Year 2000 Computing Challenge: Time Issues Affecting the Global Positioning System

Published by the Government Accountability Office on 1999-05-12.

Below is a raw (and likely hideous) rendition of the original report. (PDF)

                          United States General Accounting Office

GAO                       Testimony
                          Before the Subcommittee on Government Management,
                          Information and Technology, Committee on Government
                          Reform, and the Subcommittee on Technology, Committee
                          on Science, House of Representatives

For Release on Delivery

Expected at               YEAR 2000 COMPUTING
10 a.m.


May 12, 1999

                          Time Issues Affecting the
                          Global Positioning System
                          Statement of Keith A. Rhodes
                          Technical Director, Office of Computers and
                          Accounting and Information Management Division

                       Mr. Chairman, Madam Chairwoman, and Members of the Subcommittees:

                       Thank you for inviting me to participate in today’s hearing on the Year 2000
                       problem and its impact on the Global Positioning System (GPS). In
                       addition to being the Department of Defense’s primary radionavigation
                       system, GPS has become an integral asset in numerous civilian applications
                       and industries, including emergency services, airlines services, commercial
                       fishing and shipping, corporate vehicle fleet tracking, and surveying. It
                       also plays a critical role in communications networks and, hence, the
                       Internet. The system is affected by both the Year 2000 computing problem
                       and a problem associated with the way the system keeps track of time.
                       Today, I will discuss these two important issues, their potential impact, and
                       the status of remedial efforts.

GPS, the Year 2000     GPS was designed to support military missions, such as missile guidance
                       and search and rescue. The system consists of a constellation of 24
Problem, and the       operational satellites that are positioned so that system users can receive
End-of-Week Rollover   signals from at least 6 satellites nearly 100 percent of the time at any point

Problem                on Earth. The satellites are constantly monitored by ground stations
                       located throughout the world. Anyone using a GPS receiver can determine
                       their location with great precision. Defense began launching GPS satellites
                       in 1978 and started using the system in 1980. The system became fully
                       operational in 1995.

                       GPS is now used in numerous civilian applications and industries. For
                       example, emergency vehicles use GPS to pinpoint destinations and map
                       routes, shipping companies use the system to track movement of their
                       vessels, truck and transportation services use the system to track their
                       fleets and to speed deliveries, and airlines use GPS to develop flight plans
                       and to land planes. GPS is also being used to map roads, track forest fires,
                       assist in construction projects, and even monitor earthquakes.
                       Additionally, telecommunications companies are increasingly relying on
                       GPS receivers to synchronize their own networks, comparing their
                       reference clocks directly with a GPS receiver.

                           Instead of using calendar dates, GPS counts weeks, and seconds within a week, from precise clocks on

                       the satellites. GPS started at week zero on January 6, 1980. Because of its design, the GPS time counter

                       starts over after counting 1,024 weeks. The end of the 1,024th week will occur, for the first time, on

                       August 21, 1999. This is known as the end-of-week rollover problem in the GPS community.

                       Page 1                                                                              GAO/T-AIMD-99-187
                  GPS is affected by both the Year 2000 computing problem and an upcoming
                  end-of-week rollover. The Year 2000 computing problem is rooted in the
                  way dates are recorded and computed in many computer systems. For the
                  past several decades, systems have typically used two digits to represent
                  the year, such as “97” representing 1997, in order to conserve on electronic
                  data storage and reduce operating costs. With this two-digit format,
                  however, the Year 2000 is indistinguishable from 1900, 2001 from 1901, and
                  so on. As a result of this ambiguity, system or application programs that
                  use dates to perform calculations, comparisons, or sorting may generate
                  incorrect results when working with the years after 1999.

                  The upcoming end-of-week rollover is a problem that will occur for the first
                  time on August 21, 1999. Instead of using calendar dates, GPS counts
                  weeks, and seconds within a week, from precise clocks on the satellites.
                  This is based on how the signal codes transmitted by the satellite are
                  generated. GPS started at week zero on January 6, 1980. Because of its
                  design, the GPS time counter starts over after counting 1,024 weeks. The
                  end of the 1,024th week will occur on August 21, 1999. This is known as the
                  end-of-week rollover problem in the GPS community.

                  I will now discuss the potential impact of the Year 2000 problem and the
                  upcoming end-of-week rollover on each of the three GPS components—
                  space, control, and user—as well as the status of remedial efforts.

Space Component   The space component of GPS consists of 24 operational satellites in 6
                  orbits at approximately 11,000 miles above the earth. The satellites
                  transmit radio signals that permit adequately equipped users to calculate
                  position, velocity, and time anywhere on or above the Earth’s surface and in
                  any weather condition. They are equipped with very precise clocks that
                  keep accurate time to within 3 nanoseconds. According to the Air Force
                  Materiel Command (AFMC), the executive agent for the Department of
                  Defense in acquiring GPS satellites, all GPS satellites are Year 2000
                  compliant as well as end-of-week rollover compliant.

                  The space component also includes satellite support systems, which are
                  physically located on the ground. These systems are responsible for
                  maintaining the satellites and their proper functioning. This includes
                  keeping the satellites in proper orbits (called station keeping) and
                  monitoring satellite subsystem health and status—e.g., monitoring solar
                  arrays, battery power levels, and propellant levels and activating spare
                  satellites, if possible. While the satellite support systems are end-of-week

                  Page 2                                                      GAO/T-AIMD-99-187
                   rollover compliant, they are                not yet Year 2000 compliant, according to
                   AFMC. AFMC reports that these systems are in the process of being either
                   replaced or renovated and tested. This work is expected to be done by
                   December 1999. Workarounds have also been reportedly developed for
                   systems being replaced.

Ground Component   The GPS control, or ground, component consists of a master control
                   station, five monitoring stations, and three ground antennas located
                   throughout the world. The monitoring stations track all GPS satellites in
                   view and collect ranging information from the satellite broadcasts. The
                   stations send these data to the master control station, which computes
                   precise satellite orbits. This information is then formatted into updated
                   navigation messages for each satellite and transmitted to each satellite
                   through the ground antennas, which also transmit and receive satellite
                   control and monitoring signals. These systems are interconnected through
                   networks and also have their own information systems and equipment that
                   must be renovated for Year 2000 compliance. According to AFMC, the
                   ground support systems are now both Year 2000 and end-of-week rollover
                   compliant. Contingency plans are also in place for these systems.

User Component     The user component consists of receivers, processors, and antennas that
                   allow land, sea, or airborne operators to receive the GPS satellite
                   broadcasts and compute their precise position, velocity, and time.
                   According to AFMC, many newer GPS receivers, including all designs
                   procured for the Department of Defense by the GPS Joint Program Office,
                   have been tested and have demonstrated that they are Year 2000 compliant
                   and end-of-week rollover compliant. According to the U.S. Coast Guard
                   Navigation Center, however, the accuracy of navigation on some older
                   receivers may be severely affected by the end-of-week rollover.

                       Distance from a receiver to the satellites.

                   Page 3                                                                   GAO/T-AIMD-99-187
Activities Ongoing to   Several activities are ongoing to raise awareness among owners of older
                        GPS receivers of the upcoming end-of-week rollover problem. The U.S.
Raise Awareness of      Coast Guard Navigation Center has been assigned responsibility for being
Problem With Older      the government liaison to the civil sector for GPS. Its Internet website

Receivers               explains the potential rollover problem on older receivers and provides an
                        extensive list of manufacturers and points of contact. The Air Force has
                        provided a list, also available on the Internet, of specific receivers that have
                        been tested and found to be compliant by the Department of Defense.
                        Furthermore, the President’s Council on Year 2000 Conversion’s Internet
                        site provides links to sources of GPS Year 2000 and end-of-week rollover
                        information. These activities are important and should be useful to GPS
                        users seeking to determine whether their receivers will operate correctly at
                        the end-of-week rollover.

                        However, even with these awareness efforts, it is conceivable that some
                        organizations and users may not be aware that their GPS receiver could be
                        vulnerable to the end-of-week rollover problem. Moreover, some may not
                        even be aware that they rely on a GPS receiver as a communications
                        network tool. Because they contain precise clocks, GPS receivers are
                        sometimes used to synchronize time in communications networks.
                        Synchronization is critical to the transmission of compressed or
                        packetized voice, data, and video transmissions. Timing errors due to the
                        lack of synchronization, in fact, can lead to data loss and degradation and
                        eventually to network disruption or even complete failures. Because of the
                        interconnective and interdependent nature of networks, these problems, in
                        turn, could affect other networks and even the Internet.

                        As a result, it is vital that organizations make an effort to determine
                        (1) whether the networks they operate rely on GPS equipment as a time
                        source and (2) the potential GPS-related risks. Once the problem and its
                        potential impact are known, organizations and individual users can
                        (1) modify receivers, (2) replace them with newer models, or (3) contact
                        their service providers to ensure that GPS receivers supporting their
                        telecommunications networks are not susceptible to the upcoming end-of-
                        week rollover. Because the rollover is less than 4 months away, however,
                        organizations must take these measures as quickly as possible.

                            Digital voice, data, and video transmissions are sent in packets or cells.

                        Page 4                                                                           GAO/T-AIMD-99-187
                   Mr. Chairman, Madam Chairwoman, this concludes my testimony. I will be
                   happy to answer questions you or Members of the Subcommittees may

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