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LightSquared offers low-power olive branch to GPS

But in a few years we'll turn the knob up again

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LightSquared, the firm which so upset GPS users with their 4G-at-satellite-frequencies plan, has offered to reduce the interference by dialling back the transmission power and clear an exclusive band for precision GPS.

The wannabe network operator has already offered to shuffle away from the Global Positioning System, promising to keep itself to its lower band for the next few years, but now reckons it can mitigate the remaining problem by reducing the base-station transmission power to -30dBm, though that will scale back up to -24dBm by 2017 when a better solution will be needed.

The full proposal (pdf) has been filed with the FCC, but bearing in mind that the decibel scale is logarithmic; one step towards zero doubles the transmission power, it's a significant concession and means LightSquared will need a lot more base stations to get national coverage. But it is also a temporary concession which relies on better filters and antennas being developed, and deployed, before the power gets ramped up again.

For precision applications, which today use the LightSquared (satellite) signal to augment GPS where high accuracy is needed within the USA (such as guiding tractors and such), Light Squared is proposing to reserve 4MHz of bandwidth starting at 1555MHz - beside the existing GPS band and well clear of the proposed LTE network.

Today those precision applications can expect the signal to be anywhere between 1525 and 1559MHz, so the allocation of a specific band shouldn't be a big deal.

But that's a side issue: the problem remains that a huge number of GPS devices will be knocked out by LightSquared's proposed network. Even at the reduced power some devices will be affected when within 500m of a mast, and that power reduction will only last five years or so.

LightSquared contends there are already two manufacturers making filters capable of focusing a GPS receiver well enough to ignore the neighbouring LTE network it is proposing to build, and that better technology is coming all the time. Reading between the lines it seems the company is prepared to offer financial restitution of some sort, though it continues to argue that its really up to the manufacturers of the GPS equipment to make sure it doesn't pick up neighbouring transmissions.

The plan has received he traditionally-cautious welcome, with the director for the National Coordination Office for Space Based Positioning, Navigation and Timing telling Inside GNSS magazine "If the power is low enough that fixes the problem ... I don't know if their number is low enough".

Even then he's not pepared to commit: "Anything they do I still want to test ... it is a creative proposal, and we'll seriously consider it".

Demonstrating the complexity of the issue last week we saw the UK's Ordnance Survey blogging its concerns about LTE networks interfering with GPS.

"Will 4G Mobile Broadband Spell The End Of GPS?" asks the headline, despite the UK's 4G being deployed in entirely different bands (800MHz and 2.6GHz) and Ofcom recognising very early the potential for rezoning of frequencies reserved for satellite ground components (the trick which has made Light Squared viable in the USA) and pricing them accordingly (making a UK Light Squared commercially difficult).

Light Squared is very definitely a US problem, and one that isn't going to go away. Compromise offerings are to be welcomed, but the GPS industry has yet to coherently respond with anything other than intransigent calls for the whole plan to be scrapped. ®

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