Satellite communications provider Inmarsat and aviation standards organizations have been working on a solution to a lesser-publicized aspect of the planned LightSquared broadband wireless network–the potential for the system’s high-power transmitters to interfere with satcom as well as GPS navigation signals.
RTCA Special Committee 222 is involved in defining minimum performance standards for an antenna diplexer with the filtering necessary to prevent LightSquared base stations transmitting at L-band frequencies (1,525 to 1,559 MHz) from interfering with reception of Inmarsat aeronautical service in the same band. Also involved is the Arinc AEEC Air Ground Communication System subcommittee, developing the engineering specification for a diplexer/low-noise amplifier, a satcom subsystem that separates transmit and receive signals and amplifies the received signal.
The standards work was set in motion by events over the past seven years. In 2004, the Federal Communications Commission (FCC) issued a ruling that spectrum previously restricted to satellite use could be reused on the ground. The FCC action drove a spectrum-sharing agreement between Inmarsat and LightSquared’s predecessor company.
“Inmarsat had historically opposed this, but with the FCC ruling, we then came to this cooperation with LightSquared, who were looking at deploying” a terrestrial network, said David Kingdon Jones, Inmarsat director of product and service management. “So at least we had the ability to provide support to our customer base and to have it done in a coordinated manner.”
Inmarsat and SkyTerra Communications, now LightSquared, signed a cooperation agreement in December 2007 to enable “re-banding and efficient reuse” of L-band spectrum covering North America. According to the parties, the agreement was designed to “increase the contiguous spectrum” available to support deployment of LightSquared’s ground infrastructure.
The agreement between Inmarsat and present-day LightSquared precipitated work by RTCA and Arinc, traditional entry points in the development of aviation standards. “Once we knew essentially what the band plan was and what the frequency assignments were going to be, we could work with our avionics community and aviation community in order to define a standard to provide the necessary protection” against interference, Kingdon Jones said.
The diplexer, a common component of satcom systems, is being designed to Arinc 781, the latest industry standard for Inmarsat satcom capability, including SwiftBroadband. Kingdon Jones said Inmarsat has contracted a leading supplier, Com Dev International, of Cambridge, Ontario, to develop the diplexer in advance of direction from the standards organizations.
Preventing possible interference from LightSquared transmissions will require some but not all aircraft operators to change out current satcom diplexers. Satcom system configurations on some aircraft enable a fix “back in the RF line” in the radio subsystem, Kingdon Jones said.
“One of the uncertainties is what are the operational scenarios under which an end customer would require this,” Kingdon Jones said. “What we do know is the equipment spec we’ve got to work to. … The most likely situation that a customer could be affected (by LightSquared transmissions) is where they have their aircraft on the ground, and a base station is pointing directly at that aircraft near an airport.” There also are circumstances with older, legacy satcom equipment where interference could happen while airborne, he said.
John Broughton, vice president of product development for EMS Aviation, a manufacturer of Inmarsat high- and intermediate-gain antennas, concurred that potential GPS interference from LightSquared transmissions poses the greater threat to aviation. But he also noted the potential of the LightSquared system interfering with satcom and the need for new equipment to counter that possibility.
“What we’re looking at here is a change in the filter specs on that diplexer,” Broughton said. “It’s potentially a new device, but I don’t want to cause a panic in the streets because the scenarios in which LightSquared is going to interfere with Inmarsat receivers in a way that matters [involves] a relatively small number of people, and there’s a time-phasing [aspect] as well. And this is not an issue anywhere else but in the United States.”
Jeff Carlisle, LightSquared executive vice president of regulatory affairs, said the potential of satcom interference does not cause the same concern as potential GPS interference, “largely because there has been a longstanding process to develop the standards for diplexers on this, so Inmarsat can have the standards it needs to go through and make those changeouts. … We signed the cooperation agreement at the end of 2007, so they have been planning on this transition since then.”
In August 2010, Inmarsat announced the start of Phase 1 of the cooperation agreement with LightSquared. Inmarsat said it would immediately transition to a modified spectrum plan to increase spectrum contiguity, an 18-month process involving certain network modifications. In return, LightSquared would make a series of payments to Inmarsat totaling $337 million.
Inmarsat announced Phase 2 in January this year. Under the second phase, “Inmarsat will support a spectrum plan that increases the total capacity available through the LightSquared network.” In return, LightSquared will make payments to Inmarsat of $115 million annually, with a five-year minimum commitment period. o