[anrope]

The title of this article has it backwards. LightSquared's technology is entirely scientifically feasible.

The problem is political-ish, and the U.S. government and GPS industry carry a good portion of the blame:

The way that spectrum is allocated in the U.S., the chunk of spectrum that LightSquared got just happens to be designated as low-power, space-ground. Or at least, that's what their neighbors (GPS) are doing.

According to LightSquared, GPS makers have been lazy in their filter design, under the assumption that they wouldn't have to deal with much noise from neighbors on the spectrum.

I can understand saving cost by not designing really aggressive filters, but it would be sad to see an innovative idea like LightSquared be shut down because the GPS people don't want to clean their act up.

I hope LightSquared can trade for another chunk of spectrum that will allow them to move forward. This is really cool technology, and a great step forward.

[pithon]

GPS makers have been designing their receivers under the rules the FCC has set forth for decades, that the bands surrounding GPS are for sky level signals. I wouldn't call that "lazy".

Imagine if the NTB suddenly decided that it would be OK for people to drive tanks at 1000 MPH on existing highways. Would you call carmakers "lazy" because cars currently on the road wouldn't be safe to drive anymore? Or would it be OK to tell hundreds of millions of people to replace their cars immediately at exorbitantly higher cost?

[voodoomagicman]

It sounds like it is not so cut and dry. From the article:

Michael Marcus, who worked at the FCC for 25 years and is now a consultant on wireless technology and spectrum policy, writes that cellular base stations have been "allowed next to the GPS band since 2003" under FCC policy, yet GPS makers "paid little attention to the fact that GPS would be having a new neighbor with much stronger signals in some places than the original MSS [mobile satellite services] signals."

Marcus, who does some consulting work for LightSquared, further writes that the "GPS industry has not pressed the filter manufacturers for the latest technology," and "as a result many GPS receivers have a lingering vulnerability to strong adjacent band signals that results from GPS manufacturers ignoring policy changes made in the US almost a decade ago."

[gregatragenet]

The quote from the consultant for LightSquared is taken out of context or just plain wrong. i.e. this statement:

cellular base stations have been "allowed next to the GPS band since 2003"

The FCC started allowing an "Ancillary Terrestrial Component (ATC)" specifically to supplement areas where satellite signals may have dead zones due to, say, dense urban areas. This would also be a low-power signal. It was put in place for things like Sirius/XM satellite radio to have acceptable performance in urban areas.

ATC is intended to (1) be a small supplement to a satellite-based service and (2) have a comparably low-power signal to the signal provided by the satellites.

Once LightSquared came along and got waivers so that (1) their whole 'satellite' spectrum could be terrestrial, and (2) could run much higher powered terrestrial stations (power levels similar to cellular towers) the amount of power they were radiating for their network eclipsed the weak signals GPS units depend on to function. The design and filtering for GPS units is fine for the adjacent spectrum to be used for similar systems/services as allocated (until the last 18 months).

[CamperBob]

Unfortunately, for some technical reasons that are too arcane to get into here, extremely sharp filters are not suitable for GPS receivers.

It would have been better for everyone, including LightSquared, if the political appointees at the FCC had not tried to simultaneously ignore international frequency-allocation treaties, disregard the advice of their own engineering-educated colleagues, and break the laws of physics.

[anrope]

You're right, maybe lazy wasn't the right word.

Still, the reasons for interference are more political (FCC rules), than scientific.

There's only so much usable electromagnetic spectrum out there, so neighbors need to play nicely together. Especially when your neighbor is trying to do something awesome.

[gvb]

Well, the requirement for GPS was that they be able to filter out adjacent low power level signals (signals from space).

LightSquared is now arguing that changing the requirement to be able to filter out high power (terrestrial) signals is reasonable, and it is reasonable to retroactively apply that requirement against the millions of existing GPS receivers that were designed to meet a different requirement and thus will have to be discarded.

I would have a lot less heartache over the requirements change if LightSquared would be willing to bear the cost of their proposed change of requirements, rather than forcing me to bear the costs. This means they would need to replace my GPS receivers (a car navigation unit, a boat navigation unit, three cell phones, and an iPad). A quick mental summation is about $2,500, and that is pretty cheap compared to a lot of people out there.

[Czarnian]

You think that's expensive, think of all the military gps units that would need to be replaced. Do you have any idea how much one military-grade gps unit costs? Probably more than all the civilian gps units on your whole block combined. And just about every military vehicle in production has at least one. Aircraft have 2-3. Then there are the commercial gps units in airplanes, which also have redundancy requirements.

There is no chance that LightSquared will get their way. The cost to the military (taxpayers) would be stupendous.

[kronusaturn]

I would assume that part of the reason military units are so expensive is that they already have high quality RF filters for jamming resistance.

[CamperBob]

Such filters would be of no use against deliberate in-band interference.