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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!
Dawkins Revisited: A person is shit's way of making more shit -- Steve Barnett, anthropologist.
that is also completely worthless
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
You'd run too if your cellie called you "Riser".
More Roland fest! Why doesn't SourceForge just hire the guy? Good grief! Who's he giving blow jobs to?
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.
Maybe Computers will never be as intelligent as Humans.
For sure they won't ever become so stupid. [VR-1988]
No, no. Roland Piquepaille is a joke, we get it. It just gets old fast.
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?
Nope, it's Paris, Texas, that's the inside joke. No, not that.
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.
Get your own free personal location tracker
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.
not to get an iPhone... "I see your reality distortion field, and I raise you one electrically active ionosphere!"
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!
The ionosphere has to go down. Now all we need is some sort of death ray, hmmm.
I (think I) do, but only because "Ich sprach ein venig Deutsch", so I suppose it is required prerequisite.
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
So do trees.
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.
There are 4 boxes to use in the defense of liberty: soap, ballot, jury, ammo. Use in that order. Starting now.
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
Roland has photos of Cowboyneal doing unspeakable things to an iMac. If they stop posting his crap they photos go public!
...are the weather and GPS forecast for the next days. Good night.
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.
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.
I can tell you that railroads do not use GPS. Control of the railroads is done by dispatchers. GPS is not reliable enough to run trains. In most cases, the accuracy is not good enough to differentiate between tracks (15 feet apart) while going 60 mph.
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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