LightSquared vs. GPS: Beware of simple thinking

Incredibly, would-be wireless provider, LightSquared, is still tilting at windmills. It continues to waste everyone's time in a vain effort to transmit terrestrial 4G signals on frequencies adjacent to those used for GPS satellites. Anyone with a decent understanding of the real, analog world would have known this will never work, as we'll see in The Long View...
(by @richi )

For several years, LightSquared has been planning and building a hybrid terrestrial and satellite-based 4G network. However, about nine months ago, the FCC ordered the company not to use the terrestrial component of its service, for fear of interfering with GPS signals. Unsurprisingly, LightSquared then embarked on a campaign of indignant lobbying.

But, in my opinion, LightSquared was dead wrong to assume it could use those frequencies without stomping all over GPS. Alexander Pope was right: A little learning is a dangerous thing. And this is important, because GPS has become a piece of critical infrastructure -- essential to our way of life.

LightSquare's argument is that its transmissions aren't within the GPS frequency bands. So it opines that any interference is the fault of badly-designed GPS receivers, not of its transmitters.

It's a physical certainty; it's the nature of radio modulation. The only question is the degree to which you'll interfere with the GPS signal.

You see, in the analog world, the actual bandwidth of a signal is infinite. Of course, the vast majority of the transmitted energy is at and around the carrier frequency, but lower power "sideband" transmissions will extend far above and below the intended range. Theoretically, infinitely far, but in practice, they're irrelevant once they become quiet enough to merge with the natural background radio noise.

This isn't normally a problem, because adjacent frequency bands are usually used at similar power levels. However, in this case, the difference in received power levels are typically 50 dBm or more. Typical received power levels near LightSquared towers are -70 dBm, whereas an outdoor GPS signal is just -120 dBm (dropping to perhaps -150 dBm indoors).

In other words, the LightSquared terrestrial power is roughly 100,000 a million times stronger than the weedy GPS signal received from the satellites 12,600 miles away (don't forget that dB scales are logarithmic). So adjacent sidebands are going to cause major interference in a receiver tuned to pick out a weak GPS signal.

What about filtering?
Can the situation be improved by adding more filters to the GPS units? Not much. The problem is that even the most expensive filters aren't perfect. They will reduce the power of the GPS transmissions received, which could be the difference between getting a satellite fix and being completely lost.

And now we read that LightSquared is complaining that a recent test used "old" GPS receivers. In other words, receivers that are less sensitive and less able to reject interference than units you can buy today. But this presumably reflects the receivers that are in-use today.

We're not just talking about cheap consumer units that might justifiably have a limited lifespan. You also have to consider units certified for use in light aircraft, which cost thousands or tens of thousands of dollars.

Conspiracy theory?
Bizarrely, LightSquared even appears to be accusing regulators of conspiring against it, calling the tests "rigged." However, that's bunkum; the failure of its loud signal to co-exist with GPS's weedy signal on nearby spectrum was utterly predictable.

I suppose you can't blame LightSquared for trying: It's probably got investors screaming at it to make this idea work. Sadly, it's practically an impossibility.

And what of the FCC? Why did the regulator even allow LightSquared to entertain this idea? Why allow terrestrial use of a satellite band? It's a mystery.

The frequency was originally licensed to the company for the use of satellite transmissions. And, as I've explained, that's all it can legitimately be used for, because of the interference problems.

What do you think? Leave a comment below...

Richi Jennings, blogger at large

Richi Jennings is an independent analyst/consultant, specializing in blogging, email, and security. As well as The Long View, he's also the creator and main author of Computerworld's IT Blogwatch -- for which he has won American Society of Business Publication Editors and Jesse H. Neal awards on behalf of IDG Enterprise. A cross-functional IT geek since 1985, you can follow him as @richi on Twitter, pretend to be richij's friend on Facebook, or just use good old email: You can also read Richi's full profile and disclosure of his industry affiliations.

Copyright © 2012 IDG Communications, Inc.

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