Lori Ranson/Washington

A pledge by US broadband wholesale company LightSquared to use a different segment of its approved spectrum to develop a 4G network has failed to quell industry concerns about interference of GPS devices.

A waiver granted by the US Federal Communications Commission (FCC) to LightSquared in January to offer widespread terrestrial broadband service using a portion of the L-Band spectrum worried aircraft operators that the new network would render their GPS devices useless.

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

LightSquared has since acknowledged that one of its allotted 10MHz blocks could create GPS interference, and has proposed operating on a spectrum of the L-Band furthest away from the portion used by GPS receivers beginning at 1525MHz. It would also reduce the power used by its base station transmitters, it said.

But this has not weakened fierce opposition to its plans.

During recent testimony before the US Congress, Garmin vice-president of aviation engineering Philip Straub concluded that LightSquared's proposal of a power reduction is "immaterial as it does not affect the proposed deployed power level of +62dBm, which has been conclusively shown to cause harmful interference for many GPS receivers".

LightSpeed - (c) Boeing 
 © Boeing
 A waiver permits LightSquared to use part of the L-Band specturm

A study carried out by FAA adviser RTCA concluded that using a 5MHz-wide channel from 1526.3-1531.3MHz is "compatible with aviation GPS operations". RTCA said that further study is needed involving the 10MHz channel at 1526-1536.

However, a study carried out by the Brattle Group, for LightSquared, concluded that signals from LightSquared satellites do not cause the interference; rather, many existing GPS devices fail to adequately filter spectrum beyond allotted GPS bands.

"It is not the case that LightSquared transmissions are interfering with GPS spectrum, but rather GPS receivers are effectively utilising L-Band spectrum below the GPS spectrum allocation," said Brattle.

It proposes a retrofit of filters to GPS devices, although Straub dismissed the feasibility of this solution and warned that a filter would also degrade the performance of GPS receivers.

Source: Flight International