Year 2000 Computing Challenge: Time Issues Affecting the Global
Positioning System (Testimony, 05/12/99, GAO/T-AIMD-99-187).

Pursuant to a congressional request, GAO discussed the year 2000 problem
and its impact on the Global Positioning System (GPS).

GAO noted that: (1) GPS is affected by both the year 2000 computing
problem and an upcoming end-of-week rollover; (2) the upcoming
end-of-week rollover is a problem that will occur for the first time on
August 21, 1999; (3) instead of using calendar dates, GPS counts weeks,
and seconds within a week, from precise clocks on the satellites; (4)
this is based on how the signal codes transmitted by the satellite are
generated; (5) GPS started at week zero on January 6, 1980; (6) because
of its design, the GPS time counter starts over after counting 1,024
weeks; (7) the end of the 1024th week will occur, for the first time, on
August 21, 1999; (8) there are three GPS components that may be affected
by the year 2000 problem and the end-of-week rollover--space, control,
and user; (9) the Air Force Materiel Command (AFMC) reports that the
satellite support systems are end-of-week rollover compliant, but that
they are not yet year 2000 compliant; (10) AFMC reports that these
systems are in the process of either being replaced or renovated and
tested; (11) according to AFMC, the ground support systems and user
component systems are both year 2000 and end-of-week rollover compliant;
(12) according to the Coast Guard Navigation Center, however, the
accuracy of navigation on some older GPS receivers may be severely
affected by the end-of-week rollover; (13) several activities are
ongoing to raise awareness among owners of older GPS receivers of the
upcoming end-of-week rollover problem; (14) the Coast Guard Navigation
Center has been assigned with responsibility of being the government
liaison to the civil sector for GPS; (15) its Internet website explains
the potential rollover problem on older receivers and provides an
extensive list of manufacturers and points-of-contact; (16) 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; (17) 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; and (18) 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.

--------------------------- Indexing Terms -----------------------------

 REPORTNUM:  T-AIMD-99-187
     TITLE:  Year 2000 Computing Challenge: Time Issues Affecting the
	     Global Positioning System
      DATE:  05/12/99
   SUBJECT:  Y2K
	     Systems conversions
	     Strategic information systems planning
	     Computer software verification and validation
	     Data integrity
	     Navigation aids
IDENTIFIER:  Y2K
	     NAVSTAR Global Positioning System

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ai99187t.book GAO

United States General Accounting Office

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 10 a.m. Wednesday, May 12,
1999

YEAR 2000 COMPUTING CHALLENGE

Time Issues Affecting the Global Positioning System

Statement of Keith A. Rhodes Technical Director, Office of
Computers and Telecommunications Accounting and Information
Management Division

GAO/T-AIMD-99-187

Page 1 GAO/T-AIMD-99-187

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.
1

Today, I will discuss these two important issues, their potential
impact, and the status of remedial efforts.

GPS, the Year 2000 Problem, and the End-of-Week Rollover Problem

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

1 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 2 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 useras 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 statuse.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 3 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 2 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.

2 Distance from a receiver to the satellites.

Page 4 GAO/T-AIMD-99-187

Activities Ongoing to Raise Awareness of Problem With Older
Receivers

Several activities are ongoing to raise awareness among owners of
older GPS receivers of the upcoming end-of-week rollover problem.
The U.S. Coast Guard Navigation Center has been assigned
responsibility for being the government liaison to the civil
sector for GPS. Its Internet website 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 3 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.

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

Page 5 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 have.

(511155) Lett er

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