on GPS code Measurement. The most important thing is calculating transmission time of GPS signal from a GPS satellite to a receiver accurately in time transfer based on GPS code measurement[4]. However, there are various sources of errors that alter the accuracy of GPS receivers. The primary sources of errors that degrade GPS performance include satellite clock and orbit errors, atmospheric errors, multipath and receiver error as shown in Figure 1. 36 Copyright c 2014 SERSC
Figure 1. Calculation of Moving Distance Satellite clock and orbit errors: GPS satellites carry very accurate atomic clocks and follow very precise orbits. But drifts in both clock and orbit are inevitable and very small amount can cause significant errors in a receiver on the ground. Even though their clocks and orbits may not be adjusted, their offsets are computed by the GPS Ground Segment then sent back to the satellites. The satellites then broadcast the clock and ephemeris message to the end-user. There is some latency between the actual occurrence of the offsets and the time they are computed and broadcasted. Depending on the type of differential correction used, the effects of satellite clock and orbit errors can greatly be compensated [5-8]. Atmospheric Errors: Atmospheric errors are the most significant source of errors of GPS. With the satellites orbiting at about 20,000km above the earth, the GPS signals have to travel through the ionosphere and the troposphere layers before reaching the receiver antenna. Ionosphere is the collective term for the various layers of ionized particles and electrons found at altitudes of 80 250 km in the atmosphere. Ionization is caused primarily by short-wavelength solar radiation during the daytime. Ionospheric activities have the biggest impact on GPS accuracy [5-9]. Another deviation from the vacuum speed of light is caused by the troposphere. Variations in temperature, pressure, and humidity all contribute to variations in the speed of light of radio waves [5-9]. Multipath: Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies, mountains, trees and buildings [5-8]. Copyright c 2014 SERSC 37
Receiver Errors: Despite the synchronization of the receiver clock with the satellite time during the position determination, the remaining inaccuracy of the time still leads to an error. Rounding and calculation errors of the receiver also affect the accuracy [5-7]. CCTF(Consultative Committee for Time and Frequency) advocated in its Recommendations S 5 (2001) that the manufacturers of receivers used for timing with GNSS(Global Navigation Satellite Systems) implement the technical guidelines for receiver hardware compiled by the CGGTTS(CCTF Group on GNSS Time Transfer Standards). CGGTTS is an international standard format for time comparison and provides data about clock error with an accuracy of 1 ns or better. These data are achieved by compensating prior mentioned error budget. CGGTTS is generated by GPS timing receivers or can be produced by utilizing RINEX(Receiver Independent Exchange Format) data from geodetic receivers [10, 11]. 4. Clock Synchronization Algorithm for Pseudolite In this section, we propose clock synchronization algorithm for pseudolite. This algorithm compare CGGTTS from a GPS receiver with CGGTTS from a pseudolite receiver in order to produce clock error between GPS satellite and pseudolite. Figure 2 shows the architecture of the proposed algorithm. In this process, the same procedure can be applied to pseudolites and pseudolite receivers as in GPS time transfer. By the way, there is no need to consider the errors such as satellite orbit error and atmospheric errors, since puseolites are installed in a fixed location on the ground. Finally, synchronization station produces clock error between GPS and psueolite by comparison their clock offset and pseudolite can synchronize with GPS by adjusting the clock error. In the proposed method, it is required to determine data link of pseudolite and to define the format of broadcasting data. And pseudolite receivers which support RINEX data are also required. Thus, it can be considered that a pseudolite itself works as a synchronization station by adopting GPS receiver to the pseudolite. Figure 2. Architecture of Clock Synchronization Algorithm for Pseudolite 38 Copyright c 2014 SERSC
5. Conclusions The pseudo-range measurement accuracy of pseudolite depends on the performance of clock synchronization between pseudolite and GPS satellites. Pseudo-range measurement of pseudolite has around 300m range error, when time synchronization error of 1us. Therefore the time synchronization methods play an important part in navigation augmentation using pseudolite. In this paper, we proposed clock synchronization algorithm for pseudolite. The proposed algorithm adjusts clock of pseudolite by producing clock error between GPS satellites and pseudolite. It is a revision and improvement of the time comparison technique based on GPS code transfer in order to determine the UTC. As a future work, we consider performance evaluation of the proposed algorithm in a software simulation platform. References [1] S. Hwang and D. Yu, Clock Synchronization Algorithm for Pseudolite, Advanced Science and Technology Letters, (Networking and Communication 2013), vol. 44, (2003), pp. 36-39. [2] Two-Way Satellite Time Transfer, http://tycho.usno.navy.mil/twstt.html. [3] D. W. Allan and M. A. Weiss, Accurate Time and Frequency Transfer During Common-View of a GPS Satellite, Proceedings of the 34 th Annual Frequency Control Symposium USAERADCOM, Ft. Monmouth, NJ, (1980). [4] D. Yu, S. Yang, J. Do and C. Lee, Analysis of Tropospheric Zenith Path Delay of GPS Code Based Precise Time Comparison Technique, The Journal of Korea Society of Computer Information, vol. 17, no. 12, (2012), pp. 61-69. [5] B. W. Parkinson, J. J. Spilker Jr., P. Axelrad and P. Enge, Global Positioning System: Theory and Applications, Progress in Astronautics and Aeronautics, vol. 1, (1996). [6] B. W. Parkinson, J. J. Spilker Jr., P. Axelrad and P. Enge, Global Positioning System: Theory and Applications, Progress in Astronautics and Aeronautics, vol. 2, (1996). [7] B. Hofmann-Wellenhof, H. Lichtenegger and J. Collins, GPS Theory and Practice, 5 th Edition, SpringerWienNewYork, (2001). [8] GPS The Error Budget, http://sxbluegps.com. [9] S. J. Wormley, GPS Errors and Estimating Your Receiver s Accuracy, (2010). [10] CGGTTS guidelines for manufacturers of GNSS receivers used for timing, http://tycho.usno.navy.mil, (2001). [11] W. Gurtner and L. Estey, RINEX: The Receiver Independent Exchange Format Version 2.11, (2007). Authors Soyoung Hwang received the B.S., the M.S., and the Ph.D. degrees in Computer Science from Pusan National University, Busan, Korea in 1999, 2001 and 2006 respectively. From 2006 to 2010, she was a senior researcher in ETRI, Daejeon, Korea. Since 2010, she has been a professor of Department of Multimedia Engineering at Catholic University of Pusan, Korea. Her research interests include embedded systems and sensor networks. Donghui Yu received the B.S., the M.S., and the Ph.D. degrees in Computer Science from Pusan National University, Busan, Korea in 1992, 1994, 2001 respectively. From 1994 to 1997, she was a researcher in ETRI (Electronics and Telecommunications Research Institute), Daejeon, Korea. From 2003, she has been a faculty of Department of Multimedia Engineering at Catholic University of Pusan, Korea. Her research interests are time synchronization and mobile systems. Copyright c 2014 SERSC 39
40 Copyright c 2014 SERSC