Ionospheric Interference With GPS Signals

Roland Piquepaille writes "In recent years, we have become increasingly dependent on applications using the Global Positioning System, such as railway control, highway traffic management, emergency response, and commercial aviation. But the American Geophysical Union warns us that we can't always trust our GPS gadgets because 'electrical activity in the... ionosphere can tamper with signals from GPS satellites.' However, new research studies are under way and 'may lead to regional predictions of reduced GPS reliability and accuracy.'" Roland's blog has useful links and a summary of a free introduction, up at the AGU site, to a special edition of the journal Space Weather with seven articles (not free) regarding ionospheric effects on GPS.

Time to declare war?

[by Anonymous Coward]

The electrons in the ionosphere must be terrorists!

Oblig.

[elguillelmo (1242866)]

Tinfoil hats ahoy!

Re:

[WormholeFiend (674934)]

Tinfoil hats ahoy!

Indeed, my conspiracy theorist friend, indeed [wikipedia.org]

Dual Frequency

[Detritus (11846)]

I thought that was why the military version of GPS used two frequencies. From what I've read, it allows them to measure the actual propagation delay through the ionosphere, instead of relying on the static delay prediction model used in the single frequency mode used by civilians and those without a crypto-keyed military GPS receiver.

Re:

[Shipwack (684009)]

It's one of the reasons. The second one being, as someone else has mentioned, that one used to be more precise and encrypted, with the other being less precise and for civilians. There is no longer any distortion applied to the civilian band, and with differential GPS now available, it's a moot point (at least where DGPS is available).

Ionosphere interference is reduced by using two frequencies. The higher frequency shifts less when it enters the ionosphere. Both frequencies are compared by the receive

Re:

[afidel (530433)]

Yep, and good equipment will also use Glonass when available. I expect once the Galileo constellation is more complete you will see even higher end consumer devices using both GPS and Galileo. I was really glad when they announced that the commercial parties had abandoned the project and that it was being picked up by the EU directly, per device licensing fees would have meant it would basically go unused like Iridium.

Re:

[Criliric (879949)]

less precise everywhere actually

Re:Dual Frequency

[jrumney (197329)]

I thought they'd stopped injecting error as a routine measure a few years back. It is so easy to get around that I doubt it serves any military purpose, even for relatively unsophisticated enemies.

Re:

[DRobson (835318)]

Yeah, it appears so. In order for the random error to be useful for non-military use the error had to be somewhat uniform across large regions. So, once you established the error on one known point you were pretty right.

Also, it looks like military personnel ended up buying there own civilian units a large percentage of the time with obvious problems.

Looks like it was officially disabled around 2000 or so.

Re:Dual Frequency

[The Evil Couch (621105)]

I don't think scarcity was driving the conversion that much; I found PLGRs pretty common in the Army while I was in (1998-2005). However, the main draw to commercial GPS products was that the PLGR had a fucking awful UI and was about the size of a hardbound dictionary. The internal hardware and screen was hopelessly out of date by the time it was in common usage. Entering numbers by pressing UP/DOWN? No visual map? A control scheme that required a knowledgeable or at least technologically apt soldier to? Fuck that! If there's a navigation tool for my squad, I need everyone in the squad to be able to use it. If I'm the only one that can make use of it and I go down, it's instantly become useless.

That's not to say that it was a total piece of shit. It was water-proof and pretty durable. It was really extensible; it could be plugged into a variety of other things, which made them really useful *if* you had the proper hardware. The problem was that all the needed gear to take full advantage of it required a vehicle to transport and provide power. The PLGR was a fantastic piece of gear for anyone but the infantry. Problem is, there's a hell of a lot more infantry that needs coordination on the ground than there is anyone else. So, many of us bought our own.

Re:

[Comboman (895500)]

The PLGR has been replaced by the DAGR [wikipedia.org] which addresses many of the problems you described.

Re:

[tlhIngan (30335)]

And if I'm not mistaken, they were considering to enable it again, but the FAA asked them not to, since aircraft use it to better state their position (I'm sorry for any factual inaccuracy, but I'm just a Spaniard with a limited understanding of how the system works and the US agencies involved).

Less positioning, more navigation. GPS is rapidly becoming (if it hasn't already) a level 1 navigational device (trustable on its own). Right now, it's level 2, which means it's good for general use, but must be com

Good Grief!

[by Anonymous Coward]

More Roland fest! Why doesn't SourceForge just hire the guy? Good grief! Who's he giving blow jobs to?

Re:Good Grief!

[jacquesm (154384)]

why is his stuff getting this insane posting ratio on ./ ? Since march 21st of this year 20+ accepted submissions ??

Re:Good Grief!

[owlnation (858981)]

Agreed. Slashdot editors take note: a lot of people here do not think Roland is neither intelligent enough nor qualified to be making /. at all, but 20+ articles in a few months is a total disgrace. There are many people here who absolutely hate this guy and the off-the-wall, irrelevant, discovery-channel-level science, garbage he writes. Showing bias towards him is going to hurt you long term, it's already losing you respect.

Re:

[Psychotria (953670)]

Did you not read the link to his blog? The answer is right there at the bottom of the article:

A final note: if you own - or use - a GPS device, bookmark this excellent article.

Re:Good Grief!

[jacquesm (154384)]

Let me spell it out for you, I'll ignore your strawman about me not liking him 'because he's french', I don't know what prompted you to say that, it lowers the discussion level:

Roland has an extremely high ratio of postings and a *much* higher ratio of accepted postings. So much higher that for the longest time I figured he was a sockpuppet for one of the ./ editors. Once you start noticing and analyze the quantities of stories getting rejected from other members, the quality of those stories and how many of Rolands stories get accepted and the quality of *those* stories then you really can't help but wonder what the game is here.

The discrepancy is too large to be ignored or brushed under the carpet.

After all, the ./ firehose gives you a pretty good idea of which stories make the grade and which don't (besides of course a guaranteed placement of dupes ;) ), and it allows you to get a good idea of the average submission quality of stories that eventually don't make it.

The standards that most postings are held to would mean that *none* of Rolands postings would have been accepted, they are the very definition of blog spam.

Something is smelly here, even if I can't quite put my finger on it. Maybe it's time to do some scripting to get some real hard stats on this whole thing.

Re:

[caluml (551744)]

Let me spell it out for you, I'll ignore your strawman about me not liking him 'because he's french', I don't know what prompted you to say that, it lowers the discussion level:

I wasn't trying to troll - I just couldn't think of any other reasons off the top of my head.
However, I must say that I don't spend a lot of time analysing the balance of quality of stories to the chance of them being accepted, so I'll have to defer to your superiour knowledge in this area :)
I just look at the stories, and see if I like them. I happen to be a Radio Ham, so this one is of interest to me.

Re:

[1u3hr (530656)]

I don't see any problem with Roland's postings. Do you not like him because he is French, or some other banal reason?

I don't like him becasue he plagiarises stories from other sites, copies them to his blog, then submits to Slashdot. He's just trying, and succeeding, in pumping up his pagerank. Originally he used to ONLY link to his blog. There were many complaints about that, eventually he started also giving the original link, but he always adds his blog link as well. He's a parasite.

Re:

[caluml (551744)]

Well, that's a fair reason, I suppose.
However - why shouldn't he get a little PR for supplying Slashdot with stories?
After all, we'd be moaning in hours if there weren't any stories posted.

Re:

[1u3hr (530656)]

However - why shouldn't he get a little PR for supplying Slashdot with stories?

Every submitter gets one link, on his name. He pimps his site with the bogus "for more information" one he puts at the end (in this case the slashdot editors have, unusually, added the original link, if you compare with the firehose version.

Anyway, it's a bit like RealNetworks, there is a lot of residual mistrust after seeing how they exploited their access, and a feeling not to trust them an inch again.

After all, we'd be

Re:Good Grief!

[Frosty Piss (770223)]

I for one welcome our new Roland Overlord. May he pour hit grits down Natalie Portman's shorts, I'll take a Beowulf cluster of that! I'll bet in Soviet Russia they can't even get Roland. But one thing is for sure, he does run Linux. And all these stories of his on Slashdot almost certainly result in Profit!

Re:

[zeromorph (1009305)]

I don't want to make the Roland here, but what's your problem with Roland Pick-a-pie?

I guess Roland is just a new name for that entity Anonymous Coward that we all love and respect for its valuable contributions to our beloved slashdot. What would a day on slashdot be without goatse, first post and Roland postings

Re:

[Nullav (1053766)]

Look on the bright side: Roland's blog-spam gets the editors to edit submissions somewhat.

However, if you're a Ham....

[caluml (551744)]

However, amateur radio people such as myself rub their hands with glee, as a reflective ionosphere means good DX [wikipedia.org]:)
I check the "Space dials [rice.edu]" regularly, and can't wait for them to be in the red! 73s.

second amendment

[stripyd (614714)]

And we "trust" the GPS which the US Government controls the big OFF switch to because....?

Fortunately we have the right to bear sextants.

Now which button on this Tom Tom gives me the GHA of the first point of aries?

No Problems for me...

[Frightened_Turtle (592418)]

Doesn't cause any problems for me. Sometimes I've got just a few feet of accuracy in my position, other times it's 10's of feet. I guess it would cause issues with my home-made cruise missle, though...

Aviation has used VOR navigation for decades, developed during WWII. And the US Government has a big OFF switch for that, too. Part of pilot training is knowing how to navigate when all the fancy gadgets are offline. Because you never know when a system will fail.

I just view this as a confirmation of what I've noticed before: that sometimes the signals aren't as good as others. Fortunately, I have a computer that is capable of recognizing the situation and performing the necessary error correction on the fly. I call it my brain.

Hence WAAS

[RJFerret (1279530)]

I've used GPS receivers since 2001 almost daily (I was even featured on CBS news geocaching). A LOT has changed in that time, but WAAS [wikipedia.org] is a brilliant feature all GPSrs now incorporate, that totally adjusts for ionospheric disturbances, by broadcasting corrections from ground stations.

In geocaching, the greater the accuracy the better. For car navigation, you don't even need it, as the accuracy is better than the width of a road regardless!

This article seems to be a decade behind... -Randy

GPS designers already working on this

[Discoflamingo13 (90009)]

Ionospheric interference has always been a problem with GPS signals - but military GPS uses two signals (L1 and L2) in order to isolate the total effect, which is much easier to do if you can decrypt the P-code of the L2 signal. In the efforts to make civilian GPS more robust to interference, GPS will be introducing the L5 code [wikipedia.org] in satellites launched this year to address this problem.

Differential GPS

[spaceyhackerlady (462530)]

For various reasons (including this one), people have come up with ways to enhance the accuracy of GPS.

I've used differential GPS for several applications. Terrestrial beacon stations listen to GPS, and compare where they know they are with where GPS says they are. They broadcast these corrections and anybody in the vicinity can use them.

WAAS is a similar concept. I've played with it too.

...laura

Roland the Plogger, again

[Animats (122034)]

First, it's a Roland the Plogger story, so it's going to be wrong.

GPS accuracy is a serious problem for users who need high precision. More applications are assuming that GPS is precise to a few meters, which, often, it isn't. It's always good enough if you just need to find an airport. Below that level, error can be a problem.

Local high-precision systems, like GPS-based systems for landing, use a pseudolite, a receiver on the ground in a known location that receives GPS and broadcasts small corrections. The pseudolite is usually located near the end of the active runway, so as aircraft get closer to the runway, the error approaches zero. There's a similar setup for "precision farming", where the tractor precision is precisely known but there's a psuedolite at the side of the field.

Without a pseudolite, it's harder. Part of the problem is that there aren't enough satellites. To get a GPS lat/long fix, you need to see at least three sats. To get lat/log/elevation, you need to see four. For high-precision work (down to 15cm), you need five, plus correction signals from receiving stations (see Omnistar) that are monitoring propagation. You're lucky to see four in a built-up area, because you can only see part of the sky. If you can see five, you can measure error. Some systems use both GPS and GLONASS sats; now that Russia is building up the GLONASS constellation again, this works better. By 2009, the GLONASS constellation should be fully populated, and systems that use both GPS and GLONASS will have a better chance of seeing five sats.

Propagation problems always add delay; they never subtract from it. Propagation problems come from what the ionosphere is doing, and from reflections from big metal surfaces like buildings. In urban canyons, you're seeing mostly bounces.

This is an issue for civilian uses that assume the system has more precision than it really does. Car navigation systems that try to tell whether a car is on a freeway or an adjacent side street from GPS data alone are likely to have problems. The same problem applies to GPS systems for railroad signalling (these make me nervous) which try to tell on which track a train is running.

Re:

[PhxBlue (562201)]

GPS accuracy is a serious problem for users who need high precision.

This is a misleading statement, because it depends on the model of receiver you're using. Some newer receivers, for example, use the two GPS signals -- military and civilian -- to resolve ionospheric interference. You don't actually need to decrypt the military signal, you just have to be able to receive it. Then the receiver can adjust for the ionosphere's activity and give you a highly accurate signal.

railway control?

[fish waffle (179067)]

Isn't GPS a little overkill for railways? I suppose they may end up anywhere, but mostly they stay on the tracks, which makes them quite easy to find.

Re:

[Detritus (11846)]

Digital wont save you if the ionosphere gets hit by a solar flare. I've seen signals from satellites that were strong, but hopeleesly scrambled, when the signal path went through a stormy section of the ionosphere.

Re:

[pragma_x (644215)]

I've seen signals from satellites that were strong, but hopeleesly scrambled, when the signal path went through a stormy section of the ionosphere.

But at least you knew the transmission was scrambled since the data failed to align with the required protocol (e.g. a bad checksum, no magic number, etc.). I think this is what the OP was referring to. If you can get any data from the transmission after the signal has been processed, it's highly likely that its good data.

Re:GPS is digital!

[canavan (14778)]

The data encoded in the signal is digital, however, the location information is derived from the timing of the signal, something that changes depending on the medium (i.e. the distance within the atmosphere the signal has to travel and the precise compisition and electrical conditions therein). I thought that ionospheric corrections were something that was part of the WAAS [wikipedia.org] standard, or at least something that tended to be corrected by using WAAS. The wikipedia article lists this as part of "slow" corrections.

Re:GPS is digital!

[borizz (1023175)]

Thats not how GPS works however. The satellites hum a digital tune. The receiver hums the same tune. It then measures how much later the sat's tune is heard. With this and the speed of light you can calculate how far the satellite is from you. Get distances to three sats and you can triangulate your position.

So you might hear the tune fine, but if the ionosphere delays the tune every so slightly, your reading will be off and your position will be inaccurate.

Re:

[The Evil Couch (621105)]

Four satellites, actually. You have to resolve the position in all three dimensions, unless your receiver has an altimeter and incorporates that into its calculations.

Re:

[borizz (1023175)]

You can kind of assume where the receiver is. You get 2 possible locations with 3 sats, one will be where you are, and one will be up or down from where you are. Pick the location that is most likely and work from there. For example, the railway use in the summary pretty much guarantees that the trains will not go flying any time soon.
Aviation can go both ways, but planes do come with altimeters.

Re:

[The Evil Couch (621105)]

True enough. I was thinking about mostly hand-held units which probably don't have full contour maps for the world. Although as technology improves, that's beginning to be not so true.

Re:

[borizz (1023175)]

My cheap hand-held unit, a Garmin Etrex Legend (which is at least 7 year old technology by now, and retails for about a hundred dollars), just assumes that I am at the position which is nearest to sea level. Which is a valid assumption, considering it's a trekking GPS, and not an aviation one. But, I have had my GPS report my position as -10 meters while I was at approx. 200 meters above sea level (in France). But that usually only lasts a few minutes and a bit of common sense can rule that out as invalid.

Re:

[afidel (530433)]

Actually you can usually get a good approximation using two satellites because the other solution is either in space or inside the planet.

Re:

[SuiteSisterMary (123932)]

Interestingly enough, also, you need to take relativistic distortion into account. General relativity speeds up the atomic clocks (due to less gravity) and special relativity slows down the clocks (due to their velocity); add them together, and the clocks run about 28 microseconds slower than they would sitting beside you on Earth.

Re:

[CXI (46706)]

That's called Differential GPS, although WAAS is more commonly available on consumer units.

http://en.wikipedia.org/wiki/Differential_GPS [wikipedia.org]

http://en.wikipedia.org/wiki/Waas [wikipedia.org]

Re:GPS is digital!

[spandex_panda (1168381)]

Actually this is not true. The digital code modulated onto the radio waves is affected by the ionosphere too. The military gets 2 'code' signals on 2 frequencies, but geodetic or surveying GPS gear observes the 'phase' of the frequencies, there are L1 and L2 frequencies which are observable and you can combine them to cancel out the ionospheric effects. Observing everything, civilian code, carrier frequencies, military P codes, can give you a single point precision of a couple of cm in horizontal (an inch for you yanks) and something like 3 times that in vertical.

Just receiving a digital signal doesn't mean its right!

Re:

[cerberusss (660701)]

Just receiving a digital signal doesn't mean its right!

That reminds me... I went into a (wooden) floor store for a hygrometer. The salesgirl showed a couple of models, one of them being a digital one. So she said, "this one is digital, so it's more accurate because it shows the humidity with one decimal".

Re:

[Nullav (1053766)]

Either way, wouldn't you be a bit suspicious if you were suddenly in Antarctica?

Why not just calculate based on the reported velocity of the vehicle, 'pinging' satellites every minute or so and simply dropping anything that puts you in Antarctica?
Trains? Anything that puts a train a certain distance off the track could be dropped. The acceptable values would have to be manually defined, however. Results could also be checked against reports from evenly-spaced receiver towers, with each train constantly broa

Re:

[gnasher719 (869701)]

Why not just calculate based on the reported velocity of the vehicle, 'pinging' satellites every minute or so and simply dropping anything that puts you in Antarctica?

Because that is not what happens. TFA (and the F does _not_ stand for "fine" in this case) claims that "your GPS cannot be trusted". It can. Ionospheric interference has been well known since the initial design of GPS. It is one of several factors that introduce a few meters of inaccuracy into the GPS in your TomTom, and all these factors add up to about ten meters of inaccuracy. It doesn't put you into Antarctica - and since your GPS calculates a four-dimensional position (x, y, z and time), the chances t

Re:

[by Anonymous Coward]

No, no. Roland Piquepaille is a joke, we get it. It just gets old fast.

Re:

[Discoflamingo13 (90009)]

RAIM just gives you a measure of statistical confidence in your position based on pseudorange measurements, allowing you to exclude signals from bad GPS satellites - if all of your pseudoranges are off because of ionospheric interference, you would never know that that is your error source without additional aiding (inertial and/or radio navigation) in low satellite availability areas.