A pledge by broadband wholesale company LightSquared not to use part of its approved spectrum for a proposed 4G network has done little to sway fierce opposition from the aviation industry that LightSquared's framework will not cause severe interference of GPS-related devices.

In January 2011 the US Federal Communication Commission (FCC) granted a waiver to LightSquared to offer widespread terrestrial broadband service in a portion of the L-Band not previously allowed.

The waiver permitted LightSquared to use L-Band frequencies of 1525-1559 MHz, which borders the 1559-1620 MHz allotted to GPS devices, including those used onboard aircraft.

Concern stems from how the GPS system is constructed, said Air Transport Association of America (ATA) senior vice-president of safety, security and operations Tom Hendricks during 23 July testimony before the US Congress. Signals are generated from a constellation of satellites 20,277km (12,600miles) about the earth. GPS satellite transmitters operate a low power levels, and when combined with the long-distance travelled by GPS signals, create a need for extremely sensitive GPS receivers to pick up the weak signals.

"GPS receivers of all types are not designed to exclude such strong signals, something never contemplated before LightSquared sought its waiver to offer its new terrestrial broadband service in an area of the spectrum historically reserved for weak space-to-earth signals" said Garmin vice-president of aviation engineering Philip Straub, who joined Hendricks in addressing Congress.

A study carried-out by FAA advisory non-profit RTCA examined LightSquare's proposed signal strength - and its planned three phase deployment - on aircraft in various phases of flight including high and low altitude cruise, precision approach and airport service operations. RTCA concluded that LightSquared deployment in its first phase, which is closest to the 1559MHz GPS band, "is expected to be complete loss of GPS receiver function".

On 20 June LightSquared unveiled plans to only operate in the 10Mhz lower band of its allotment from the FCC that begins at 1525MHz, "which is farthest away from the L1 band used by GPS operators", said company executive vice-president of regulatory affairs Jeffrey Carlisle on 23 June.

RTCA is its analysis concluded that terrestrial base station operations at the lower 5MHz of the LightSquared spectrum "is compatible with GPS operations". However, it stated further study is needed for a deployment in the lower 10MHz band LightSquared is proposing. RTCA concluded its study prior to LightSquared proposing the MHz adjustment.

But Hendricks of ATA stressed while LightSquared's latest proposal "may be feasible it is fraught with technical challenges not yet fully understood. Significant research and modelling is required to fully define this mitigation and conclusively prove whether it would achieve the desired effect".

Source: Air Transport Intelligence news