Can I count on my GPS during an extended blackout?

February 18, 2008 7:20 PM

Say there's another huge blackout like the Northeast had a few years ago, and let's say it lasts two months... Will a GPS unit in my dashboard and powered by my car still be able to connect to the satellites it needs to, to direct me on my way?
posted by CarlRossi to technology (20 comments total)

Yes. Why wouldn't it?
posted by Pater Aletheias at 7:21 PM on February 18

if your car runs, it'll run.
posted by joshgray at 7:22 PM on February 18

The satellites are self powered and broadcast a signal that is picked up by the receiver in your car. It uses signals from multiple satellites to get a fix on your location. There isn't anything on the ground that the receiver relies on so a blackout won't have any effect on its operation.
posted by mmascolino at 7:23 PM on February 18

Yes, the car charges the battery in the gps. The gps signal is not dependant on local power resources. Of course if you run out of gas you cant charge the gps device. Perhaps a hand-crank for the voltage it runs on would be a better bet.
posted by damn dirty ape at 7:23 PM on February 18

Cell phones get signals from transmission towers that are on the ground, and depend on local power, though, right? I wonder if that's what the asker had in mind -- wondering whether GPS units are like that, dependent on local ground towers in the same way?
posted by LobsterMitten at 7:31 PM on February 18

Only if you can get gas to your car... and since most gas stations get the gas into your car with an eclectric pump I would say no, unless you have your own stash of gas or don't drive your car farther than your tank will take you.
posted by crewshell at 7:31 PM on February 18 [1 favorite]

Yes. The GPS network's ground stations -- the only vulnerable part in such a situation, since the satellites are solar powered -- are located around the world and are operated by the NGA (an agency of the U.S. DoD). The master station is (IIRC) located in Colorado, but there are a number of redundant backups. I think there's even one on Ascension Island in the middle of the Atlantic Ocean, and similarly remote places.

I'm not sure how long the system would function accurately for, if we went totally "hands off" (say, the world was decimated by a superplague, etc.), but a regional blackout wouldn't kill it.
posted by Kadin2048 at 7:32 PM on February 18

The GPS satellites in the sky, called Navstar, are self-powered, probably nuclear-powered but because they're military in origin it's actually hard to dig up confirmation of that.

If the US Government decides that it's in their interest to degrade or stop providing the civilian GPS signal to a certain area or to the entire Earth, they can do that. They might do that, for example, if they thought an enemy was firing GPS-guided missiles at New York.

If not, your in-car GPS will work fine. Here's a page with a little more detail.
posted by ikkyu2 at 7:32 PM on February 18

Unless your car is purely electric, it will derive the energy needed to power your GPS from its gasoline supply, via its engine; a blackout would have no effect on the process.

Now, it's very possible that a two month failure of the power infrastructure would hinder your ability to replenish that gasoline, but I imagine the gas stations would eventually affect some sort of work-around.
posted by Iridic at 7:33 PM on February 18

Some GPS units use a dual system, some puesdo-gps units use just cell signals, but in that case its not a real GPS so this question does not apply. The prior situation exists in cities where a direct line of sight to the satellite is not obtainable (like New York) so in this case, you could lose that functionality of the GPS unit.
posted by crewshell at 7:33 PM on February 18

I could have asked better, yes, "Are there any ground support stations necessary to GPS and will my car GPS etc etc..."

I'm not sure how long the system would function accurately for, if we went totally "hands off" (say, the world was decimated by a superplague, etc.), but a regional blackout wouldn't kill it.

There's that great question over at The Straight Dope about how long the power would last after a zombie plague hits. So using your "hand's off" situation there, any idea if the GPS would work if all the power went out from lack of workers/electricity due to a super-plague?
posted by CarlRossi at 7:50 PM on February 18

I remember watching a BBC show called "What on Earth is Wrong with Gravity" a few weeks ago. In it, they took a visit to the main GPS headquarters in Colorado where they communicate with the satellites and update their internal clocks. Apparently time is a bit slower in orbit than it is on the surface of the earth and they have to adjust the clocks by as much as a second or two per day.

With all the stuff they were doing, it appeared to be a manual rather than an automatic process.

If there wasn't anybody adjusting the internal clocks of the satellites, I think that their times would drift further and further from the atomic clocks on earth and become more useless over time.

This is, of course, assuming that the clock adjustment communications are not automatic, but even if they were, I think the GPS network would only function as long as the main Colorado center had power so it could communicate with the satellites.
posted by lockle at 8:16 PM on February 18

LORAN used ground-based transmitters. GPS relies exclusively on satellites.
posted by Class Goat at 8:25 PM on February 18

A basic GPS receiver just receives signals directly from the satellites, but many/most receivers also use differential-GPS signals like WAAS to enhance their accuracy. If the ground-based parts of the WAAS system were to die, then your GPS receiver would fall back to plain GPS, and wouldn't be able to provide you with quite as accurate a fix. It'd know which block you were on, but not which lane you're in.

OTOH, I suspect the WAAS stations are built to survive the occasional disruptions in utility services and have backup generators and so on available (since they're used for things more important than just in-car navigation).
posted by hattifattener at 9:01 PM on February 18

(There are ways to distribute DGPS info other than WAAS — like encoding it on a commercial radio station subcarrier, or shortwave, or even post-processing a GPS receiver's log file — but I think WAAS has pretty much superseded earlier methods.)
posted by hattifattener at 9:04 PM on February 18

The GPS satellites in the sky, called Navstar, are self-powered, probably nuclear-powered but because they're military in origin it's actually hard to dig up confirmation of that.

Every image of them online shows them with big solar arrays.
posted by Cool Papa Bell at 10:38 PM on February 18

If there wasn't anybody adjusting the internal clocks of the satellites, I think that their times would drift further and further from the atomic clocks on earth and become more useless over time.

While I'm sure the atomic clocks aren't perfect, they're pretty darned accurate. I suspect that a more significant source of error would be the growing uncertainty in the satellites' positions.

Interestingly enough, the newer Block IIA satellites seem to have a "UHF crosslink" that lets them continuously monitor their distances from their neighbors in orbit, which allows them to maintain a 16-meter accuracy threshold for 6 months with no ground contact whatsoever. (Global Positioning System: Theory and Applications, via Google Book Search)
posted by teraflop at 11:09 PM on February 18

Apparently time is a bit slower in orbit than it is on the surface of the earth and they have to adjust the clocks by as much as a second or two per day.
--lockle

Is this because of the satellites' speed? Relativity and all that?
posted by philomathoholic at 11:38 PM on February 18

From this page at LSU on relativistic effects on the GPS system :

Relativistic effects on satellite clocks can be combined in such a way that only two corrections need be considered. First, the average frequency shift of clocks in orbit is corrected downward in frequency by 446 parts in 10^12 . This is a combination of five different sources of relativistic effects: gravitational frequency shifts of ground clocks due to earth's monopole and quadrupole moments, gravitational frequency shifts of the satellite clock, and second-order Doppler shifts from motion of satellite and earth-fixed clocks. Second, if the orbit is eccentric, an additional correction arises from a combination of varying gravitational and motional frequency shifts as the satellite's distance from earth varies. This correction is periodic and is proportional to the orbit eccentricity. For an eccentricity of .01 , the amplitude of this term is 23 ns. Due to a shortage of computer resources on satellites in the early days of GPS, it was decided that this latter correction was to be the responsibility of software in GPS receivers. It is a correction which must be applied to the broadcast time of signal transmission, to obtain the coordinate time epoch of the transmission event in the ECI frame.


IIRC, the BBC program actually stated that the relativistic effect of the reduced gravitational force on the satellites would introduce errors that would translate to several km of positioning error per day if uncorrected.

In addition, GPS is a military system. You can be sure that they have their own back-up generators for any essential ground stations, so any non-civilization-changing blackouts will not have an effect upon the functioning of GPS.
posted by Jakey at 2:23 AM on February 19

philomathoholic: Speed and location. There's the special-relativistic time dilation because the satellites are moving. There's also a general-relativistic effect that clocks run slower when they're deeper in a gravity well; atomic clocks in labs are accurate enough to see this effect with only a few meters' change in height. (There's also an effect called frame dragging, in which the rotation of the earth drags spacetime around after it — but that effect too small to be important to GPS, although it is measurable.)
posted by hattifattener at 12:09 PM on February 21