The LightSquared Kerfuffle
We had a great discussion about the ugly problem of GPS interference last week and why it can’t be oversimplified. Yesterday, venture capitalist Philip Falcone – a major financial 
backer of wireless startup LightSquared – slammed the GPS coalition for opposing LightSquared’s plans to use satellite spectrum for terrestrial wireless broadband. Falcone reiterated the same talking points used by LightSquared regarding GPS devices lacking the proper filtering, and GPS device misuse of spectrum that was allocated to LightSquared. The problem, however, is that he didn’t address the fundamental issue of what we’re going to do about all those GPS devices that will experience interference.
First let us summarize the undisputed facts of this debate:
- There are 500 million GPS receivers in this country.
- GPS receivers lack adequate filtering systems because they weren’t required to have them.
- LightSquared’s proposed terrestrial transmitters (if approved by the FCC) will interfere with those millions of GPS devices. This includes GPS receivers used in aviation.
- It will be extremely expensive to replace the existing GPS receivers, and no one is volunteering to write the check.
Conflicting assertions from some major players:
- If LightSquared switched to using its lower frequencies, the company claims that “only” 200,000 GPS receivers will be affected.
- The United States Global Positioning System Industry Council (USGIC) says: “There is no question, however, that many types of GPS receivers and GPS-dependent applications will suffer harmful interference from LightSquared’s operations in the lower channel, even when transmission power is limited to one-tenth the authorized limit that was used in the testing”.
- Extensive testing has not been done on the newly proposed spectrum in the lower block.
- Even if the 200,000 claim is true, no one is offering to write a check to pay for the replacement of those 200,000 receivers.
The engineer’s view on filters:
- It’s not just the problem of replacing GPS receivers, it may not even be possible to build a good enough filter.
- The problem with real-world radio filters is that they can only reduce noise. When they do reduce the noise, they also reduce the passband signal which is needed by the GPS receiver.
- I have asked many engineers if it is possible to build a filter that can filter out a neighboring signal that is a billion times stronger than the GPS signal and do it in a way that doesn’t make the GPS signal unusable. The only response I have received from engineers is that such a filter is not possible.
- Even if such a filter existed – and it was affordable – how would we address the problem of replacing the existing GPS receivers?
The reality, therefore, is not nearly as simple as Mr. Falcone would have us believe, and there will be GPS interference no matter what portion of the spectrum LightSquared uses. Given this harsh reality, it’s unclear how anyone – even those sympathetic to the company – can support LightSquared’s plans.
[Cross-posted at High Tech Forum]
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