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Old 24-06-2011, 08:08   #1
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GPS Signal Interference in US

The saga of Lightsquared's attack on GPS in the US took a turn for the better at a FCC hearing yesterday but we're not out of the woods.

For those not following it, LightSquared received permission in January to deploy 40,000 high power transmitters for ground based internet service. Unfortunately this was in a band directly beside the GPS band. The band had previously been designated for low power earth to satellite transmissions. Garmin and other GPS vendors discovered that the high powered LightSquared equipment could knock out GPS around the LightSquared transmitters. The problem was much worse for aviation but we need to worry too. A Garmin executive testified yesterday that the Lightsquared transmitters are "like a lawn mower in a library"

It's incredible that the FCC would let this get started. I guess there are no ham radio guys left in the FCC who understand how radio works. Fortunately, the uproar got them to delay it -- but not reverse it. The Wall Street Journal reports that LightSquared has mounted a large lobbying effort to go ahead.

Several congressmen including Ton Petri who heads the Congressional Aviation committee spoke out yesterday against LightSquared at the FCC meeting.

If you are a US voter, it would be a great time to drop a note to your congressman about how important GPS is to your family's safety. Mention LIghtSquared specifically. I don't think we can trust the FCC to do the right thing without Congressional pressure.

In other countries, you might want to be on the lookout for spectrum grabs by well funded companies who don't know what it's like to be near a submerged rock ledge in the fog.

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Old 24-06-2011, 08:17   #2
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Re: GPS signal interference in US

but how would ground based transmitters affect GPS signals on the water?
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Old 24-06-2011, 08:27   #3
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Re: GPS signal interference in US

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Originally Posted by gonesail View Post
but how would ground based transmitters affect GPS signals on the water?
But there are far more land based users these days than us, the suffering minority of off shore users.

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Old 24-06-2011, 08:35   #4
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Re: GPS signal interference in US

Lightsquared was proposing to broadcast their service from satellites according to some news links about the subject. http://en.wikipedia.org/wiki/LightSquared
- - That would definitely wipe out or severely degrade use of GPS satellite signals. Most recently Lightsquared has suggested moving their allotted frequency to a different band to solve the interference problem.
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Old 24-06-2011, 11:02   #5
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Re: GPS signal interference in US

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but how would ground based transmitters affect GPS signals on the water?
Coastal marine traffic would be affected. If LightSquared's transmitters were at full power, your GPS receiver could be overwhelmed when trying to lock onto a satellite signal which was 120 dB (1 billion times) lower than LightSquared's. Out beyond line-of-sight it probably won't be a problem.

You'd complain to your rental car company if they fined you for abusing their car by going 140mph as reported by their GPS tracker device, when it was really caused by LightSquared. If you were a pilot, you'd complain as well.

I'm not a radio designer, but from what I remember from my EE electronics courses, designing a bandpass filter with such a steep cutoff (~100dB) in the "narrow" range between 1.6 & 1.7 GHz is not trivial nor cheap...
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Old 24-06-2011, 11:24   #6
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Re: GPS signal interference in US

I was part of a group that took a look at a similar system (that was never realized) about 4 years ago, so I have some background with this problem (and I'm a radio and software engineer). I wanted to clear up a few things.

First, they're already operating their satellite system, with 2 geosynchronous satellites in operation. The received signal strength at terrestrial heights is so small as to not interfere with the adjacent GPS bands. That part works exactly as everyone originally intended, and business model for Lightsquared is as untenable as we all thought when this was proposed. Hence the push for a broad terrestrial system.

Second, as Beausoleil says their terrestrial repeaters will definitely interfere with land, air, and sea GPS reception. Lightsquared has tried to rig the tests by providing transmitters running at only a fraction of the proposed power output, and even in those tests GPS reception has been destroyed or severely degraded out to a radius of 1-2 miles. I've seen simulations that show loss of GPS reception out to 10 miles or more from the full power transmitters. So gonesail, any transmitters near a coast could easily wipe out GPS reception around critical coastal and harbor areas, and probably fairly far out to sea as well given the amount of signal skip over water.

Obviously "fixing" the millions of GPS receivers currently in use is not practical, but it may also be impossible. Despite what the Lightsquared CEO says, building an analog filter into a GPS radio to allow their system to work would probably be impossible. You simply need way too much noise rejection, on too steep a curve, with almost perfect pass-through in the GPS band. Any practical filter I've seen that would effectively filter out the Lightsquared interference would radically degrade GPS sensitivity. It may be possible digitally, but would require a fairly hefty amount of processing, which would have a major impact on both the cost and battery life of GPS'. I've tried to implement similar filters on a DSP, and it's very difficult to get that much rejection with that little signal loss.

We all need to put pressure on the FCC to deal with this appropriately, which would be denying Lightsquared their permits. It is simply not going to work, and I don't believe that the folks at Lightsquared are so stupid as to believe it will (although you never know). My guess is that their strategy is to pressure the FCC into granting them a juicy chunk of spectrum in a different band, essentially for free, but that's pure speculation on my part.
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Old 24-06-2011, 12:00   #7
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Re: GPS signal interference in US

It's amazing that part of US government would even consider allowing something that will destroy or damage a system operated by US government. Does FCC not talk to DOD? Does DOD have nothing to say about that?
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Old 24-06-2011, 12:34   #8
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Re: GPS signal interference in US

There is a rumor that alot of people in the FCC cant explain how this was approved.
Sometimes in DC its not what you know but who your roommates were. Somewhere someone was pushing this.
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Old 24-06-2011, 13:25   #9
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Re: GPS signal interference in US

The chances of interference with GPS are minimal, and subject to constant monitoring, and LightSquared won't be allowed to build a network until the issue has been entirely resolved.
See ➥ LightSquared and GPS

“... The startup's new proposal, in which it would step away from the frequencies that it said cause the most interference with GPS (Global Positioning System), still needs regulatory approval and hasn't even been presented to the U.S. Federal Communications Commission yet ...
... Though details are scarce, LightSquared presented an outline of its plan in a press release on Monday. It would abandon, for now, a 10MHz band that sits near frequencies commonly used by GPS devices, and in its place would use a lower 10MHz band that is farther away. In the new frequencies, LightSquared's network would not interfere with GPS except for "a limited number of high precision GPS receivers," the company said ...
... The Coalition to Save Our GPS, which includes GPS vendors Garmin, Magellan and Trimble, as well as FedEx, Caterpiller, the Air Transport Association and others, dismissed LightSquared's claims of having solved the interference problem.

"Confining its operation to the lower MSS band still interferes with many critical GPS receivers in addition to the precision receivers that even LightSquared concedes will be affected," the group said in a statement attributed to Jim Kirkland, vice president and general counsel of Trimble. "It is time for LightSquared to move out of the MSS band ...”

More ➥ New LightSquared Plan Faces Attack, Uncertainty | PCWorld Business Center

“... Concerns of GPS makers and a number of government agencies have prompted the House Appropriations Committee to attach an amendment to the Fiscal Year 2012 Financial Services and General Government bill restricting LightSquared's implementation of a hybrid terrestrial/satellite 4G wireless broadband network.

The amendment was approved by voice vote and will now move to the House floor along with the bill.

Republican Reps. Steve Austria (Ohio) and Kevin Yoder (Kan.) introduced the amendment, which would prevent the FCC from spending any money to permit the service "until the Commission has resolved concerns of potential widespread harmful interference by such commercial terrestrial operations to commercially available Global Positioning System devices."
More ➥ Appropriations Committee Conditions FCC Funding for LightSquared - 2011-06-23 23:28:10 | Broadcasting & Cable
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Old 24-06-2011, 14:51   #10
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Re: GPS signal interference in US

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Originally Posted by brak View Post
It's amazing that part of US government would even consider allowing something that will destroy or damage a system operated by US government. Does FCC not talk to DOD? Does DOD have nothing to say about that?
Yes, I can't believe the US Military and USCG are not all over this.
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Old 24-06-2011, 17:24   #11
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I am far from an expert in telcom stuff. Imagine if you will Verizon owning rights to large amounts of frequency. Other telecoms want to open up the frequencies . Verizon is content. Now add in a company that has access to a frequency but can't use it in a really profitable way. They pry the door open with a satellite based com system that became more of a 4g land based array. The land based stuff was an augment and has become a staple to the System. Add in the roomy of a prominent politico and the difficult but profitable way the frequency are sliced up and you have a small threat to gps but a great foot in the door to a huge profit in a wanted competition for wireless services.hopefully I said that so it's not just jumbo jumbo. I don't even know if it's really a bad thing. Just that there are a bunch in the FCC scratching their heads wondering how the initial approval got through. And s rumor about the class buddy.
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Old 25-06-2011, 22:11   #12
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Re: GPS signal interference in US

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\ And s rumor about the class buddy.
I've heard it's much more than that, from a friend who handles FCC licensing and regulations. I don't want to delve into politics here, but the talk is about the large amount of buddy time that the Lightsquared CEO has spent not just with the chairman of the FCC, but with one or more people on the President's close staff. There seems to have been a connection directly to the Whitehouse that has enabled this nonsense this far.

As far as the FCC is concerned, congress has had to step in way too many times in the past 15 years - through a succession of presidents from both parties - to override decisions the FCC appears to have made for purely political reasons. Par for the course unfortunately.
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Old 26-06-2011, 14:47   #13
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Re: GPS signal interference in US

Which is true world-wide, not just within the USA. Politics anywhere in the world follows pretty much the exact same processes where "favors" are traded. So nothing new, it just means the electorate needs to be aware and watching so it doesn't get too far out of hand.
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Old 09-08-2011, 16:33   #14
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Re: GPS signal interference in US

I just recieved a reply from Jon Sarbanes (D, MD) who is involved with this issue.
Quote:I strongly believe that technology—including broadband, digital communications and other information technology – is an important tool for the economic and social advancement of individuals and communities. I favor policies that seek to reduce the digital divide, which refers to the uneven growth of Internet service and other technologies, so that all Americans can share in the benefits that these technologies provide. For every dollar invested in broadband, it is estimated that the economy sees a ten-fold return on that investment. In this tough economic climate, an intelligent, innovative telecommunications policy offers a path to recovery. By investing in our nation's physical and electronic infrastructure, we will save or create jobs for three million Americans, strengthen our nation's economic backbone, and allow commerce to flow more freely to all parts of our country. We are literally building stronger communities and a more secure America. (Unquote)

I agree the concept is good but the reality of signal loss, the reason for my letter, is hardly addresses in this response. Smells like a backroom deal.
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Old 09-08-2011, 16:39   #15
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Re: GPS signal interference in US

If that the totality of the response, then I would say he totally ignored your question. Which is what being a "good" politician is all about.
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