Global Positioning Devices: What Kinds of Thing Interfere with Their Signals?

I am wondering what, if anything, would interfere with the tracking ability of one of these devices? Would something so simple as going under an overpass create invisibility, or would this require something stronger like a lead-lined brief case?

GPS needs line-of-sight to work properly. An overpass or even trees can be enough to disrupt the signals - but usually only for a short time if you’re moving or only blocking part of the sky.

There are newer techniques to intermittently get really low power signals in those cases or inside some buildings (around the edges anyway) - but with a great reduction in accuracy.

Forgive me if I am telling you something you already know, but the way you word it makes me think that you think that the gps devices are emitting signals which are then tracked elsewhere. That is completely false.

GPS devices are passive. They recieve multiple signals from satellites and perform triangulation calculations to determine their own position on earth based on those signals.

The concept of “invisibility” doesn’t apply, but “blindness” can.

I guess it depends on what’s “invisible”. I assumed the OP meant the satellites are invisible to you.

ExpiringMind: are you planning on stealing a car?

VD: That would be a resounding “No” on the car theft idea. However, I am considering using it as a plot device, assuming it makes technical sense.

Nerd: Actually, I did not know that. That gives me something to think about. Could you please expand on the “blindness” concept?

Thanks to you both. You may have saved me a trip to SPY Headquarters…

GPS devices are indeed passive and listen to satellites. But properly speaking they don’t “triangulate” - they essentially determine the difference in the time a signal takes to arrive from pairs of satellites. This places them on a “hyperbloid of revolution” (basically, a 3-dimensional hyperbola) with respect to the positions of the two satellites. With several different satellite pairs to work with, it’s possible to calculate a single position where the receiver is located. (And more satellites typically yield better accuracy.)

This is obviously a simplification. But “triangulation”, while even simpler, simply isn’t happening with GPS.

I’ve used a GPS while hiking and just general fooling around and they tend to be pretty finicky. If you’re inside a building, you’re unlikely to get a signal unless you’re very near a window. Heavy tree cover will definitely interfere with reception. If you don’t lose reception completely, you will see the probable error in your position increase substantially. You might go from 30 or so feet of accuracy to several hundred feet. Even putting the GPS in your pocket can be enough to block the signal. This is with a good quality (but not state-of-the-art) Garmin GPS with an external amplifying antenna.

To qualify the “blindness” vs “invisibility” distinction, I imagine that if you were using a GPS receiver as a tracking device, you’d need a transmitter component to communicate the coordinates. For example, the Garmin Rino ( is a combination radio/GPS that can transmit coordinates. With such a system, you’d have two opportunities to fox the mechanism, either block the GPS signal or the transmission of the coordinates. Given the relative strength of the signals, though, it would probably be easier to block the GPS transmissions.

I can tell you from experience that GPS units used in car nav systems can be blocked by widow tints that contain metal (some do). Along with the building that have been mentioned.

Also if you are using a car based GPS in your story, some cars now a days have systems that can track a car if it is stolen. (GM ON-Star or Volvo On Plus for example) The in car cell phone has its own GPS. If the car is stolen, the car can call home with the location of the car.

Typically a GPS will only use 3 or 4 sats in it’s calculation (the WAAS sat might be a 5th). Most GPS can receive signals from 12 sats but will only use max 4 to get your position. It will still work w/ 3 but it will assume your altitude and give you what is called a 2d position.

When the signal is week, due to obstructions, the error (can be displayed on most units) will increase. A strong lock might give you an error of 12 to 20 ft (less w/ WAAS) while a week one can give you 200 ft or so. IF you are able to block all sats but 2 (or less) you can stop the unit from tracking. Some units might estimate that you are continuing along your same corse for a while but eventually loose the lock.

As for jamming, IIRC Sadam Husane bought about 4 gps jamming units from Russia, our missles went right to them.

A normal GPS receiver needs a minimum of 4 satellites to calculate a position in 3 dimensions. Many receivers use a “Best 4” scheme, but “All in view” is becoming common, as this scheme offers better accuracy.

Thanks also to Xema, Finagle, Rick, and Kanicbird. I now have a better idea of how it could be used for tracking, and what kind of interference might be added to keep things rolling.

In a nutshell, it sounds as though GPS trackng is ellusive enough for my protagonsits to be hard to find, yet not so hard to find that it would not work as a plot device. Granted, that is not a particularly original idea. <grin> I was thinking GPS because anyone can buy one, as opposed to the high-end stuff demonstarted in “Enemy of the State.”

And to think, I anticpated this thread might just lay here dead like clingy child’s goldfish. <grin>

As kanicbird mentioned, it is apparently possible to buy a commercial off-the-shelf GPS jammer. If your character is in Europe or an American city with lots of shipping (and potentially smuggling) such as New York or Boston, then they could buy one from the Russian mob. It’s a noise jammer, so it’s not going to report a “false” position–it’s just going to make local receivers give junk readings.

The problem with using one would be that you conceal yourself from GPS receivers by transmitting a very strong signal. If the person or persons tracking you by GPS realize they’re being jammed, they can–with equipment not much more sophisticated than what your hero would be using–triangulate on the source of the noise.

A very sophisticated jammer would have several transmitters and contain a hard-coded copy of what a GPS transmitter (satellite) broadcasts. Because it’s much closer to any local receiver than the satellite, it can transmit a much weaker signal and still completely drown out the true signal.

I don’t know exactly what info is in a GPS packet, but I imagine transmission time is part of it. By transmitting carefully faked GPS packets, one could theoretically convince a commercial-grade receiver to display the wrong positional info.

If your heroes are using a GPS receiver to follow someone, the receiver will need to be hidden with the target, and your heroes will need to couple it with a radio transmitter that sends the GPS receiver’s readings to their laptop/tricked-out A-Team Van/PalmPilot.

At that point, you’re better off buying a (Dept. of Corrections surplus?) regular locator bug from the Russian Mob. It’ll be smaller, cheaper, and do the job just as well if not better.

Hope this helps.