General Says Fixing LightSquared Interference to Military GPS Ops Could Take 10 Years
September 15, 2011
WASHINGTON: It could take a decade or more for LightSquared
to mitigate the impact of its proposed network on military global positioning
systems, a general testified before lawmakers today.
Gen. William Shelton also told members of the House Armed Services Strategic
Forces subcommittee the filters most recently proposed by LightSquared to
eliminate interference with high-precision GPS receivers could cause other
“We believe the precision of those receivers would be impacted in the presence
of that filter,” said Shelton, commander of the U.S. Air Force Space Command.
“There is a center frequency and harmonics that go out... Clipping off those
harmonics decreases the accuracy of those receivers. If there’s something else
magic out there, we don’t know.”
Shelton was one of five witnesses at the hearing entitled “Sustaining GPS for
National Security.” All were from various federal agencies involved in vetting
LightSquared, the proposed 4G wireless broadband network using satellite and
terrestrial components in L-band spectrum, adjacent to GPS operations. Tests
convened earlier this year indicated LightSquared interfered with GPS
receivers. LightSquared offered a modified plan that it said left only
high-precision receivers affected, and offered to make filters for those.
Most of the questions were directed at Shelton, who was skeptical about the
filter fix, as were members of the subcommittee. Its chairman, Michael Turner
(R-Ohio), asked Shelton how it would work, though both had clearly discussed
the matter in a previous classified briefing.
“The filters are something that you would have to do, not that they would
have to do, right? It doesn’t go on LightSquared’s system, it goes on your
system?” he asked Shelton.
“Every precision receiver would have to be retrofitted,” the general replied.
“How that might affect the overall platform that it’s on, is an unknown.”
Turner continued. “And anytime you’re modifying these systems, you add the
issue of vulnerability to the systems and all type of unintended consequences
that we can’t be certain of, including the enormous cost that you would be
“Enormous cost, time, integration, testing to thoroughly ring out these filters
if they’re technically feasible. And even with that,” Shelton said, “we believe
that the precision of those receivers would be impacted even in the presence of
He told subcommittee ranking member Loretta Sanchez (D-Calif.) that the cost
would likely run into the billions, and that it would take “a decade ore more”
to affect a fix.
A fix for which there should be no need, as far as LightSquared is concerned. In
its modified proposal with the Federal Communications Commission filed in June,
LightSquared blasted GPS manufacturers for not building receivers with
sufficient rejection capacity. LightSquared said GPS manufacturers did so in
violation of FCC policy. Shelton testified otherwise.
“The frequency bands we’re talking about here, by FCC rulings in the past
has always been intended to be a ‘quiet neighborhood’ so that GPS could coexist
with other signals of the same magnitude,” Shelton told the chairman. “GPS is a
very weak signal coming from space.... If you add signals of a similar strength
to GPS, that’s not a problem for the receiver. However, if you put a rock band
in that very quiet neighborhood, it’s a very different sort of circumstance.”
In his written testimony, the general said LightSquared’s proposed network for
40,000 terrestrial transmitters would produced received signal strength 5
billion times stronger than a GPS signal.
The “quiet neighborhood” Shelton mentioned referred to the original usage
designation of LightSquared’s spectrum--that of Mobile Satellite Service. Eight
years ago, the FCC added a provision allowing MSS providers to integrate an
Ancillary Terrestrial Component to their networks. The ATC was supposed to fill
in gaps in satellite coverage, which was to remain the primary form of service.
In 2004, the commission granted ATC authority for spectrum licenses that later
came under LightSquared’s purview. LightSquared asked for a modification of the
ATC integration rule in November 2010 so it could deploy terrestrial-only devices
on its spectrum, essentially using satellite coverage to fill in ground-based
gaps. The FCC gave its approval on the condition that the network did not
interfere with GPS operations. The commission reiterated its position yesterday
in a Public Notice calling for more testing of LightSquared’s systems. (
See “FCC Delays
Shelton said the regulatory circumnavigation caught the Air Force “off guard.”
He testified that more than 100 types of high-precision GPS receivers were
tested at Holloman Air Force Base in New Mexico in April, including 29 military
devices used in ground- and air-based missions, weapons and unmanned aerial
“The test results demonstrated empirically that the LightSquared signals
interfere with all of the types of receivers in the test,” Shelton said in
Aviation GPS receivers operating 7.5 miles from LightSquared transmitters lost
signal, with degradation as far out as 16.5 miles, he said. High-precision
geological survey receivers picked up interference as far away as 213 miles and
lost signal at 4.8 miles.
The tests primarily were based on LightSquared’s original proposal rather than
the June modification. The June plan calls for the network to launch on the
lower 10 MHz of its spectrum--not adjacent to GPS--at half the originally
proposed power. Shelton said limited tests were conducted on the lower 10 MHz
that showed unacceptable interference to the 33 devices used for testing. However,
he said, those limited tests did not “constitute a sufficient evaluation of
LightSquared’s deployment plan.
~ Deborah D. McAdams