Opinions Differ on Mitigation Options for LightSquared Interference
WASHINGTON — If startup firmis allowed to deploy its hybrid satellite-terrestrial communications network, any interference caused to GPS signals could not be mitigated by modifications to GPS receivers, a GPS hardware manufacturer said May 18.
The issue at hand is whether wireless terrestrial broadband communications — a high-power application — can exist in a frequency band adjacent to one used for the low-power application of space-based position, navigation and timing, namely GPS. Jim Kirkland, general counsel for Trimble Navigation of Sunnyvale, Calif., said clever engineering alone cannot solve the interference problem and thus GPS and the LightSquared network as currently envisioned cannot coexist.
That notion is disputed by Reston, Va.-based LightSquared, which already has spent hundreds of millions of dollars launching a satellite and leasing spectrum from another firm with the intent to begin operating its network by next year.
“When there are statements that no mitigation is possible on the receiver side, well we know that’s not true sitting here today,” said Jeff Carlisle, LightSquared’s vice president of regulatory and public affairs. “We know certain companies have designed perfectly resilient receivers … where no interference is going to occur.”
LightSquared and its predecessor companies have been crafting a plan since 2001 to deploy a satellite-terrestrial communications network. The firm in 2005 was granted authority to deploy an unlimited number of 1.6-kilowatt ground stations across the country by the Federal Communications Commission (FCC), which included provisions meant to ensure users would only use the terrestrial network where the satellite service was unavailable. The company was prohibited from selling smartphones that could only access its terrestrial network. The FCC in 2010 increased the maximum allowable ground station power to 15.8 kilowatts.
In November, LightSquared’s new owners, Harbinger Capital Partners of New York, submitted a new proposal to the FCC to deploy as many as 40,000 ground stations across the nation. It also sought to sell, via its resellers, terrestrial-only smartphones alongside its satellite phones and dual-mode phones.
The FCC granted LightSquared a provisional waiver in January. Among the stipulations, LightSquared would not be allowed to sell terrestrial-only phones for significantly cheaper than dual-mode phones. The company also would have to prove that its network would not interfere with GPS receivers, which are tuned to listen for faint signals in frequencies close to those of the LightSquared network.
The FCC chartered a Technical Working Group of government and industry experts to study the interference issue. The group, which convened in March, is conducting laboratory and field tests on many different types of GPS receivers that will continue through the end May, Carlisle said at an event here hosted by the Center for Strategic and International Studies. The group’s final report is due to the FCC by June 15, which will then have to decide whether to allow LightSquared to deploy its ground stations.
Meanwhile, the U.S. Air Force recently completed field testing at Kirtland Air Force Base, N.M., using LightSquared hardware and a number of different military, civilian and commercial GPS receivers. Preliminary results of this testing indicate that the LightSquared network could interfere with all types of GPS users, Air Force Gen. William Shelton, commander of Air Force Space Command, told the Senate Armed Services Committee May 11.
Kirkland said the GPS community had long believed that LightSquared’s network would be disruptive to GPS receivers, but it would impact far fewer users if it was still planned to be a primarily satellite-based service. The most recently proposed plan from LightSquared is a “sea change” that flips the original business model upside down, making it a primarily terrestrial-based network, which the FCC has consistently sought to prevent, he said.
“I can’t say it often enough that if you look at the FCC’s orders, they consistently say this will be a primarily satellite[-based] service,” Kirkland said. “There’s a very good reason for that, which is that this billion-times power differential [between received GPS signals and transmitted LightSquared terrestrial signals] creates a physics problem, not an engineering problem.”
Carlisle believes that an adequate set of solutions to the interference problem can be found if both sides come to the table with an open mind. LightSquared, for example, will install filters on its ground stations to ensure they do not broadcast any signal at GPS frequencies. The problem is that certain GPS receivers are tuned to look for signals outside of the range of GPS frequencies and depending on many factors, such as distance from a ground station, the receivers could be interfered with, Carlisle said in a May 19 interview. The solution is to better design future GPS receivers or outfit them with filters that block out LightSquared frequencies. It could also be possible to retrofit some existing GPS receivers with filters, though it may be cost-prohibitive for many consumer GPS devices such as cell phones that are typically replaced at regular intervals, Carlisle said.
“We know going forward that it’s perfectly possible to design front ends and implement filters in a way that will allow these receivers to look at just the GPS band like they’re supposed to,” Carlisle said. “On existing units, that is no doubt a more difficult issue. The question is how many of them are there and is it economically feasible.
“If the most efficient and fair solution is not to do a receiver-side solution, we haven’t taken any of the possible transmission solutions off the table. We can look at ways of implementing our service, how we do it, when we do it, using what spectrum. We can talk about all of that with the Technical Working Group. But I think you do need to come to the table with an open mind and talk about what’s possible.”