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Ionospheric Interference With GPS Signals

kdawson posted more than 5 years ago | from the trusting-your-garmin dept.

Space 127

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.

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127 comments

Hans Reiser ESCAPES !! (-1, Offtopic)

Anonymous Coward | more than 5 years ago | (#23743883)

He's on the run !!

Re:Hans Reiser ESCAPES !! (0)

Anonymous Coward | more than 5 years ago | (#23743997)

You'd run too if your cellie called you "Riser".

Re:Hans Reiser ESCAPES !! (-1, Offtopic)

Anonymous Coward | more than 5 years ago | (#23744031)

Am I the only one who understands the joke?

Re:Hans Reiser ESCAPES !! (2, Informative)

Anonymous Coward | more than 5 years ago | (#23744309)

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

Re:Hans Reiser ESCAPES !! (0)

Anonymous Coward | more than 5 years ago | (#23744411)

Nope, it's Paris, Texas, that's the inside joke. No, not that.

Re:Hans Reiser ESCAPES !! (0)

Anonymous Coward | more than 5 years ago | (#23745921)

I (think I) do, but only because "Ich sprach ein venig Deutsch", so I suppose it is required prerequisite.

Time to declare war? (5, Funny)

Anonymous Coward | more than 5 years ago | (#23743911)

The electrons in the ionosphere must be terrorists!

Re:Time to declare war? (0)

Anonymous Coward | more than 5 years ago | (#23744559)

YOU'RE a terrorist.

4th reply (0, Funny)

Anonymous Coward | more than 5 years ago | (#23743927)

that is also completely worthless

Dual Frequency (4, Interesting)

Detritus (11846) | more than 5 years ago | (#23743941)

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:Dual Frequency (0)

Rakshasa Taisab (244699) | more than 5 years ago | (#23744159)

I thought the military used a second frequency because the public one is intentionally made less precise in war zones.

Re:Dual Frequency (0)

DRobson (835318) | more than 5 years ago | (#23744265)

If I understand it correctly, there is a predictable pattern of error introduced into the publicly available signal. If you know the key of the day you should be able to remove the introduced error. Also, I believe there is an ability to introduce more substantial error at will. Take it with a grain of salt, vague recollections of wikipedia a few nights back.

Re:Dual Frequency (3, Informative)

jrumney (197329) | more than 5 years ago | (#23744405)

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:Dual Frequency (3, Interesting)

DRobson (835318) | more than 5 years ago | (#23744451)

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 (1)

alx5000 (896642) | more than 5 years ago | (#23744695)

Wikipedia [wikipedia.org] agrees with you. The main problem with error injection was the proliferation of commercial-grade GPS receivers among military personnel: the US Army couldn't afford to equip everyone with a military-grade receiver, so the soldiers started buying them themselves.

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).

Re:Dual Frequency (4, Interesting)

The Evil Couch (621105) | more than 5 years ago | (#23744961)

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:Dual Frequency (2, Interesting)

tlhIngan (30335) | more than 5 years ago | (#23749009)

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 compared against another source of navigational information (VORs, etc). The reason is, GPS is cheap compared to maintaining the entire network of VORs and NDBs (and sometimes LORAN) equipment.

The other issue is that if they degrade the GPS accuracy, there is a huge loss - a number of airports have instrument approaches that rely on GPS, and good GPS units can often be used at lower minimums than regular ILS approaches (not landings, though, for obvious reasons). Airports are happy with GPS approaches because they're cheap, they avoid having to maintain expensive ILS equipment. Thus there are a number of GPS-approach-only airports (the requirements are quite strict - WAAS must be available, and enough GPS satellites must be available to compensate for satellite irregularity. Aviation GPSes that are certifiable for instrument approaches have calculators that can tell you if an approach is possible at the destination based on current GPS almanac data).

If they started degrading GPS again, the impact on aviation would be quite significant.

The other thing is WAAS. FAA wanted a way to compensate for GPS signal degradation, so they had WAAS put in, which broadcasts correction data... from the GPS satellite! (That's why most modern GPS receivers can pick up WAAS easily - the satellite is already transmitting the information, so picking up the WAAS information is trivial). Of course, if you degrate the main GPS signal and don't degrade WAAS, the whole exercise is pointless.

(DGPS requires an external receiver - higher end units have a bidirectional serial port so they can transmit NMEA data to a host, but also receive a DGPS correction data from a DGPS receiver, which is why almost no GPS come with built in DGPS - it doesn't come "for free" like WAAS does).

Re:Dual Frequency (0)

Anonymous Coward | more than 5 years ago | (#23750563)

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.

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.
You're both right: military GPS receivers use two frequencies, and so are able to completely remove ionospheric error. Selective Availability (SA), the feature that injected errors into the unencrypted "civilian" signal has been off since 2000. However, civilian receivers are still only able to use that one signal, which exists only on one frequency, and so are unable to correct for ionospheric error the way military receivers are. There are actually some very high-end civilian receivers that are able to use the encrypted military signals to correct for ionospheric errors without decrypting them, but they're extremely expensive. The good news is that the next version of GPS will add a second civilian signal on a second frequency, allowing civilian receivers to completely eliminate ionospheric error the way military receivers already do. Some of these new [block IIF] satellites are already flying, but not enough to provide full coverage.

Re:Dual Frequency (2, Insightful)

Shipwack (684009) | more than 5 years ago | (#23744533)

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 receiver, and a correction applied.

Re:Dual Frequency - Not just for the military (1)

mkramer (25004) | more than 5 years ago | (#23746653)

We've designed civilian receivers that use the L2 signal for correction, too, using the carrier wave, avoiding having to decrypt P code.

Not something you'll find in your Garmin or iPhone 3G, but not horrible uncommon in high-end survey equipment.

Re:Dual Frequency - Not just for the military (2, Interesting)

afidel (530433) | more than 5 years ago | (#23747403)

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.

Good Grief! (4, Interesting)

Anonymous Coward | more than 5 years ago | (#23744047)

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

Re:Good Grief! (4, Interesting)

jacquesm (154384) | more than 5 years ago | (#23744145)

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

Re:Good Grief! (5, Interesting)

owlnation (858981) | more than 5 years ago | (#23744323)

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:Good Grief! (1, Funny)

Anonymous Coward | more than 5 years ago | (#23744569)

Showing bias towards him is going to hurt you long term, it's already losing you respect.
Respect? Slashdot "Editors"? Been drinkin' tonight?

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23746645)

It's obvious the Slashdot editors don't care that people don't like Roland. Note how the delete any tagging of Roland articles. To me that's a clear sign of bad faith.

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23746923)

It has gone beyond bias. Now the editors are explicitly shilling for Roland's "other" blog by mentioning it in editorial comments following the story posting and describing it as having "useful links."

Although elsewhere in this thread someone questioned another's dislike of Roland as based on the fact that he is French, I will go on record by stating that my dislike is based on the quality of his writing and inexplicable ability to have low-quality submissions hit the main page.

Blackmail (1)

dreamchaser (49529) | more than 5 years ago | (#23748625)

Roland has photos of Cowboyneal doing unspeakable things to an iMac. If they stop posting his crap they photos go public!

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23750507)

discovery-channel-level science
there's no need to insult Discovery channel by comparing it to Roland!

Re:Good Grief! (1)

Psychotria (953670) | more than 5 years ago | (#23744515)

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! (0, Troll)

caluml (551744) | more than 5 years ago | (#23744755)

I don't see any problem with Roland's postings. Do you not like him because he is French, or some other banal reason?
Submit your own super-interesting stories if you have better ones.

Re:Good Grief! (4, Interesting)

jacquesm (154384) | more than 5 years ago | (#23745247)

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:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23745807)

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.

Have you read a lot of the stuff that's sitting over on the firehose? If by "standards" you mean "writing quality that would make a third grader shameful," then yes, by all means it is of "high standards." Sorry, but a lot of the writing found over there is abysmal. At least with Roland's writing, it actually is written well. He may not be the brightest techwriter out there, but at least he has the ability to write. Which is a lot more than can be said for most other bloggers.

Re:Good Grief! (1)

caluml (551744) | more than 5 years ago | (#23746555)

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:Good Grief! (2, Insightful)

1u3hr (530656) | more than 5 years ago | (#23745365)

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:Good Grief! (1)

caluml (551744) | more than 5 years ago | (#23746591)

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:Good Grief! (1)

1u3hr (530656) | more than 5 years ago | (#23747253)

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 moaning in hours if there weren't any stories posted.

Look at the firehose. There is no shortage of stories.

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23745515)

He's French?

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23745707)

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

No, but you gave me another reason to dislike him!

My bet is, slashdot editors own shares in some sites - count the number of networkworld posts. Its like slashdot is the frontpage for nww and roland.

Re:Good Grief! (4, Funny)

Frosty Piss (770223) | more than 5 years ago | (#23744179)

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:Good Grief! (1)

zeromorph (1009305) | more than 5 years ago | (#23744943)

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:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23747029)

Don't forget Greek and corporophagia!

Re:Good Grief! (0)

Anonymous Coward | more than 5 years ago | (#23747033)

Yes, but does he blend?

Re:Good Grief! (2, Funny)

Nullav (1053766) | more than 5 years ago | (#23744189)

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

GPS is digital! (0)

VincenzoRomano (881055) | more than 5 years ago | (#23744055)

We can reasonably rely on GPS as the signals sent are digital. If you receive them, then they are (supposed to be) correct.
The only problem could be when you don't receive any signal. But this is a different story.

Re:GPS is digital! (1)

Detritus (11846) | more than 5 years ago | (#23744283)

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:GPS is digital! (1)

pragma_x (644215) | more than 5 years ago | (#23747017)

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! (1)

crumley (12964) | more than 5 years ago | (#23748981)

If you can get any data from the transmission after the signal has been processed, it's highly likely that its good data.

The real problem isn't so much distortion of the signal in the ionosphere, which seems to be what you are getting at. The problem is that variations in the ionosphere change how long it takes for signals to get through the ionosphere. This is obviously a problem for GPS since it relies on timing the signals in order to calculate positions.

Re:GPS is digital! (3, Insightful)

canavan (14778) | more than 5 years ago | (#23744327)

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.

The ionosphere can change rapidly (1)

Anonymous Coward | more than 5 years ago | (#23744693)

The ionosphere has a daily cycle plus variations due to the sun's radiation. Also, the ionosphere varies depending on location. For instance, the E layer is highly variable depending on time and position. It is very hard to correct for.

For those who don't know how the ionosphere affects GPS, here's a quote from one of the articles tfa links to:

However, the GPS signals must transit the ionosphere to communicate with ground receivers. This transit introduces signal propagation errors because the ionosphere affects the propagation speed and direction of all radio signals (including GPS). In addition, electron density irregularities in the ionosphere can introduce amplitude and phase fluctuations, a process known as scintillation.
http://www.agu.org/journals/sw/swa/free/newarticle/?id=2008SW000400

Re:GPS is digital! (0)

Anonymous Coward | more than 5 years ago | (#23749601)

This is exactly right.

WAAS and DGPS both use ground based stations (of known location, obviously) to determine GPS error and correct for it. A modern GPS unit with WAAS turned on will query the WAAS sat to get a correction matrix for its location based on all the nearby ground stations that are reporting to the WAAS sat.

Re:GPS is digital! (4, Interesting)

borizz (1023175) | more than 5 years ago | (#23744367)

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:GPS is digital! (1)

spandex_panda (1168381) | more than 5 years ago | (#23744383)

nice. The interesting thing is that over an area of a few kilometers, the error from ionosphere is the same, so if we get a station with known coordinates and calculate the difference between these and the GPS coordinates with the ionosphere error, we can send this error to a 'rover' and correct it to get much better accuracy!

Re:GPS is digital! (2, Informative)

The Evil Couch (621105) | more than 5 years ago | (#23745011)

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:GPS is digital! (2, Informative)

borizz (1023175) | more than 5 years ago | (#23745165)

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:GPS is digital! (1)

The Evil Couch (621105) | more than 5 years ago | (#23745259)

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:GPS is digital! (1)

borizz (1023175) | more than 5 years ago | (#23746107)

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. Still, you're right. You need 4 sats for a true "3D" position fix.
I think people who depend on correct altitude information should spend some more money and get a GPS with a barometric altimeter.

Also, I am not really interested in my altitude. I live in the Netherlands, where the highest elevation around in most places is a curb. :)

Re:GPS is digital! (1)

afidel (530433) | more than 5 years ago | (#23747467)

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

Re:GPS is digital! (1)

SuiteSisterMary (123932) | more than 5 years ago | (#23747767)

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.

Trilateration, not triangulation (1)

ndoggac (1305845) | more than 5 years ago | (#23748031)

I know I'm splitting hairs here, but FYI, any GPS position is calculated using trilateration, not triangulation.

Re:GPS is digital! (5, Informative)

spandex_panda (1168381) | more than 5 years ago | (#23744369)

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:GPS is digital! (1)

cerberusss (660701) | more than 5 years ago | (#23745595)

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:GPS is digital! (0)

migloo (671559) | more than 5 years ago | (#23744427)

Positioning relies not on the contents of the signal but on its delay reaching your receiver. Being digital is irrelevant. Ionospheric propagation speed variations introduce an error of 300m for each microsecond difference in the signal reception time.

Re:GPS is digital! (1)

Nullav (1053766) | more than 5 years ago | (#23744467)

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 broadcasting its ID.
People? A lot of us use phones for GPS, rather than dedicated receivers. It doesn't seem at all unreasonable to fall back on cell towers.

Actually, I think #1/3 are being done.

Re:GPS is digital! (1)

gnasher719 (869701) | more than 5 years ago | (#23745049)

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 that any drastic inaccuracy like that would put you on the _surface_ of the earth in Antarctica are quite small.

And even if a signal is totally messed up, as soon as you receive five signals the GPS _knows_ that something is wrong.

Re:GPS is digital! (1)

polywaffle (827427) | more than 5 years ago | (#23746315)

I know my gps has a setting that keeps the position locked to the road, so assuming that im always going to be using it for driving it works for correcting really obvious errors.

Re:GPS is digital! (0)

Anonymous Coward | more than 5 years ago | (#23744479)

...And because your post is stored in digital format in slashdot's database, you must be correct.

What about timekeeping? (1, Interesting)

Anonymous Coward | more than 5 years ago | (#23744395)

Is this likely to affect GPS based reference time sources?

My understanding is that you need to see a constellation of 4 sattelites to get accurate time. Use 3 to pinpoint your exact position, and then use that knowledge, and your knowledge of the 4th sattelite's position, to compensate for the delay in receiving the time signal.

If the precision of your position lock is degrated or unreliable, would the decreased precision of the reference time be enough to cause problems?

Re:What about timekeeping? (0)

Anonymous Coward | more than 5 years ago | (#23745045)

That is not the way GPS time works. The assumption is that the time in your receiver is never accurate, so a time offset must always be calculated. With 4 received satellite signals you have 4 equations for 4 unknowns (x,y,z and time offset), a system that can easily be solved.
So you always need a cluster of 4 satellites minimum for an accurate position (and time). A degraded signal will decrease the accuracy of both position and time; it is not the inaccuracy in position that causes the inaccuracy in time.

However, if you're a Ham.... (4, Interesting)

caluml (551744) | more than 5 years ago | (#23744585)

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.

This could be dangerous ... (1)

casab1anca (1304953) | more than 5 years ago | (#23744697)

... for instance, if you're trying to locate some place in an emergency - you might be led astray by a wrong signal. That's the problem with technology - although it can do amazing things, you never know when it'll fail. (or to put it the Murphy way - it'll fail when you need it the most)

Re:This could be dangerous ... (1)

camperdave (969942) | more than 5 years ago | (#23750327)

It's not as if this is new. This ionospheric interference has always existed. If you had a ten metre accuracy before, you'll have a ten metre accuracy still.

What *IS* new is that scientists are using this GPS inaccuracy to map the ionosphere.

Mid-latitudes (1)

mokeyboy (585139) | more than 5 years ago | (#23744717)

Its been a while since I last was doing this for a living (http://www.physics.uq.edu.au/sp) but in general, this does not does have a significant effect in the mid-latitude regions of the world (think temperate climate regions). In equatorial regions, the effects on GPS are more likely to be associated with the troposhere (rainstorms and the like). Yes, there are high-latitude regions (auroral storms) that face problems but I usually operated under the assumption that this was generally: 1) more important to nuclear subs lurking in the poles 2) of diminishing consequence when rated against inductive current effects such as suffered by electric production systems (http://arc.iki.rssi.ru/mirrors/stern/Education/FAQs6.html#q81). Space weather events are more likely to be severe in impact because relativistic particles embed in satellite electronics and cause havoc, rather than Earth bound events.

One more reason.... (0)

Anonymous Coward | more than 5 years ago | (#23744783)

not to get an iPhone... "I see your reality distortion field, and I raise you one electrically active ionosphere!"

second amendment (2, Funny)

stripyd (614714) | more than 5 years ago | (#23745265)

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?

Aviation uses RAIM (1)

JayFlatland (125245) | more than 5 years ago | (#23745349)

For years, aviation has been using technology called RAIM [wikipedia.org] . With enough satellites, position is over-specified and can be checked for self-consistency.

Re:Aviation uses RAIM (1)

Discoflamingo13 (90009) | more than 5 years ago | (#23746931)

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.

No Problems for me... (3, Insightful)

Frightened_Turtle (592418) | more than 5 years ago | (#23745549)

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 (3, Insightful)

RJFerret (1279530) | more than 5 years ago | (#23746755)

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

Re:Hence WAAS (1)

vjoel (945280) | more than 5 years ago | (#23747427)

For car navigation, you don't even need it, as the accuracy is better than the width of a road regardless!
Not true. Google for "lane-level accuracy". How do you know if you are in an exit/turn lane? On a service road parallel to a freeway? In a lane with a stalled car ahead? (Ok, that's safety, not nav, but the car companies are anticipating precise enough sensor data to do safety apps.)

Re:Lane Level Accuracy (1)

ndoggac (1305845) | more than 5 years ago | (#23747893)

Lane level accuracy will have to come from embedded circuits in the roadway. Companies are already working on having an embedded IC's in the roadway reflectors that will tell your car which lane it's in with much greater accuracy and integrity than GPS with any space based augmentation system can. As the prices drop on the integrated circuits to pennies per, this technology becomes more viable. Of course your car's computer system will use both GPS/SBAS and the roadway sensors in tandem to achieve greater accuracy/reliability.

Re: Ionospheric Interference With GPS Signals (0)

Anonymous Coward | more than 5 years ago | (#23746905)

So do trees.

The ionosphere causes several effects (1, Informative)

Anonymous Coward | more than 5 years ago | (#23747039)

First off, this is old news. It has been studied for over 25 years (I was peripherally involved with this in the early 1980s).

There are two major ionospheric effects: delay and phase variations. The ionosphere is a region above the earth's atmosphere in the altitude range from about 200 km to a few thousand km. In this region there is a very low density of atoms and a significant fraction are ionized by solar radiation. The presence of electrons, combined with the earth's magnetic field, has a significant effect on radio wave propagation.

The first effect, delay, is related to the electron density in the ionosphere and, since GPS uses delay to determine distance, this affects the navigation accuracy. However, this effect can be removed by using dual-frequency receivers and it can be modeled and mostly compensated for by using differential correction systems such as WAAS. Note that WAAS only models large- and some medium-scale disturbances in the ionosphere so it will be degraded if there are strong medium- and small-scale disturbances.

The second effect is random phase variations, called phase scintillation, caused by small-scale ionospheric disturbances. If severe enough it will cause the GPS receiver to lose lock on the signal. This can somewhat be compensated for by designing a robust synchronization system in the GPS receiver.

GPS designers already working on this (1)

Discoflamingo13 (90009) | more than 5 years ago | (#23747041)

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.

LORAN - still in use. (1)

bodland (522967) | more than 5 years ago | (#23747173)

I have a unit on my sailboat. GPS died a few years back. The cira late 70's device is still going strong. It is cool retro tech. If not a little geeky along the HAM radio lines.

With SBAS, yes you can!! (1)

ndoggac (1305845) | more than 5 years ago | (#23747799)

I wanted to point you to our FAA website showing the near-realtime performance of the WAAS/GPS system. WAAS already provides error corrections for ionospheric interference as well as satellite clock and ephemeris corrections to any user tracking the WAAS geosynchronous satellite. GPS III and the corresponding L5 civil frequency will remedy this issue for users with capable receivers, but a GPS III constellation is decades away. Almost all of today's commercial receivers are WAAS capable. We have been studying GPS and WAAS performance including ionospheric activity effects for over 14 years in this office, through a solar cycle maximum and minimum. Our quarterly reports have entire sections dedicated to ionospheric study. I look forward to the increased accuracy, etc GPS III will provide 20 years form now, but I feel you neglected to mention that WAAS (and other SBAS) provides a lot of this functionality in the present to the majority of users in the world. http://www.nstb.tc.faa.gov/ [faa.gov] Space Based Augmentation System (Include WAAS, GAGAN, EGNOS, MSAS) A WAAS capable receiver will work with any of the SBAS's and vice versa.

Differential GPS (1)

spaceyhackerlady (462530) | more than 5 years ago | (#23748487)

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

Following our news... (0)

Anonymous Coward | more than 5 years ago | (#23748999)

...are the weather and GPS forecast for the next days. Good night.

Re: (1)

clint999 (1277046) | more than 5 years ago | (#23749645)

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 standard, or at least something that tended to be corrected by using WAAS. The wikipedia article lists this as part of "slow" corrections.

Roland the Plogger, again (2, Informative)

Animats (122034) | more than 5 years ago | (#23750241)

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.

l2 is the solution (1)

heroine (1220) | more than 5 years ago | (#23750671)

Obviously shares of GPS stocks are down today, but L2 was supposed to solve ionospheric interference. Go to Iran & get yourself some L2 goodness.
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