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Space Science

Ionospheric Interference With GPS Signals 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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Ionospheric Interference With GPS Signals

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  • by Anonymous Coward on Wednesday June 11, 2008 @03:53AM (#23743911)
    The electrons in the ionosphere must be terrorists!
    • Actually, there is some speculation that the Van Allen belts were the result of high altitude nuclear explosions in the 1950s. The Van Allen belts were discovered in 1958.

      They are not terrorists, they are the result of the cold war superpower arms race.

      • by Kagura ( 843695 )
        Actually, there is some speculation that the Van Allen belts were the result of high altitude nuclear explosions in the 1950s.

        Yeah, and there is some speculation that large-scale structure in the universe is actually governed by electric and magnetic influences, rather than gravity. :) But let's get a grip, the Van Allen belts are caused [wikipedia.org] by natural processes. Although entirely separate from the Van Allen belts, if you read the page on the Starfish Prime [wikipedia.org] nuclear test, it explains:

        There have been nuclea
  • Oblig. (Score:3, Funny)

    by elguillelmo ( 1242866 ) on Wednesday June 11, 2008 @03:53AM (#23743917)
    Tinfoil hats ahoy!
  • Dual Frequency (Score:5, Interesting)

    by Detritus ( 11846 ) on Wednesday June 11, 2008 @03:59AM (#23743941) Homepage
    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 the military used a second frequency because the public one is intentionally made less precise in war zones.
      • Re: (Score:2, Informative)

        by Criliric ( 879949 ) *
        less precise everywhere actually
    • Re: (Score:2, Insightful)

      by 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
      • 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: (Score:3, Interesting)

          by 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.
          • it would basically go unused like Iridium.
            You would be surprised what is about to be tried with Iridium. The US Air Force owns it you know.

    • Actually even civilians can use the second frequency to determine ionospheric delay. You do not have to actually decrypt the military signal just to use it to check the delay of the L1 signal.
  • Good Grief! (Score:4, Interesting)

    by Anonymous Coward on Wednesday June 11, 2008 @04:21AM (#23744047)
    More Roland fest! Why doesn't SourceForge just hire the guy? Good grief! Who's he giving blow jobs to?
    • Re:Good Grief! (Score:5, Interesting)

      by jacquesm ( 154384 ) <j&ww,com> on Wednesday June 11, 2008 @04:35AM (#23744145) Homepage
      why is his stuff getting this insane posting ratio on ./ ? Since march 21st of this year 20+ accepted submissions ??
      • Re:Good Grief! (Score:5, Interesting)

        by owlnation ( 858981 ) on Wednesday June 11, 2008 @05:02AM (#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: (Score:1, Funny)

          by Anonymous Coward

          Showing bias towards him is going to hurt you long term, it's already losing you respect.
          Respect? Slashdot "Editors"? Been drinkin' tonight?
        • by ozbird ( 127571 )
          "a lot of people here think that Roland is neither intelligent enough nor qualified to be making /. submissions at all" - fixed that for you (unless you are a sneaky Roland fan exploiting a double negative.)
      • 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: (Score:1, Troll)

        by caluml ( 551744 )
        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! (Score:5, Interesting)

          by jacquesm ( 154384 ) <j&ww,com> on Wednesday June 11, 2008 @07:34AM (#23745247) Homepage
          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.

          • by 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.

            • no need to do much analyzing, just use the slider in the firehose, it will give you an excellent idea of the number of quality postings vs the number of lousy ones.
        • Re: (Score:3, Insightful)

          by 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.

          • by 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.
            • by 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

    • by Frosty Piss ( 770223 ) on Wednesday June 11, 2008 @04:42AM (#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!
      • 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: (Score:3, Funny)

      by Nullav ( 1053766 )
      Look on the bright side: Roland's blog-spam gets the editors to edit submissions somewhat.
  • by Anonymous Coward
    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?
  • by caluml ( 551744 ) <slashdot@spamgoe ... m.org minus poet> on Wednesday June 11, 2008 @05:52AM (#23744585) Homepage
    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.
  • ... 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)
    • 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.
  • 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
  • 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?

  • 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.
    • 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.
  • by Frightened_Turtle ( 592418 ) on Wednesday June 11, 2008 @08:06AM (#23745549) Homepage

    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 (Score:3, Insightful)

    by RJFerret ( 1279530 ) on Wednesday June 11, 2008 @09:46AM (#23746755) Homepage
    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

    • by vjoel ( 945280 )

      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.)
      • 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 greate
      • by dfm3 ( 830843 )

        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?

        By looking out the window? ;-)

        Seriously, though, don't forget that the signal received by your automotive/handheld GPS unit is not the only source of error when trying to orient yourself in relation to an object. Your maps have to be incredibly accurate, too.

        Most of the consumer grade mapping solutions available for automotive GPS usage are only accurate down to a certain point- in my experience this is anywhere from 30-150 feet for Garmin's City Navigator series of maps. Some of their older map products can be much worse (some topo software was scaled off of paper maps, for example, and only claims to be accurate to within 800 feet). I too am an avid geocacher, and when a cache hider sloppily takes an inaccurate waypoint, it doesn't matter how accurate the signal is for you, you'll be off.

        Now, imagine the best case scenario, where your lane data is accurate to within 10 feet and your GPS unit reports 10 feet accuracy (I've seen my 60Csx report as low as 6 feet, but that's dubious). Combine your error with the map error, and you could be as much as 20 feet off- making it difficult for the GPS unit to be certain as to which 16-foot-wide lane you are in.

        Everything I've ever read about lane-level accuracy involves embedded sensors in the roadway (RFID, perhaps?) to pinpoint your position.

  • It is easy to get the facts wrong on this subject. Scintillation is not unmodeled delay in the GPS ranging signal. It is wide swings in the signal amplitude. There is an explanation at http://gps.ece.cornell.edu/SpaceWeatherIntro_update_2-20-08_ed.pdf [cornell.edu]
  • by Anonymous Coward
    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 e
  • 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.
  • 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.
  • 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 st
  • 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.


  • by Animats ( 122034 ) on Wednesday June 11, 2008 @01:00PM (#23750241) Homepage

    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: (Score:3, Interesting)

      by 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.

  • Obviously shares of GPS stocks are down today, but L2 was supposed to solve ionospheric interference. Go to Iran & get yourself some L2 goodness.
    • by Arimus ( 198136 )
      Err.... if you want people to use the L2 signal on Navstar then you'd better get hold of the keys for it :)

      Not sure either if the L1/L2 frequency shift alone will counter all affects.

      (Actually not noticed a significant difference on our keyed/unkeyed gps receivers in the lab - all of which have the same signal feed (gps amplifier/repeater feeding the rf to each PLGR) since the SA was reduced to effectively 0 a few years ago.)
  • by fish waffle ( 179067 ) on Wednesday June 11, 2008 @01:26PM (#23750741)
    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.
    • It's a good off-the-shelf solution for knowing where your rolling stock is. Train schedules are incredibly complex and are laid out to the second, so having a high degree of accuracy of the locations of your locomotives with no initial R&D investment is a hell of a deal.

      I mean, you could say the same thing about cars- they mostly just stay on the road, right?

    • by PhxBlue ( 562201 )
      I suspect the rail industry uses GPS more for the timing signal than the navigation signal. If you hook a rail switch up to GPS and tell it you want it to switch at a given time of day, you know it's going to switch at that time -- not five minutes before or after.
  • is what you're listening for.

    The cool part of this article is that scientists are now using GPS receivers (cheap, ubiquitous) to study events in the ionosphere, which used to require fixed ground-based ionosondes or worse yet sounding rockets.
  • so, anyone remember the atmospheric nuke tests? massive electrical disturbances were generated.
  • My handheld will track at least 12 satellites and also coordinate that with a ground signal for an extremely accurate civilian-signal based location -- usually under 2 or 3 meters possible error in many places, but the unit will pinpoint within 30cm/1 foot or less. I don't see it jump around much at all except when the sampling rate gets lowered.

    I'm not sure this problem is as big as it's made out to be.
  • Subject line says it all. This isn't news to anyone who has a freakin' clue about RF systems.

1 1 was a race-horse, 2 2 was 1 2. When 1 1 1 1 race, 2 2 1 1 2.