LightSquared’s spectrum concessions do little to sway GPS advocates

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A pledge by broadband wholesale US 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 of this year to offer widespread terrestrial broadband service using a portion of the L-Band spectrum roiled the GPS industry, and raised concern among aircraft operators that the new network would render their GPS devices useless.

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. The company was also granted permission to use a higher power setting to broadcast signals from its satellites.

GPS signals are generated from a constellation of satellites 20,277km (12,600 miles) about the earth. The Air Transport Association of America (ATA) has stressed GPS satellite transmitters operate at low power levels, and when combined with the long-distance travelled by GPS signals, create a need for sensitive GPS receivers to pick up the weak signals.

LightSquared has since acknowledged that one of its allotted 10 MHz blocks could create GPS interference, and has proposed operating on a spectrum of the L-Band farthest away from the portion used by GPS receivers beginning at 1525 MHz. LightSquared also said it would modify its FCC license to reduce the maximum authorised power of its base station transmitters by over 50%.

But those concessions have not weakened fierce opposition to LightSquared's plans.

During recent testimony before the US Congress Garmin vice-president of aviation engineering Philip Straub concluded 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".

The Coalition to Save Our GPS dismissed the LightSquared offer to operate from a lower block of frequencies in the L-Band spectrum, stressing that only limited test results for low band operations are available.

A study carried out by non-profit, FAA advisor RTCA concluded that using a 5MHz-wide channel from 1526.3-1531.3 MHz is "compatible with aviation GPS operations for all the representative scenarios".

However, RTCA stated further study is needed of a LightSquared deployment scenario involving the 10MHz channel at 1526-1536, which is the span LightSquared is now proposing to operate in.

LightSquared sponsored its own study carried out by the Brattle Group concluding 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.

Brattle also argued that by not deploying receivers with sufficient filters, the GPS industry has enjoyed an advantage of lower costs at the expense of new L-Band users by tying up at least 20MHz of spectrum outside the GPS allocation.

While traditionally GPS receivers have been able to function with filters that do not adequately filter all radio frequencies neighbouring the spectrum allotted to GPS, Brattle said given time, "there is a solution for the GPS industry to remedy their interference problem by installing more precise filters into GPS receivers used in the US".

Two groups of GPS devices exist, said Brattle, those needing replacement during the next several years and permanent devices requiring retrofit. For those needing replacement during the normal course of business, Brattle stated the added cost of an improved filter is $0.30.

But the cost for retrofit is obviously higher, and Brattle said based on 224,475 active general aviation aircraft in the US using a wide range of GPS applications, "numerous estimates would be necessary to determine the cost".

Straub of Garmin stressed no filters are available to protect GPS signals and the aviation community has not bee able to conduct testing since no prototype is available.

However, Straub warned that to achieve the high level of rejection required to counteract the high-powered LightSquared transmissions from the GPS receiver, "any filter must potentially reject some of the GPS signal as well. Improving out-of-band signal rejection comes at the cost of other performance requirements that are critical to the operation of the GPS receiver".

The waiver granted to LightSquared by FCC was conditional on an analysis done by LightSquared and the industry to determine effects of the frequency award on land, air, space and sea GPS systems.

LightSquared received a two week extension from the FCC to publish the study, which should be available during the next few days.