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									                            OFFICE OF THE MANAGER

                         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.
                                                                              TIMING AND
                                                                              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
band noise from electrical devices, and ultra
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 
potential to decrease timing precision or even          gps.html.
cause receivers to lose signal lock.                 3. The U.S. Naval Observatory
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 
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
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   
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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