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.

Re:Dual Frequency

[Criliric]

less precise everywhere actually

Re:Hans Reiser ESCAPES !!

[Anonymous Coward]

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

Re:GPS is digital!

[spandex_panda]

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:Dual Frequency

[jrumney]

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:GPS is digital!

[The Evil Couch]

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!

[borizz]

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:Oblig.

[WormholeFiend]

Tinfoil hats ahoy!

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

The ionosphere causes several effects

[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 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.

Roland the Plogger, again

[Animats]

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.