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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.
The electrons in the ionosphere must be terrorists!
Tinfoil hats ahoy!
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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.
Mea navis aericumbens anguillis abundat
More Roland fest! Why doesn't SourceForge just hire the guy? Good grief! Who's he giving blow jobs to?
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.
Mea navis aericumbens anguillis abundat
No, no. Roland Piquepaille is a joke, we get it. It just gets old fast.
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.
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.
Just receiving a digital signal doesn't mean its right!
like phosphorescent desert buttons singing one familiar song
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!
like phosphorescent desert buttons singing one familiar song
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?
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.
I just read Slashdot for the articles.
However, amateur radio people such as myself rub their hands with glee, as a reflective ionosphere means good DX :)
I check the "Space dials" regularly, and can't wait for them to be in the red! 73s.
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For those who don't know how the ionosphere affects GPS, here's a quote from one of the articles tfa links to:http://www.agu.org/journals/sw/swa/free/newarticle/?id=2008SW000400
... 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)
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.
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.
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And even if a signal is totally messed up, as soon as you receive five signals the GPS _knows_ that something is wrong.
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.
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.
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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. With enough satellites, position is over-specified and can be checked for self-consistency.
Badgers? Badgers! We don't need no stinkin' Badgers!
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.
Whew! This water sure is cold!
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The ionosphere has to go down. Now all we need is some sort of death ray, hmmm.
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.
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.
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.
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 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
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
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.
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 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.
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.
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Actually you can usually get a good approximation using two satellites because the other solution is either in space or inside the planet.
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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.
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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/ Space Based Augmentation System (Include WAAS, GAGAN, EGNOS, MSAS) A WAAS capable receiver will work with any of the SBAS's and vice versa.
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.
I know I'm splitting hairs here, but FYI, any GPS position is calculated using trilateration, not triangulation.
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
That's called Differential GPS, although WAAS is more commonly available on consumer units.
http://en.wikipedia.org/wiki/Differential_GPS
http://en.wikipedia.org/wiki/Waas
Roland has photos of Cowboyneal doing unspeakable things to an iMac. If they stop posting his crap they photos go public!
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.
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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.
Obviously shares of GPS stocks are down today, but L2 was supposed to solve ionospheric interference. Go to Iran & get yourself some L2 goodness.
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.
The GPS receiver do not need to pick the most likely location of the two. Because it does not really calculate positions at all. It calculates your *new* position, based on an assumed position (which it updates every time it calculates a new position). This is why a cold start of a receiver can take a long time.
Anyway. If I could just demolish all these damned surrounding buildings, I would always have more than 3 sats available when I go out the door.
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.
Actually, no you can't.
If you know your distance to one sat, and you know that sat's location, you can draw a mental sphere with radius your_distance around it. You must be somewhere on that sphere.
Now, if you know your distance (and location of) of another sat, you have another sphere where you can be on. Obviously, you cannot be in two places at the same time. So the intersection of those spheres must be where you are.
The problem is this: Intersect two spheres and you get a circle! You can be on any spot in that circle. This does not give you a complete position.
You can assume a third sphere: Earth. If the earth were perfectly round (and you were on it), you would have two possible positions (intersect three spheres and you get 2 points).
Aside from the 2 possible locations problem, the Earth isn't round. It has mountains and valleys. This means that the earth sphere is not well defined, and can intersect along multiple places with the circle. Even if you had altitude info (which you do not get from 2 sat GPS) and good contour maps of the Earth, you can not be sure where the correct intersection point is (there might be multiple, if you are in a valley, for example).
All this combined is why 2-sat GPS does not give you a position. All it could reliably tell you is which hemisphere you are on.
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.
You're correct, I was thinking 3 satellites vs 4, it's been a while since I did this stuff =)
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Subject line says it all. This isn't news to anyone who has a freakin' clue about RF systems.
+++OK ATH
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