Additional government testing of GPS receivers for interference from LightSquared’s broadband-wireless network under a revised deployment plan is getting under way at Holloman AFB, N.M.

The U.S. Federal Communications Commission (FCC) ordered the additional work after tests confirmed there would be severe GPS interference from the original deployment plan, but indicated the problem would be reduced significantly if LightSquared used only the lower of its two frequency bands, the one farthest from the GPS satellite signal.

The new tests, being conducted in an anechoic chamber at Holloman, are intended to determine whether widely used navigation and cell phone GPS receivers are still susceptible to overload interference from LightSquared’s revised lower-power, lower-frequency terrestrial transmitters.

The FAA has issued a notice to airmen warning of GPS tests being staged between Oct. 24 and Nov. 18 near Alamogordo, N.M. The agency says GPS signals “may be unreliable or unavailable” within a radius of 349 nm at Flight Level 400, decreasing to 185 nm at 50 ft. above ground level.

Testing is to be completed by Nov. 30 and is critical to LightSquared’s business plans as the FCC has said it will not approve commercial deployment of the company’s terrestrial network until GPS interference issues have been resolved.

High-precision GPS receivers, which original tests showed would suffer unacceptable interference even from lower-band transmissions, are not being tested as LightSquared is still working with suppliers on development of antennas and filters to protect these devices, used in agriculture, construction and scientific sectors.

As the tests get under way, the war of words between LightSquared and the GPS community continues to escalate. LightSquared has accused GPS manufacturers of sloppy receiver design and of benefiting from an $18 billion subsidy through the government’s investment in GPS.

Responding, the Save Our GPS coalition claims LightSquared has benefited from a $10 billion “spectrum windfall” because it paid $2 billion for rights to satellite frequencies that would have been worth $12 billion had they been auctioned for terrestrial services, as per normal FCC practice.

LightSquared should pay the “huge cost” of modifying or replacing those receivers affected by interference from its transmitters, says Jim Kirkland, VP and general counsel for GPS manufacturer Trimble. “They should not be allowed to shift the cost to industry or the … taxpayer,” he says.

Although the initial, limited testing conducted in the lower band indicated that aircraft GPS receivers did not suffer significant interference from the lower-frequency transmissions, Kirkland claims the FAA is in “active dialogue” with LightSquared on “potential adverse impacts.”

The FAA will not comment, but Jeff Carlisle, LightSquared’s executive VP-regulatory affairs and public policy, says “The FAA is continuing to analyze our operation against the minimum standards for aircraft, and we are supportive of that process.”