Analysis: Sorting out the LightSquared GPS interference mess

Can filters in GPS receivers solve the problem?

In theory, better filters in general GPS equipment and blocking filters in high-precision equipment could allow GPS and LTE to co-exist peacefully. In practice, no one knows, since such filters haven’t yet been implemented in commercial devices.

According to Brock Butler, Spirent’s director of wireless location technologies, there are several candidate designs for such filters, but none of them have been vetted. Butler contributed to the GPS technical working group that produced the FCC interference study filed last month, participating in the testing group focused on cellular GPS receivers, but he said no such filter designs were tested. Those designs are just that, designs. Butler said the working group was tasked with testing the LTE network’s effect on commercially available devices, and until those filters make their way into handset, navigation consoles and avionics gear, there’s no way for the GPS industry and LightSquared to gather adequate data.

“We would certainly like to get our hands on those devices to see how the filters perform, but up to this point none of the candidate designs have been used,” Butler said.

If LightSquared and the GPS industry were able to develop commercially feasible filters, they would be mostly targeted at new devices (retrofitting high precision GPS devices may also be possible). That still leaves hundreds of millions of extant GPS receivers prone to interference from the LTE network. LightSquared has proposed to address this problem by giving up half its network, moving its deployment to the lower portion of the L-band where its transmissions are less likely to wash out nearby GPS (CP: LightSquared proposes swapping half its network for FCC approval).

There’s still plenty of disagreement between LightSquared and the GPS industry over whether that measure is enough to protect the multitudes of low-precision receivers out there. But LightSquared and the GPS industry actually agree that those revamped deployment plans would still have harmful effects on high-precision gear. Those receivers are reaching much further into L-band to get to those augmentation signals, landing them smack in the middle of LightSquared’s spectrum.

In order to protect that high-precision gear, what’s needed is a filter that blocks out the portion of LightSquared’s spectrum in between the GPS and the augmentation bands. LightSquared and its GPS counterparts disagree about whether such a commercially viable filter is possible with today’s technology. And even if they were able to identify the proper filter, it would have to be retrofitted into more than 200,000 high-precision devices. Who’s going to bear that expense?

In the end, there are a lot of ‘ifs’ determining whether a LTE and GPS can co-exist in the L-band. If LightSquared cuts its deployment in half, if a viable filter solution is identified to protect high-precision receivers, if that solution can be retrofitted to those receivers at reasonable expense, and if LightSquared or GPS device makers could decide who would pay for it, then all parties and happy and LightSquared and the GPS industry can go on their merry ways, right?

Wrong.

Even if all of those obstacles could be overcome, there would be no way to prove that the fix actually worked. That’s because LightSquared and the GPS Industry still disagree on the most fundamental principle: what constitutes harmful interference?

Next: Can LightSquared and the GPS industry find common ground?