What's to be done with GPS?
- By Gregory Slabodkin
- Apr 14, 1997
DOD plans for all military aircraft to use GPS for navigation by 2000 and the
military's growing dependence on GPS-guided smart bombs have heightened Pentagon concerns
about the vulnerability of the navigation system to year 2000 glitches.
First showcased during Operation Desert Storm, GPS has become the source for precise
and accurate targeting information for the Tomahawk cruise missile, Joint Direct Attack
Munition, Army Tactical Missile System and Joint Standoff Weapon.
"The most significant system today that is not [year 2000] compliant is GPS,
which would have more impact than anything else," Emmett Paige Jr., assistant
secretary of Defense for command, control, communications and intelligence, recently told
the House Government Reform and Oversight Subcommittee on Government Management,
Information and Technology. "Yet I have no doubt that GPS will be ready along with
all the other weapon systems and command and control systems in the Department of
The GPS year 2000 problem is threefold and reflects the three components of the
navigation system: the space segment, the ground control segment and the user segment.
GPS consists of 24 operational Navstar satellites mounted on six orbital planes that
continuously broadcast navigation signals to ground stations. Specialized computers built
into inexpensive, portable GPS receivers in turn derive highly accurate position and
velocity information by correlating data uploaded to the satellites from ground stations.
According to documents provided to Congress earlier this month by Paige's office, the
year 2000 problems within the space segment can be found in two pieces of ground
equipment: the Bus Ground Support Equipment vehicle checkout stations and the Boeing
Mission Operation Support Center (MOSC).
Software to correct the year 2000 problem in the Bus Ground Support Equipment
vehicle checkout stations already exists, and DOD will install it during the normal
systems maintenance lifecycle.
But the MOSC date code problem lies in its underlying commercial products. So DOD
will replace MOSC with the Integrated Mission Operation Support Center (IMOSC), which it
expects to finish in December 1999.
But GPS JPO is working to push the completion date up at least six months to June
1999. The IMOSC project is part of a $1.3 billion GPS Block IIF satellite contract that
DOD awarded to Boeing Co. last April.
The GPS ground control segment consists of six monitor stations, four ground
antennas and a master control station. The software needing date code fixes generates the
uplink code to the satellites. It was written in the 1970s and uses only two-digit date
The original plan was to replace the old code as part of a modernization of the
ground control segment, or Architecture Evolutionary Plan (AEP). But schedule delays have
pushed AEP's operational beginning to mid-2000.
"It looks like that schedule is slipping out," Reaser said. "The
replacement system may or may not be there in time for year 2000 rollover. So what we're
going to do is upgrade the current system to be Y2K-tolerant as sort of the backup plan
which may become our primary plan."
GPS JPO officials have decided to rewrite some of the existing legacy code for the
ground control segment at a cost of $7.6 million. Lockheed Martin Federal Systems has been
assessing the code and will rewrite it under an existing maintenance contract. Until then,
GPS JPO will incrementally integrate modifications as part of its normal software
Though the GPS user segment does not have a year 2000 problem per se, it does have a
clock overflow problem, the Z-count rollover. This rollover occurs every 1,024 weeks; the
first one comes in August 1999.
GPS' user segment consists of the antennas and receiver-processors that provide
positioning, velocity and precise timing to the users, such as the Army handheld GPS
receivers and Navy shipboard receivers. Although GPS JPO established specifications for
GPS receivers, some manufacturers did not account for the Z-count rollover in the
satellites' atomic clocks, which synchronize the navigational signals.
To date, the only GPS receivers that GPS JPO has identified as having the Z-count
rollover problem are older Rockwell-Collins 3A airborne receivers and 3S shipboard
receivers with Link 40 software.
As part of scheduled depot maintenance, users must manually reset affected
receivers. Those with flash memory or removable programmable read-only memory can be reset
easily to accommodate the rollover. Users will have to replace altogether any receivers
that cannot be reset.