The LightSquared vs GPS problem, what now?
I learned a lot at the 2011 NMEA Conference & Expo, and got a first peek at some wonderful new marine electronics, but I’ll start my coverage with the scene above. Here on Panbo we started discussing GPS interference caused by LightSquared’s proposed new U.S. broadband cellular network back in March, and the entry now has 111 comments, many vigorously opposed to the LS plan. Plus the Coalition to Save Our GPS has become a powerfully united and outspoken voice, and the whole darn mess has recently become a political football. So it seemed fairly brave of Geoffrey Stearn — who is LS’s VP of Spectrum Development — to step into a room full of marine safety authorities (U.S. GMDSS Task Force) and NMEA members all of whose work depends on reliable GPS positioning…
But engineers and other technical folk tend to be deliberative and mild mannered, and Stearns seemed to fit that mold as well. There was no yelling. But there certainly was some skepticism about LightSquared’s latest “solution” to the interference problem. As I understand it, this is the third plan LS has proposed, or the second modification since testing proved beyond doubt that Plan A would adversely affect all sorts of GPS receivers over substantial areas around LS ground transmitters. I think the diagram below, extracted from a Deere filing to the FCC, nicely illustrates the situation even though it seems to depict LS’s original proposal.
Note the extreme signal power differences between the transmitters using the two LS bands and the adjacent GPS satellites. Note too how our “low” precision GPS receivers can filter the signal more tightly than what’s possible when receivers are designed to produce position accuracy to within inches. And finally understand that those high precision receivers also often use extra input from systems like Deere’s StarFire that are even more tangled up in the two bands of spectrum that LightSquared acquired from Inmarsat.
Below is Geoffrey Stearn’s slide depicting LightSquared’s latest proposal, which is to only use the lower of its two bands and to transmit at much lower power than originally planned (-30 dBm to be specific). That supposedly takes care of interference issues with low precision GPS receivers and a company called Javad GNSS claims to have a filtering solution for high precision receivers. And it’s worth noting that Javad Ashjaee is quite a GPS engineering talent. So is LightSquared going to happen? Well, that remains to be seen…
For one thing, the senior GPS engineer who Garmin flew to the conference specifically to rebut Stearn’s address says that Garmin is not at all convinced that the latest LS plan won’t interfere with standard narrowband GPS receivers. The arguments get into pretty technical areas like propagation auditing and standards, but I certainly got the point that we all tolerate weak cell signals much more easily than we could handle poor GPS reception. Obviously a lot more testing of LightSquared’s latest plan has to happen. And in my view it shouldn’t be rushed, even if LS is under a financial gun.
In fact, I asked Stearns in a breakout session if LS has a Plan D in case it is not allowed to transmit any cellular data from ground stations on either of its L bands. While he danced away from that answer, he did respond to my followup saying that LS had simply not realized that there would be interference issues when it conceived Plan A. An expert later told that that that answer was not only BS but “documented BS”.
Personally I remain convinced that massive disruption of GPS in the U.S. will never happen because it would outrage so many users. And Stearns did confirm that saltwater marine users are the least likely to see problems because part of LS’s deal with Inmarsat is careful transmission controls along the coastline in order not to disturb the latter’s maritime services. But I was also struck by the questioning of Jule Rutstein of Bethel Marine Electronics. If Plan C or some further compromise is indeed activated, how are guys like Jules and his team going to diagnose it? They already get calls about intermittent GPS failures that can very hard to figure out. Maybe a map of LightSquared transmission towers will become a new piece of everyone’s trouble shooting kit?
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