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LightSquared Says GPS Tests Were Rigged
itwbennett writes "Would-be cellular carrier LightSquared claims that the company's LTE network was set up to fail in GPS interference tests. 'Makers of GPS (Global Positioning System) equipment put old and incomplete GPS receivers in the test so the results would show interference, under the cover of non-disclosure agreements that prevented the public and third parties from analyzing the process,' LightSquared executives said on a conference call with reporters Wednesday morning."
Garmin GPS-12 13(?) years old.
Nagivo 3100, closing on 4 years old.
In addition, many GPS receivers in general aviation aircraft are _significantly_ more expensive than domestic units, and are not replaced merely because the battery wears out.
If you follow the link in the earlier story, 69 of the 92 GPS receivers had issues. That's either a lot of interference or a lot of older GPS units.
And even if it's old equipment, in my opinion it's still fair game, provided they're not all some obscure model that sold only a couple hundred units.
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They own a slice of wireless spectrum which is supposed to be used for satellite communication, and they want to use it for ground based cellphone communication.
No, they bought a band that is designated for low power satellite communication and are trying to get the designation changed to allow high power terrestrial communication. They aren't an innocent bystander in this mess.
So LTE needs a federal license which requires proof that their network transmissions do not interfere with GPS receivers. Well, lets see, apparently the GPS equipment worked when the LTE network wasn't on and when it was turned on the GPS had issues. So what LTE is saying is that everyone with old GPS receives has to upgrade them because their network causes issues with them so that they can get a FCC license in order for their network to be deployed everywhere. Are they assuming that people buy all new electronics every year? I mean especially testing this on a military base, when I was in the military I used computers that were designed before I was born. I have a 30 year old television myself, if LTE decides to make a network that stops my television from working isn't that their problem. The whole purpose of the FCC license is to ensure that someone doesn't put new equipment into use that will stop the use of old equipment. Okay, maybe not the only purpose, but that one is at least important.
So LTE's network failed in real world conditions and they are blaming GPS manufactures for that failure. I don't think they have a case because the GPS manufactures likely did not go back in time and put in circuits to stop their equipment from running if they detected LTE's network. It's probably a good thing it wasn't raining either or they would have to sue God for conspiring against them.
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That is completely false. Just FALSE.
It doesn't matter what sort of equipment was used or what claims Lightsquared is making. It comes down to the simple fact that there is currently no way to reliably demodulate and decode a signal sitting down at -130 dBm while you're experiencing interference trillions of time stronger as a result of sidebands from a base station.
It's a fundamental concept that all time limited signals mathematically have infinite bandwidth. However, the FCC defines bandwidth by the region where 99.99% of the power resides. Let's say you have a 150W base station. That would mean up to 1.5e-2W is outside the targetted frequency band. Now lets assume about .001% of that power resides on top of the band where your signal of interest is coming in. That would mean 1.5e-7W is on top of your signal of interest or (-38dBm). For reference, the signal at -130dBm is roughly equivalent to 1e-16W.
Disclaimer: The numbers above are general estimates used for illustration purposes. Actual conditions may vary, but it is unlikely that they will vary in such a way that will let you reliably recover your signal of interest.
Your -130dBm signal is
LS is full of it. I used to do testing of this nature for the Navy. I know many of the people who would have done this testing for the USAF. Never in 6 years of working in that field did we ever require a contractor who had submitted equipment for test to do so with no knowledge of what the test would be. They are blowing smoke to cover their asses in the hope that "the right people" won't know any better.
Troll much?
GPS receivers are designed to filter out the neighboring frequencies, when the neighboring frequency sources are satellites transmitting at power levels comparable to GPS satellites.
That is how that portion of the spectrum was designed and allocated. LightSquared is trying to use terrestrial transmitters at these frequencies. GPS receivers were never designed to filter out their signal from neighboring sources that are literally a billion times more powerful.
You don't know anything about RF transmission and why there have always been transmission power restrictions on the allocated spectrum. The spectrum allocation was specifically designed to prevent this exact situation from occurring.
If true, the use of units without filters may be enough to invalidate the tests. It would be similar to testing a microwave for radiation leakage, with the door removed.
Except it's not true - an old GPS receiver without modern filters is still a GPS receiver. The military still uses receiver that were made WHEN THE NETWORK WAS CREATED for fuck's sake - that's not going to change, it's a network meant to assist in our national defense from foreign nations - ie: TO SERVE THE SOLE PURPOSE OF THE GOVERNMENT - these morons with political ties deserve to be run out of business.
Here is good info on the issue:
http://www.forbes.com/sites/danielfisher/2011/12/21/falcones-lightsquared-faces-enemies-on-all-sides/
Prove anything by multiplying Huge Number times Tiny Number
As I understand it, it's not so much bleeding as lack of filtering. Lack of filtering because the chunk of spectrum was slotted for low powered satellite to earch communication, not the high power they want to use it for.
They are trying to get permission to use it for a purpose it was never intended for, as part of that they have to prove that the change won't interfere with anything.. and of course.. it is interfering with stuff.
I really do feel for them.. it's a pretty shitty deal.. but it's not like they bought a chunk of land to build something and can't get rid of the squatters. It's like they bought a chunk of land in a residential area and are trying to put up a skyscraper.
it's not about filters, nor is it about "GPS listens outside its band"
GPS receivers have "wide open" front ends and always have for good engineering reasons:
1) Spectrum planning ensured that there's no high power signals in adjacent bands (i.e. the adjacent band is also for satellite signals)
2) "brick wall" filters are heavy, expensive, large, and have bad effects on the inband signals (see, e.g. any digital audio application since CDs started being sold 30 years ago). Your cellphone has GPS that is as small as it is partly because you can use a fairly wide open front end that doesn't require a lot of filtering.
3) GPS signals are below the noise floor, allowing use of 1 bit ADCs in receivers, reducing cost and complexity in receivers.
There's quite a bit of arguing about what is an appropriate propagation model from L2 terrestrial transmitter to GPS victim. L2 would like to use a conventional communication model. GPS folks would like to use a jammer/interference model. The difference isn't in the "mean power" but is in where the outliers are. For comm, your concern is that your worst case low power deviation is still high enough that you can "close the link" (i.e. not drop the call). For interference, your concern is that the worst case high power deviation is still low enough that it doesn't interfere with your link. The problem is that in urban environments, the distribution isn't uniform and is highly skewed (lots of reflecting surfaces and multipath.. distance isn't as big a factor as just the number of bounces). There's lots of deviations below the mean, but small ones, and relatively few deviations above the mean, but they are huge (e.g. "hot spots"). We're talking 15-20 dB difference between the 5% low end and the 5% high end
There's also arguing about what "performance degradation" is acceptable. L2 would like to claim that 6-8 dB is ok, while GPS industry would like to use 1dB. That's because communications uses error correcting codes and such, and can tolerate dropouts and degradation. GPS is more like radar, and relies on measuring the timing of the signal, and doesn't have as much in the way of error correction or error tolerance, so they've historically used the radar standard of 1dB degradation. The GPS industry is a bit stretching here, because with new receiver designs (which might consume more power and be bigger) they could probably deal with the worse interference environment. But that's a 10-20 year kind of project.
So the tests were fair, with published test criteria, and only now, a week from their deal with Sprint expiring (after a 30 day reprieve) they're starting to raise these questions.
It comes down to the fact that there is currently no way to reliably demodulate and decode a signal sitting down at -130 dBm (roughly the strength of a GPS signal in some areas) while you're experiencing interference trillions of time stronger as a result of sidebands from a base station.
It's a fundamental concept that all time limited signals mathematically have infinite bandwidth. However, the FCC defines bandwidth by the region where 99.99% of the power resides. Let's say you have a 150W base station. That would mean up to 1.5e-2W is outside the targetted frequency band. Now lets assume about .001% of that power resides on top of the band where your signal of interest is coming in. That would mean 1.5e-7W is on top of your signal of interest or (-38dBm). For reference, the signal at -130dBm is roughly equivalent to 1e-16W.
The numbers above are general estimates used for illustration, but lets say that only .0000001% of the base stations power falls into the band where your signal of interest resides. That interfering signal is still sitting at -68dBm while your GPS signal is sitting at -130dBm.
And that's for a retrofit. What does one do (if something must be done) about units like a Garmin G1000 or the Avidyne units that have been installed in Cirrus planes for years? Factory-installed units intended to be core to aircraft operations are even more expensive than that.
You can never go home again... but I guess you can shop there.
If Joe sells you an off road vehicle then you try to get it licensed to be street legal do not get pissed at Joe because it will not be allowed on the road.
Radios are very complicated. Till I got my current job I had no idea how little I knew about them. Get education on the subject. They are attempting to use this spectrum in a way in which their current license specifically prohibits. Also it is prohibited for a very good reason. They are trying to change their license and it is that change that is causing the problems.
Why is it so hard to only have politicians for a few years, then have them go away?
Lightguard's allocated spectrum is supposed to be for SATELLITE use, they want to use it on the ground which was not included as part of their licensing agreement.
Good-bye
Actually, you mean RNP (Required Navigation Performance) which are a set of approaches that are more efficient, but require that the plane have onboard a minimum set of equipment. And one of this is dual RAIM-locked GPS units.
A RAIM-locked GPS is a receiver that can see more than the 4 minimum GPS satellites - and all aviation GPSes have utilities that can take a location (destination) and time and calculate whether or not a RAIM lock is achievable (it depends heavily on the satellite configuration at that point in time).
Primary purpose of RAIM is to help the GPS decide if a satellite is "out of whack", which is essential if you need to figure out your position accurately.
RNAV is slightly different - it requires a flight management system that basically generates a GPS-like path by taking in multiple navigation sources like VORs and NDBs and calculating a virtual track based on your position relative to those navaids. So you're not flying navaid to navaid, you're flying a course through but using the navaids to cross-reference your position continually.
These days, a combination of RNAV, INS (Inertial navigation system) and GPS are used altogether to get very accurate positioning required for RNP. (RNP dictates the minimum performance your navigation equipment can have - you can always use better equipment to fly the RNP approaches more precisely).
See my other comments on the GPS 12 for an example. Similarly, there are *tens of thousands* of GNS 430\530 GPS\NAV\COM units in aircraft around the world, and those had a time on market of over a decade. They'll have support for years to come as well. At $15-20k each, people aren't going to run out to replace them.
So ASSUMING Lightsquared, operating 100% within their spectrum, and not interfering with GPS frequencies, could still interfere with GPS because the GPS Unit itself is using part of Lightsquared's spectrum for filtering purposes.
They can't filter out LightSquared's signal. It'd be the metaphorical equivalent of trying to spot a candle flame standing next to a searchlight. It's just not physically practical. Worse still, LightSquared managed to get their spectrum at a huge discount exactly because it was technically unsuitable for the purpose they're trying to use it for now and the rules forbade that use - and then somehow managed to lobby the FCC into ignoring the technical side of things and let them go ahead anyway.
There is always some leakage of the local oscillator through the other mixer port. There may also be some leakage of the downconverted intermediate frequency (IF).
In my youth I made a "police detector" using this principle. It worked quite well.
The PNT government website has the complete test results for the first round of testing here:
http://www.pnt.gov/interference/lightsquared/
Look for "Final Report of FCC-Ordered Working Group"
I'd expect you'll find the test report for the second test to be available at the same site, once it is released.
~1500 watts was the official LS broadcast power for its towers, until after the parameters for the second round of tests were finalized.