US Air Force Space Commander William Shelton
Best. Job title. Ever.
LightSquared has signed a deal with Javad GNSS that will see GPS filters capable of resolving interference issues, within two weeks. That's remarkable, not least because US Air Force Space Commander William Shelton recently claimed that such filters would cost billions and take decades to install, if they could be made to work …
Better filters on LightSquared's transmission won't help. They don't intrude on the licensed GPS band anyway. The problem lies in that cheap GPS receivers don't bother to exclude the non-GPS bands from their radio receivers. The rules from the FCC and similar bodies round the world are quite clear; if you don't design equipment to reject other people's legitimate transmissions, that's your problem not theirs.
LightSquared are, I suspect, commissioning the design of these filters to demonstrate that the GPS industry is lying in their claims that such filters are not possible. If LightSquared get these filters going (and there is not particular electronic reason why they won't) then the GPS industry will be obliged to shut up and start designing their mass market equipment properly, just like the rules say they should.
What's more, if LightSquared lay claim to and enforce the rights to the filter design the GPS industry might have to pay a license fee to them for every new GPS receiver built. That will fill LightSquared's coffers nicely. It would be *quite* ironic, especially when it this is a problem soley of the GPS industry's making.
"The rules from the FCC and similar bodies round the world are quite clear; if you don't design equipment to reject other people's legitimate transmissions, that's your problem not theirs."
Err, no, practicality is a huge part of the frequency allocation that regulatory bodies do. Broadcasting is an excellent example of this. The likely spurious reception of the receivers used in a particular area exclude many possible frequencies for people to put their TV or radio transmitters. Things like intermodulation and IF images are important as well as "adjacent-channel interference".
This is particularly important for satellite downlinks. Transmitters in space run off of solar cells so the links are generally engineered with little extra margin. They are thus very susceptible to interference. The normal practice is to assign guard bands on frequencies adjacent to downlink bands where terrestrial transmitters are not allowed.
There is a tradeoff between loss and how sharp you can make a filter. The filter that is delivered will most likely have too much loss to be practical for the application ... unless of course there has been some sort of recent breakthrough that no one has heard of yet...
If LightSquared's transmitters were so terrible that "Things like intermodulation and IF images" were causing them to radiate outside their allocated band then they wouldn't have a leg to stand on. But by all accounts that's not the case here; no one is saying that LightSquared's transmissions will intrude in an unlicensed or unreasonable way into the allocated GPS band.
The interference of GPS receivers is caused by not defining their receiving band with a filter good enough to reject out-of-band signals that will, if LightSquared start operating, be commonly encountered. All receivers are vulnerable to high power interfering signals causing non-linear responses within the receive chain components. That's why when you design a radio you take a look at the expected operating environment (maybe glance at the frequency allocation tables) and decide how much filtering is going to be needed against legitimate and commonplace adjacent signals. And you are right - guard band practicality is an important factor for the allocating authorities.
It's worth questioning whether today's filtering requirement for GPS is so very different to that theoretically required years ago when the 1525-1559MHz band was allocated to satcoms. To set the historical context for the current debate I think that it is important to analyse the practicality of the previous use of the 1525-1559MHz band for satcoms long before LightSquared came along. And the issues are not generally evident from the online material covering this topic. For example, the last slide at:
shows the frequency allocations and a stylised filter response. It suggests that "Low Power On Earth Satcom Emissions" don't cause interference with GPS, and that "15kW Base Station Emissions" do. However such an analysis is valueless without considering the distance between the interference source and the GPS receiver.
So consider this: a single LightSquared 15kW base station may indeed cause operating problems for a large number of poorly filtered GPS receivers over a wide area. However the low power 2W-ish transmission of a satcoms mobile phone would still have been able to cause problems, just over a much shorter distance (perhaps tens of meters?).
And consider where the satcoms mobile phone might have been used; on a train, on a plane, in a car with SatNav, etc. etc. If they had become as ubiquitous as terrestrial mobiles are today they'd be everywhere all the time. It is perfectly possible that the satcoms phone itself could have caused equally significant problems for GPS just by being physically close to critically important GPS receivers. Anyone who has ever placed a GSM mobile phone near a set of loudspeakers and heard the ticketer-ter ticketer-ter sound it makes will understand my point, especially considering that a set of loudspeakers isn’t designed to be a highly sensitive radio receiver like GPS.
What I seek to show above is that the question of how much guard band to have between GPS and satcoms was just a relevant then as it is now between GPS and LightSquared. It is highly likely that practicality of the chosen guard band was considered long ago by the FCC when the satcom band was first allocated. That consideration ought (hindsight?) to have taken into account a close encounter between a GPS and a satcom phone. That scenario is, from the point of view of a single GPS receiver, not so very different to a more distant encounter with a LightSquared base station. So if the guard band then was considered to be practical, why not now? Maybe the FCC didn't consider such a scenario back then, or maybe they never imagined that everyone and everything would be using GPS receivers for some purpose or other.
By endeavouring to develop an adequate filter LightSquared are seeking to show that the FCC got it right and that the GPS industry are too damned lazy and cheap to do their own jobs properly. The irony of the GPS industry having to license a filter design from LightSquared would be memorable...
However, if the GPS industry is proved right and that filter with adequate out of band rejection can't be build then that really would mean that the FCC got it wrong, and arguably always had got it wrong long, long ago when the 1525-1559 band was first allocated for satcoms.
In both outcomes it's not LightSquared's fault (though you could argue that they should have known better than that). I don't think that it would really be the FCC's fault either. The bands were allocated to satcoms long before anyone thought that every mobile phone, car, etc. would have a GPS receiver in it, so the need for miniaturisation of filters with sharp cut-offs didn't exist. It's not really the GPS industry's fault either. Hardly anyone was actually using the satcoms band. If they had been then we'd have sorted this out years ago. But the solution (whatever it turns out to be) is going to cost a lot of money, and that's always going to come from the customers one way or other.
Some people have pointed out that this is a purely North American problem. But we'd all like our phones (including its GPS) to work properly when we go there.
Just a quibble. Intermodulation and IF images happen entirely in the receiver. There isn;t anything you can do at the transmitter other than avoiding frequencies and combinations of frequencies that can stimulate these sorts of problems.
This in the end comes down to a matter of opinion. The battle between the users of satellite downlinks and terrestrial transmitters is ongoing. This is merely a single battle...
Man who stands to make a ton of money making filters to allow GPS and LightSquared to cooperate says he approves of LightSquared. What a surprise! And how do they propose to address all the GPS units in current existence? Unless the filters are fitted to the phone and not the GPS?
So what magic filters are these? Lightsquared could filter their signals so the out-of-band power that leaks into the GNSS band is 'negligible', but that's not easy or cheap, especially if band-edge phase performance is important. However there is still lots of power in their own band which cheap GPS receivers won't necessarily filter effectively, and will get aliased by the digital sampling process into the signals the GPS software has to sort out.
Now that GPS has had augmentation and integrity features added by SBAS services such as WAAS & EGNOS, it can be used as a primary navigation aid by aircraft, so that's going to have a severe impact on the aviation industry if GPS can be randomly unreliable due to interference, continent-wide in the US.
"Now that GPS has had augmentation and integrity features added by SBAS services such as WAAS & EGNOS, it can be used as a primary navigation aid by aircraft, so that's going to have a severe impact on the aviation industry if GPS can be randomly unreliable due to interference, continent-wide in the US."
Anyone suggesting that GPS is usable as a primary navigation aid for aircraft hasn't done enough failure mode analyses. Any radio system is susceptible to interference / deliberate jamming. They've merely done a commercial analysis that say's it's cheaper than, for example, an inertial navigation system. It's especially worrying if the GPS equipment manufacturers haven't bothered to secure their receivers against legitimate out of band signals.
The aviation industry is already experiencing significant problems with kids (presumably kids) shining laser pointers at aircraft coming in to land. Given the ease with which GPS jammers can be bought, how long before some kid tries one out near an airfield for a laugh?
"But LightSquared reckons better filters are all that's needed, and now claims it will have 25 working prototypes of such filters within the next couple of weeks."
Before LightSquared mentioned before that they wouldn't interfere with GPS at all and welcomed the GPS industry to the tests. The tests clearly showed that there was interference, so this wouldn't be the first time, the second time or even the third time that LightSquared has lied.
If they filters were commercially available in a month, it wouldn't change a thing, how many devices would need to be replaced? Car NAV's, portable NAV's, phone, aircraft, etc. Sorry, but unless you are talking about a decade long plan before you light your network up, it won't help.
What Gen. Shelton is freaking out about is the degradation of his high end guidance systems used on military aircraft, autopiloted drones, guided ordinance, ...
LightSpeed's cheap transmitters are splattering all over the GPS frequencies. This significantly decreases the accuracy of GPS carrier phase correction processing. Centimeter accuracy suddenly becomes 10 meters or worse.
That might be ok for a nuke, but won't cut it when he's poking a small FAE into a cave entrance or through your bedroom window.
It is more of a case that cheap GPS units are keeping civilians out of his airspace.
MIL GPS usage is either hardened against jamming or has a fallback such as INS, a Pilot or map.
Read the previous notes. Lighsquared does not operate outside its licenced spectrum. If it did, then the FAA would be locking them up and throwing away the key. The problem is shortcuts in the design of gear that uses the adjacent spectrum means that they detect Lightsquared frequencies and suffer.
A suitable analogy is White Trash complaining that middle class Blacks or Asians have moved in next door. Just because you were nearby first, doesn't mean you have the right to lower other people to your standards.
And another thing, as GPS units are receivers, their makers do not pay licence fees for the spectrum they do use. Sine the US turned off significant signal degradation under Clinton, they have access to the high accuracy signal in real time. They are freeloaders watcing their gravy train slowly derail.
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