LightSquared blasts GPS naysayers in FCC letter LightSquared, the US operator hoping to camp beside the GPS frequencies, has written to the FCC accusing the GPS industry of failing to comply with the Department of Defence standards. The letter is in response to the industry complaints that LightSquared is putting lives and property at risk by insisting on setting up a 4G …
Let them build the ground-based kit. And stop there. This ground-based kit will only affect stupid rednecks and hillbillies, and leave us civilized Europeans alone. A satellite deployment by these oafs would affect civilized areas as well. ;)
Bring it on, I'm wearing my asbestos long-johns anyways. ;)
//Svein
If the GPSes involved just included car satnav units, that'd be one thing. But what if the GPSes involved also include aviation units? Now it's not just a car taking a wrong turn, but an aircraft flying who-knows-how-far off course. Sure, there are other means of navigation available to planes, but those means are basically in the process of being decommissioned in favor of a purely satnav-based system. Oops. It'd also be nice to know how exactly LightSquared determined what GPS units are in that 200,000 bunch.
"Eat this", because that's what we could have if LightSquared's 4G network is really as detrimental to GPS as it sounds.
One assumes that US military GPS kit is built properly, otherwise LightSquared wouldn't have made it this far.
It is rather tricky trying to receive signals at -160dBm if there's something nearby radiating +30dBm, I challenge anyone to build a useful filter that would let one pass and attenuate the other by 160dB.
I'd say the fault is in letting the frequency bands be reallocated from satellite service. Had the relevant people understood the technical arguments and stuck to their guns, the US wouldn't be stuck with the argument.
Exactly as Number6 points out, this *was* a satellite band that was fine to co-exisit along side GPS. The idiots at the FCC/gov moved so the LightSquared could make money without analysing the full implications of this.
Maybe militart GPS are OK, but if so I bet they are fitted with coke-tine sizes filters and use way more power to get the necessary phase noise performance to meet this sort of thing.
Easy to settle though: have LightSquared build a GPS demonstrator that can fit in a decint sized phone with similar power & cost (no more than say 25% more than industry norm) to show how easy it is to do. If they can, they have a case, if not then they should STFU.
Of course, it depends on where the nearby transmitter is. The most likely source of the most troubling transmissions would be a customer's Lightsquared mobile phone, not the Lightsquared base station. You don't have to go very far from any base station before the received signal strength is really quite low. 1/r^2 is a powerful attenuator! The mobile phone, being so close to the customer, is almost always the more powerful transmission relatively speaking.
You always *can* build a filter with the required level of isolation, but for very high levels of isolation it is going to be physically quite big and/or expensive. This would not be ideal for a GPS receiver which is by definition a portable device. Military equipment doesn't mind filters being expensive, but large filters can be as much a no-no as for civilian applications.
It is very difficult for the frequency planners to know what to do. They could allocate frequency bands in such a way so that there's adequate guard bands either side to protect against any conceivable transmitter. But that would be enormously wasteful of bandwidth, and it depends on an accurate understanding of what technological developments there may be in decades to come. Not a straight foward task, and something that humans have proven to be very bad at. Remember the 640k RAM limit?
But in this particular case I think that even the original use of the adjacent bands for a satellite mobile service was arguably wrong. Sure, the satellites for that service were a long way away (in orbit, in fact), so their transmissions were never going to be powerful enough on the ground to affect GPS receivers. But those satellite mobile phones themselves would have to be quite powerful to be received by their satellites. It's quite possible that they would have interfered with GPS receivers in much the same way as Lightsquared's transmission apparently does. I suspect that this has been going on all the time. But seeing as the original satellite mobile service is defunct it sounds like it wasn't so popular in the first place. So maybe the problem was always there, just not badly enough for it to come to widespread attention.
So if that is the case, who is to blame for the mess? Lightsquared? Not really, they've acquired the rights to a band; they're not transmitting outside that band, so they're sticking to the rules. However, I do agree with the view that Lightsquared should have known better. In effect they trusted the FCC to know their stuff when they asked for the band. Was that commercially a wise choice? Probably not. And I bet that the receivers on Lightsquared's mobile phones are just as vulnerable to out of band interference [similar bands, similar electronic constraints to achieve significant out of band filtering]. They're getting away with it too, because the adjacent band is GPS, which has no terrestrial based transmitters. I bet that if GPS were replaced with some sort of mobile telephony service, Lightsquared would be complaining just as loudly as the GPS crew are today.
How about the frequency planners at the FCC? Depends on whether they were obliged to consider mass market cheap GPSs when the bands were allocated all those years/decades ago. Back when the bands were first considered (in the 1980s?) it would hardly have been imaginable that we'd have mobile phones, never mind mobiles with GPS in them. And the rules on operating bands are quite clear. If you receiver is open to receiving transmissions from adjacent bands that's your problem, not the FCC's.
How about consumers and their want of cheap, small GPS receivers? No not really, they're the not the designers of the equipment they've bought.
How about the GPS receiver manufacturers? Largely yes - they've got away with ignoring the effects of transmissions in adjacent bands for many years now when they had no right to assume that those bands would be forever quiet. The FCC rules (and the rules from frequency planners everywhere else in the world) are very clear in black and white on paper about that, and always have been. And if the manufacturers had paid strict attention to the FCC rules then we would likely not now have things like GPS in phones.
So what happens now? Personally I don't think that the GPS manufacturers deserve to get away with it. However, the real question is does the paying public deserve the right to access the GPS service in the way they do? Yes, they do! Affordable, convenient and functional GPS does make the lives of the general public better in a very big way, and improving the lives of the public as a whole must be a governmental goal.
I think that the FCC should buy out Lightsquared's band allocation (a pricey proposition), recover the cash through a one off levy on any GPS manufacturer with (currently) non-compliant kit, and place bigger guard bands either side of GPS.
GPS is speared rectum with about 40dB processing gain, so filtering is possible, although not cheap. Non-portable military receivers have front-end filters - usually because they sit on a platform right next to something radiating very close at high power (e.g. SSR ).
The point I sought to make in my previous post is that any mobile phone service, be it satellite or terrestrial, has a quite powerful transmitter (i.e. the mobile phone itself) in exactly the wrong location. It's right next to the customer, and presumably very close to the customer's GPS receiver. That is far more significant to a GPS receiver than a base station a few miles away or a satellite in orbit.
The subsequent point I made is that presumably that has always been the case long before Lightsquared came along, but the old satellite mobile bands that Lightsquared took over were never popular enough for the problem to come to widespread attention. Only now that GPS is a mass market and that Lightsquared are reviving usage of the adjacent band has the problem been highlighted.
It would be interesting to know if a Lightsquared mobile phone has a GPS receiver that works when the phone is also transmitting to the Lightsquared network (for example when using something like Google Maps). If it does then the GPS industry is clearly talking horse shit.
Presumably a GPS receiver in a Lightsquared phone, if it needed to, could time-slice it's reception to only turn on the GPS receiver when the phone's radio is not transmitting and vice versa. The radios already get turned on/off for power saving, so this should be possible. This is not the case, however, for an existing GPS receiver that does not know about Lightsquared.
I have worked on 3G testing and we test of ACLR (Adjacent Channel Leakage Ratio) of 50dB.
Even if the new signals were at -50dBm (same as terrestrial TV) the ACLR for GPS would need to be more than double typical industry standard.
Yes, the change from satelite to terrestrial should never have been allowed and Lightsquared must have been aware of this problem at the beginning.
If all GPS receivers will really be able to to pick up the Lightsquared base stations so easily, and if the signal from those ((Fixed Location, Terrestrial)) positions is so much more powerful, USE THEM.
Both systems are probably nothing more than an ugly firmware patch away from using the Lightsquared system to precisely and quickly triangulate a position. There is already such system in place for assisted differential GPS. Cellphones use their own towers for much the same purposes, and there is the Google/Skyhook wifi map as well.
And for Frack's sake, the GPS manufacturers will be able to sell another zillion units, just like the US digital TV switch. This is pretty much a win for all parties, except those of us that have to eat buying a new receiver.
That's...not how GPS works. In basic terms, the satellite sends out a timing signal, and the ground receiver analyzes the signal to determine how long the signal took to reach it, and uses triangulation from other GPS satellites to determine a 2- or 3-dimensional location. Of course, this process fails to work when the receiver assumes the satellite is in space, but is actually a ground base station somewhere significantly closer. This is an over-simplification, but you can see how it's not just a matter of a "firmware patch" to fix.
There are some ground-assisted differential GPS systems out there, but that requires specialized equipment and a very precise geographical position reading of the base station(s). The other type of assisted GPS system out there--WAAS--uses additional geostationary satellites that broadcast a correction signal to WAAS-capable receivers (correcting for atmospheric effects, among other things). But even that uses ground stations with very precisely-known locations to pick up the GPS signals, in order to calculate the local correction factor. Such precise geographical measurements don't come cheap.
As for cell-phone assisted geolocation, have you ever noticed how imprecise it is?
I don't know the extend of the interference LightSquared presents to GPS, but if it affects aviation GPS units, those cost several thousand dollars, with some units costing more than many new cars. So yeah, I think some people would be pretty annoyed at having to replace THOSE.
1. Triangulation requires more than one transmitter, not just one. Probably you can make this work with reception from more than one base station, but this adds complexity and requires more than one base station in range.
2. The question is what happens to existing GPS receivers, not whether it's possible to build a different receiver in future that is immune. Specifically all of the people with SatNav in cars or phones - "buying a new receiver" is not good if it means "major car repair expense", for example.
IMHO LightSquared would be well advised to make sure they do not break existing kit, otherwise they and/or the FCC may find themselves defending their argument that they are not responsible in court. Regardless of the fact that they may legally be in the right (and I am not a fan of these type of lawsuits) this is not a good outcome. Deploying a system that breaks existing GPS receivers, if that's what happens, would be worst.
Actually I would not be surprised if this letter is Lightsquared's way of saying that they would be OK with using different spectrum to not break GPS, but they are hoping to get different spectrum and/or some money from the GPS makers and/or FCC out of any change.
I'm pretty certain that both standard accuracy public channels that Galileo transmits on coincide with the GPS L1 channel and so will also be affected. I think the Chinese COMPASS system also has this issue, maybe not GLONASS though.
Basically, this will affect all free use of public sattelite location systems.
The FCC is no longer run by (nor do they seemingly employ) RF engineers. It's all lawyers now. These bands should not have been allowed to change from satellite to terrestrial-based service. The company knew what they were buying, and from what I've read, did so in bad faith, always planning to claim they couldn't make the sat service viable, and wanting to change to ground-based. The politicians get their handouts, and the people get screwed. Business as usual, 2.0.
Your analogy is way off. This situation is more aptly described as follows:
A building houses a legally licensed lab containing highly sensitive audio gear that listens for cricket whispers (in the 20 kHz range) miles away. LightSquared buys the adjacent lot, knowing it is zoned as "super quiet use only", lobbies the town to change the zoning, and commences to operate an open-air rave 24x7. While the music doesn't contain 20 kHz frequencies, it so vastly overwhelms the lab's sensors that they can no longer hear crickets.
As Number6 said, there are not effective, inexpensive methods to filter out that level of noise. It's likely the mil community has methods, but the kit would cost way more than any consumers would spend.
No, they also use the coarse acquisition band at 1.5GHz (which commercial GPS use) along with another at 1.2GHz for precision correction of ionospheric delay.
The only reason(s) the military are not complaining probably are:
(1) They have very well built equipment designed to be robust against jammers and adjacent transmitters. This requires very tight filters, which in turn demand very high unloaded Q-factor of the resonant elements. Typically that means large and expensive. It also requiers other RF parts that are more costly and power-hungry as well (high dynamic range LNA/mixer, low phase noise local oscillators, etc). Not going to fit in your phone/sat-nav any time soon.
(2) They are likely to be operating far away from most probably users of LightSquared equipment, and in an emergency on USA territory where it was an issue, they would simply disable the network one way or another.
Galileo still only being built - not operational.
Last time I checked (which was a while back) the operators of GLONASS (ex Soviets) were struggling to maintain a full constellation. I was on a research ship in the1990's which had an Ashtech GG24 GPS/GLONASS receiver. Just for a test I put it in GLONASS only mode. Did not get much coverage. The ship was operating between the UK and the high Arctic at that time.
Even if filtering can fix this, consider that GPS is now built in to phones, cars, watches. My latest compact camera has one in. It is also becoming an SPOF for the US transport industry. Lightsquared may point the blame at the GPS receiver vendors, but it seems to me that if they want to change the use of the adjacent bad, they get to pay for all the upgrades and replacements of the existing devices.
DOD was one of the earliest objectors to LightSquared. The full details of their tests are, naturally, classified.
LightSquared wants to use, for terrestrial purposes, a band that is internationally set aside for satellite use only (at substantially lower power levels than LightSquared wants to use on the ground).
The precision GPS users (military, aerial navigation, precision agriculture, civil engineering to name a few) are the worst-impacted, and they're using multi-thousand-dollar receivers, usually many of them.
Glonass and Galileo are affected too, as they use the same part of the spectrum that the American NavStar GPS uses. And if LightSquared manages to pull this off, count on them, or competitors, to want to go world-wide with this, so don't pretend that an ocean makes you safe from this.
LightSquared is most analogous to light pollution. You may not notice that you can't see as many stars when that street light goes up nearby, but when they build a brightly-lit shopping mall there, you'll start to see. And the astronomers would have noticed the street light. Precision GPS is the astronomers in this case. Give LightSquared what they want and you'll start to see the problems yourself. Tests to date haven't used the full spectrum LightSquared wants to use, haven't used their signal at full power, and have only used a single tower, not the forest they want to build. And they're still showing massive disruption to the adjoining frequencies.
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