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.

Comments

Dual Frequency

[Detritus]

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

[DRobson]

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

[The Evil Couch]

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 The internal hardware and screen was hopelessly out of date by the time it was in common usage.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

[tlhIngan]

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).

Good Grief!

[Anonymous Coward]

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

Re:Good Grief!

[jacquesm]

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

Re:Good Grief!

[owlnation]

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!

[jacquesm]

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:GPS is digital!

[borizz]

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.

What about timekeeping?

[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?

However, if you're a Ham....

[caluml]

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.

Re:Dual Frequency - Not just for the military

[afidel]

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.

Re:Roland the Plogger, again

[PhxBlue]

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.