Current U.S. plans to upgrade the global positioning system (GPS) navigation
satellite constellation include both the replacement of aging spacecraft and
enhancements to the signals transmitted to Earth.
Users of GPS, civilian and military, rely on the radio signals for timing and
positioning information. Military systems advertise 16-meter accuracy for its
GPS receivers, but today the technology approaches the "single digits,"
said Air Force Col. Peter F. Hoene, deputy system program director at the GPS
Joint Program Office.
The United States will spend $875 million during the next five years to upgrade
the GPS constellation, currently of 27 satellites, and to provide a stronger
military-only signal that would be less vulnerable to enemy interference than
the weaker civilian signal.
During a recent conference of the Precision Strike Association in Fort Belvoir,
Va., Hoene said the government's goal is to make GPS more accessible to civilians
but also more robust for military applications.
Last month, the Clinton administration directed the GPS office to turn off
the current military-only frequency, called "selective availability"
and offer it to civilians.
This means the Defense Department will stop degrading the GPS signal available
to the public, which should improve the accuracy of civilian receivers up to
tenfold.
Navy Vice Adm. Herbert A. Browne, deputy commander of U.S. Space Command, recently
told reporters that the information technology industry is likely to benefit
the most from the more accurate civilian signal because of the timing precision
that is now available. He noted, however, that the military still has the ability
to prevent an adversary from using that signal, if desired.
Ten years ago, he said, "all of us [in uniform] opposed doing away with
selective availability. But we all endorse it today."
Military applications of GPS, meanwhile, are driving the Pentagon's long-term
plan to make the signals more jam-proof, replace some of the older satellites
and upgrade the receivers used in ships, aircraft, land vehicles and those employed
by foot soldiers.
By 2002, civilian users will have access to two frequencies from the two signals
currently in use, the L1 and L2. A new signal for civilians, called L5, will
be a "safety-of-life" signal. It will be similar in structure to the
current military code and will be approximately four times stronger than the
L1 signal. The L5 signal will be implemented on the modified Block IIF satellites,
with the first launch in 2005.
For the military, there will be a new M-code line, with increased power and
the ability to jam enemy use. The current 27-satellite constellation consists
of seven Block IIs, 18 Block IIAs, and two Block IIRs.
There were three launches planned for 2000. The first one was scheduled last
month, the second is planned for June, and the third in December. The satellites
to be launched are basic, not modified, IIRs.
Twelve Block IIRs will be modified by Lockheed Martin Missiles and Space, Valley
Forge, Pa. This effort will add a new code on L2, the second civil signal, and
the military M-code signal on L1 and L2. The first of the modified Block IIRs
are scheduled to launch in 2003 and the last is planned for 2006.
The Boeing Company, in Seal Beach, Calif., will modify six Block IIF satellites
currently in production, and up to six more IIF satellites, if purchased. Boeing
will add a civilian code on the L2 signal, the M-code signal on L1 and L2, and
a new civil signal on L5. The first of the 12 modified Block IIFs are planned
for launch between 2005 and 2010.
By 2016, there will be 18 M-code satellites in operation. "We expect the
anti-jam resistance [will be] much greater" in these signals, Hoene said.
Looking ahead, the joint program office is working on concepts for a new program
called GPS III, he said. This program aims to project "the next 30 years
of GPS."
In the short term, the Defense Department wants all GPS receivers used by the
military services to be upgraded with the so-called SAASM technology. SAASM
is the selective available and anti-spoofing module. It will become mandatory
by October 2002. Any system that is not upgraded by that time, said Hoene, will
need a special waiver from the office of the Joint Chiefs of Staff.
SAASM is designed to provide additional anti-jam protection.
One of the dilemmas that will confront users, Hoene said, is how to schedule
their equipment upgrades. The first M-code production receivers will not be
available until 2008. So program managers will have to decide whether to convert
to SASSM in 2002 and again to the M-code in 2008.
"We may accelerate production of some M-code receivers that will be "day-one
platforms" for critical platforms that "really want the M-code capability,"
he added.
To expedite and minimize the cost of upgrading receivers, there is a product
being developed called GRAM-SAASM. That essentially is a "GPS on a card"
that includes receiver and security devices all in one piece of hardware. It
is designed to support all platforms, Hoene said.
The prime contractor for the M-code upgrade work is Lockheed Martin Space Systems,
in Sunnyvale, Calif. A major subcontractor is ITT Aerospace, of Fort Wayne,
Ind. The company developed a software-based technology called waveform generator,
which allows GPS to put several signals on a single carrier wave-thus allowing
for a new military and a new civilian channel.
The technology also enables changes in encryption from the ground by sending
new algorithms up to the satellite, explained ITT spokesman John Kirkwood. The
waveform generator is accompanied by a power amplifier, which adds power to
any signal seeking to override jamming signals. The power amplifier is controlled
through the waveform generator.
One of the largest purchasers of new receivers in the near future will be the
U.S. Army. The service plans to buy 137,000 defense advanced GPS receivers,
known as DAGR. These will be SAASM-compliant.
Under the DAGR 2000 program, the Army will receive card replacement units that
will include the M-code upgrade, in order to avoid replacing the entire box.
Today, the mainstay of user equipment in the Army is the PLGR (precision lightweight
GPS receiver), said Lt. Col. George Eveland, Army product manager for GPS. There
are 70,000 fielded. "Not all the force currently is GPS-equipped,"
said Eveland in an interview. "One reason we want to go to a new receiver
is to free up PLGRs, which will go to the portion of the Army that currently
is not GPS-equipped."
The Army plans to introduce the new receivers by 2002, said Eveland.
As back-up to GPS-depending on the application-the Army uses Doppler-based
systems and inertial systems, especially in aviation. But by and large, said
Eveland, "GPS is a reliable system."
The PLGR doesn't have any specific anti-jam capabilities other than filtering,
said Eveland, but the DAGR has additional protection against jamming.
PLGR works on the L1 frequency and DAGR will work both on L1 and L2. The plan
is to buy the DAGRs and then transition to an M-code receiver when it becomes
available in about 2009.
The Army DAGR procurement is worth about $300 million, said Eveland. Other
platform managers also are buying GPS receivers in the form of GRAM-SAASM cards.
Because the same receivers will be used by all services, the joint program office
makes purchases of user equipment. Waivers can be requested, however, for service-unique equipment.