GPS system is distributed in six orbital planes of 24 satellites of the constellation. GPS satellites orbit height of 20000km, the satellite is equipped with 10-13 high-precision atomic clock. A master ground control station and multiple stations on a regular basis on the constellation of satellites for precise determination of the location and time to the issue of satellite ephemeris information. Users to use GPS receivers to receive four or more satellites at the same time the signal, can determine its latitude and longitude, height and precise time.
OFFICE OF THE MANAGER NATIONAL COMMUNICATIONS SYSTEM TECHNICAL AUGUST 1999 NOTES TECHNOLOGY AND STANDARDS DIVISION VOLUME 6, NUMBER 2 NS/EP Implications of GPS Timing by LeeAnne Brutt The Global Positioning System (GPS) is a satel- tains an even greater level of accuracy, with a lite-based positioning and navigation system predictable horizontal accuracy to within 22 that is funded and operated by the United States meters, 27.7 meters in the vertical direction, and Department of Defense. Although originally timing to within 100 nanoseconds. However, developed for use by the U.S. military, GPS now this higher level of service is secured through supports thousands of civilian users worldwide encryption and is only available for use by U.S. and is employed in a wide range of applications. military and other approved users. This technical note focuses on the use of GPS for GPS is composed of a space segment, a control telecommunications network timing and segment, and a user segment. The space synchronization. The following sections describe segment consists of a constellation of 24 satel- the GPS system, how GPS calculates and trans- lites along with several spares (figure 1). Each mits its timing signals, and how GPS timing is satellite orbits the Earth once every 12 hours on used in networks. Lastly, potential National one of six orbital planes. The GPS constellation Security/Emergency Preparedness (NS/EP) is posi- implications are discussed. tioned such that OVERVIEW OF GPS at any GPS provides two levels of operation: standard given positioning service (SPS) and precise positioning time, 5 to service (PPS). SPS is available to all users, free of 8 satel- charge, on a continuous, worldwide basis. It is lites are able to provide a predictable 95 percent posi- visible tioning and time transfer accuracy, which trans- from any lates to approximately 100 meters in the point on horizontal direction, 156 meters vertically, and Earth. timing to within 340 nanoseconds. PPS main- Figure 1. GPS Constellation The control segment is a system of monitor GPS TIMING stations, ground antennas, and a master control The standard international reference for accurate station. The monitor stations measure signals time and frequency is known as Coordinated from all visible satellites. The accumulated data Universal Time, denoted UTC. Developed in is then processed by the master control station to 1970 by the International Telecommunication calculate satellite orbits and to update satellite Union, official UTC time is generated at the navigation messages, including clock correc- Bureau International des Poids et Mesures, tions. The revised information is transmitted located near Paris, France. However, UTC is not back to the satellites via the ground antennas, directly available as a real-time clock. Therefore, and finally data is transferred over radio signals many timing centers worldwide generate a local- to GPS receivers. The receivers comprise the ized estimate, which is accurate to within 100 user segment, which is the portion of the GPS nanoseconds of UTC. The time scale generated at system that converts signals into timing and the United States Naval Observatory (USNO) is positioning information for users. one example. Known as UTC (USNO), the USNO Master Clock’s approximation of official To perform positioning and timing calculations, UTC is used as the timing reference for GPS. GPS employs a triangulation technique. With this method, GPS receivers measure and GPS keeps its own system time that is derived compare the travel time of radio signals sent from a composite clock consisting of all opera- from four visible satellites with known posi- tional satellite clocks and the USNO timing tions. Three of the measurements are used to standard. Each GPS satellite contains four calculate the receiver’s position in 3-dimen- atomic clocks (2 cesium and 2 rubidium), sions, and the fourth is used to determine time. offering a very high level of precision. The These signals, known as pseudo-random code satellites transmit clock information as part of (PRC), are unique to each satellite. This unique- the signals that are sent to the monitor stations. ness allows all the signals to broadcast over the The master control station then gathers the data same frequency. to calculate timing errors and to make appro- priate clock corrections. When the revised One caveat to this method of computation is timing signal is uploaded to the satellites, GPS that GPS users often reduce their costs by system time can be broadcast to the receivers employing receivers with less accurate clocks. during the satellite navigation message. Although this has the potential of introducing error into positioning and timing calculations, As part of its error analysis of the satellite timing GPS avoids this problem by taking an extra signals, the master control station compares the satellite range measurement during the trian- satellite clock times with the timing standard gulation phase. This allows the system to generated at the USNO. GPS system time is correct any timing offset and thus maintain steered to remain within one microsecond of UTC GPS’ overall high level of accuracy. (USNO). However, GPS does not allow for leap Nevertheless, the triangulation technique seconds, as does UTC, because any discontinuity requires that each of the satellites transmit its would offset the receivers. As a result, GPS time is PRC in a highly synchronous manner. A timing ahead of UTC by several seconds. The receivers error of just 1/1000 of a second would produce compensate for this difference automatically a measurement error of nearly 200 miles. during their signal conversions, and so the timing Therefore, precise timing is a critical element information that is passed to the user is in fact a for the proper implementation of GPS. very close approximation of UTC (USNO). Figure 2 illustrates the GPS timing sequence. NETWORK TIMING AND SYNCHRONIZATION Precise time dissem- ination is critical for the synchronization of telecommunica- tions networks. Within both wireline and wireless systems, consistent pulses and time intervals are used to manage information flow through the network nodes. In particular, the Public Switched Network (PSN) relies on accu- rate timing informa- tion for the proper digital transmission of voice and data. Because of the high degree of accuracy of the GPS system, special-purpose GPS receivers are often employed as a timing source. Figure 2. GPS Timing In addition to its use in telecommunications, The GPS system records both the number of GPS is used in other applications as a timing seconds that have passed in a given week and reference for wide-area synchronization. the number of weeks that have elapsed since the These include electric power systems, distrib- GPS time zero point (established at midnight uted computer networks, banking (for money (UTC) on January 6, 1980). The GPS week transfers and bank time locks), manufac- number cycles every 1024 weeks. After week turing, and metrology. 1023, the week number count is reset to 0, during what is called the week number rollover NS/EP VULNERABILITIES (WNRO) or end of week (EOW) rollover, Because GPS is commonly used as a timing approximately every 19.6 years. The first GPS source for telecommunications, any system rollover will occur just before midnight on vulnerabilities concern the NS/EP community. August 21, 1999. One limitation of GPS is its susceptibility to interference. GPS signals are extremely weak, with satellites transmitting at power levels segment, in particular, the receivers, will vary which measure only -160 dBW at the receiving according to manufacturer and model. It is antenna. This compares to the amount of light unclear what problems will occur as a result of that can be seen from a 25-watt bulb at a receiver noncompliance. One scenario is that user distance of 10,000 miles [Ref. 1]. As a result, equipment may experience delays while locating the GPS signals can be affected by both inten- GPS satellites or while making position and date tional and unintentional sources. calculations. It is also possible that satellites might not be located at all. Even if receivers do The deliberate interference with GPS signaling is properly access the satellite signals, they might known as "jamming." It has been shown that, display inaccurate timing and position informa- using readily available materials, a one-watt tion. Although it is uncertain how widespread jammer can be constructed to tamper with GPS such problems might be, it is expected that only reception from a distance of more than 60 kilo- older equipment could be affected. Newer meters. Even the best of receivers are susceptible models have been programmed to properly to jamming. Additionally, signals whose funda- account for the date and time transitions. mental frequencies are within the bandwidth of a GPS receiver could unintentionally cause prob- REFERENCES lems. Possible sources of interference include 1. Herskovitz, Don. (May 1997). “And the Compass emissions from both ground-based and aeronau- Spun Round and Round: The Coming Era of tical satellite communications equipment, wide- Navigation Warfare.” Journal of Electronic Defense http://www.jedonline.com/jed/html/new/may97/ band noise from electrical devices, and ultra cover.html. high frequency (UHF)/very high frequency 2. Dana, Peter H "Global Positioning System (VHF) communications. Regardless of the inten- Overview.” tion, interference with GPS signaling has the http://www.utexas.edu/depts/grg/gcraft/notes/gps/ potential to decrease timing precision or even gps.html. cause receivers to lose signal lock. 3. The U.S. Naval Observatory http://tycho.usno.navy.mil. Another NS/EP concern is that the use of GPS 4. Trimble Navigation Limited. “All About GPS.” as a timing reference could be jeopardized by http://www.trimble.com/gps/howgps/gpsframe.htm. selective availability. This is the military’s 5. Allen, D., Ashby, N., and Hodge, C. (March 1998). current practice of introducing intentional “Fine Tuning Time in the Space Age.” IEEE random error into the SPS signal to limit hostile Spectrum use of the service. However, telecommunications http://www.spectrum.ieee.org/spectrum/mar98/ features/utc.html. applications rely on real-time outputs. By adding random errors to the signal, peak-to- 6. Langley, Richard B. (November 1998). “The GPS End-of Week Rollover.” GPS World. peak variations on the order of hundreds of http://www.gpsworld.com/0299/1198innovation.html. nanoseconds can result, thereby affecting synchronization. This problem will be alleviated with the scheduled elimination of selective For more information on availability in 2006. GPS timing, contact: The pending date and time changes due to LeeAnne Brutt WNRO and the year 2000 (Y2K) have also caused National Communications System some concern in the NS/EP community. The GPS Technology and Standards Division (N6) Joint Program Office has certified that the space 701 South Court House Road and control segments are WNRO/Y2K Arlington, Virginia 22204-2198 compliant. However, the compliance of the user (703) 607-6200
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