Are GPS signals scrambled for military bases?

Factual Questions
1

I know that cell phone signals are cut out; what happens with GPS signals?

I’m preparing a proposal for an environmental study on an Army base that will include the mapping of certain features. A handheld GPS would normally give us coordinates within a fine enough margin for error (<4 m), but it occurs to me that such resolution might not be possible on a base if the satellite signal is scrambled for any coordinates within base boundaries. As a subcontractor we are several steps removed from the engineers who will be evaluating the proposal, and none of the intermediaries we can ask knows the answer.

Now, we have been asked to be as specific as possible in terms of what information we will provide, so I could really use at least a ballpark idea of what’s possible. Can anyone shed a little light, so that I don’t promise something that can’t be delivered? Alternatively, if it’s considered so serious a security issue that it can’t be mentioned here, please just let me know and I’ll figure something out. Thanks.

2

AFAIK, Military GPS uses the same band (if perhaps a different frequency) as civilian receivers, so jamming ours without jamming theirs would seem to be quite a chore. Besides, if Mr. Rouge State wanted to lob a GPS-guided munition of some sort at a US base, jamming the signal just within the base itself wouldn’t help a whole lot.

3

Okay, in the famous words of the warden in Alien3, “this is rumour control.”

While I do not know the situation in the Army, I can say that within the Air Force there is no magical “tin foil beanie” covering the whole base, or a “signal fence”, or a “1920s-style comm jammer” that turns off any signals - GPS, phone, etc.

I don’t know where you got the info about cell phones. Mine has worked fine on bases such as Petersen, Bolling AFB, Andrews AFB, F.E. Warren, USAF Academy, Wright-Patterson AFB, LA AFB, Dover AFB, Lackland AFB, and more. It also functioned just fine on the Naval Academy grounds, inside the Pentagon, at Aberdeen Proving Grounds, and all over the Redstone Arsenal.

As for the issue of “scrambling” GPS, it’s software based in your receiver and the satellite, and is independent of your position on the globe. Military receivers can receive the completely accurate GPS signal. Civilian receivers are given a key that unlocks only part of the encrypted signal, and turns the rest to gibberish. I believe that during the Clinton administration, civilian receivers were allowed access to the “full monty” and now have equal accuracy with military receivers.

There may be a very very few places that jammers are erected, but I’ve been in “secured” facilities where the prohibition against cell phones was enforced by a desk clerk with a night stick, not by a black box.

I expect Airman Doors will be along shortly with much the same story (or, to my chagrin, to correct me).

4

… and there’s no such thing as a rouge state; only Reds, pinkos, and countries run by maroons.

5

The cell phone experience was my colleague’s - we could never reach him while he was on base (most frustrating!), and when he asked whether the cell phone signal was being jammed somehow, he was told it was. Maybe base personnel were having a joke at his expense, I don’t know. Btw, this particular base is currently heavily involved in the Iraq war effort; don’t know if that would make any difference either.

6

It is possible that there is some Army communications gear on that base that interferes with cellular phone traffic, I guess, but I can’t imagine deliberately jamming cell phones on a base within the continental US. As for “heavily involved with Iraq”, I, too was “heavily involved” (at a desk) working on Baghdad time, which on the East Coast is 2200-0600. I was prohibited from using my cell phone within my building, but I know that on Sept. 11, people who had brought their phones into the building (against the regs) were allowed to call their loved ones and ensure they were safe; the installation commander turned a blind eye.

My bet is communications interference, and not active jamming. But, as for GPS, I don’t think it’s effective or practical to try to jam it over the entirety of a base. Unless your surveying project will be covering particularly sensitive areas (and the base facilities manager would probably tell you if you needed to know) then you should be okay.

7

Another possible explanation for not having cell phone coverage on base might be a lack of nearby cell towers. If it’s a large base, and the cell phone providers don’t have permission to put up towers on the base, there would be huge gaps in the coverage.

8

GPS is primarily a military tool, made available for public use for personal/comercial navigation, etc. While the military doesn’t normally restrict or interfer with civilian capabilities, they have the means to do so at will. There are also different levels of GPS signal strength, with the strongest being the most accurate. The common reciever one can buy in Radio Shack, while extremely accurate, is nothing compared to the military signal. AFAIK, only in extreme situations would the military actually “mess with” somebody elses’ signal unless they are a terrorist or the like.

You should be fine with your endevours. If you have problems, it might be because of an outside, non-GPS related influence.

Hope this helps.

9

I’ve never seen a GPS service interruption when flying over a military base.

10

There aren’t too many bases state side that are more involved with Iraq than Mac Dill AFB or even Ft. Bragg. My cell phone works fine on both of them. In fact, I had a cell tower a block away from the barracks on Bragg.

I think they were just messing with your friend. He needs to get a better service provider.

11

I work at a large airport with two different flavors of military (Air National Guard and USCG). Cell phones work poorly at best.

Is it:

  1. Secret gov’t jamming to prevent you from making a phone call from your office without using your office phone or any of the other fifty-three phones in the building?

  2. Lack of cell phone towers because i) airports and military bases are generally big, ii) Uncle Sam couldn’t care less about your phone reception and iii) pilots hate towers near the runway?

The consipracy is sexier, but I’m going with 2.

P.S. [quasi-physics B.S.] I’m also in a cheap gov’t building (metal frame, metal shell). I will now say some important sounding words. Faraday cage. Radio reception. Reflection/ absorbtion/ grounding of radio signals. I have no idea what these terms mean. [/quasi-physics B.S.]

–kdeus

12

I’ve ridden thru Ft. Dix in NJ (on rt 539), and my GPS worked fine.

13

I used to fix military GPS systems.

The signal is transmitted on two different frequencies, the lower fidelity civilian channel and the higher fidelity military channel( L1 and L2). The higher fidelity channel can be scrambled or not scrambled at will by the DoD. This happens at the satelite level, and can only be turned on and off one hemisphere of the planet or so at a time. Until recently, this channel was always scrambled, and the unscrambled civilian channel was deliberately degraded.

Since I stopped working on them before they unscrambled them, I am unsure if they also increased the fidelity of the civilian channel, or even if they are capable of doing so. In any case, the capability to degrade the signal on one base does not exist at least at the satellite level. That doesn’t mean that someone can’t spoof an erronious signal and totally screw up your hand-held GPS however. Some military GPS units have anti-spoofing devices that would allow it to still work even with deliberate interference, but I somehow doubt that the military would send out bad signals to mess with people near its base, even if they could.

14

The CDMA technology used by most cell phones in the U.S. was based on something developed for the military, and is very hard to jam. Any cell phone jammer installed at a military base would have to broadcast noise over the entire cellular and PCS spectrum, and it would probably disrupt service for the nearest city, not just the base itself.

15

If you have permission to do this job on the base, I rather doubt they will actively work against you completing the job.

I have done mapping jobs on military bases before and since it was for them, they were right helpful.

If you are getting interference, tell them and they will make arraignments to stop it so you can finish your job.

If you are not in contact with the powers that be on the base, I strongly urge you to do so. They might be a bit upset if they don’t know you are going to be running all over their base.

16

I recently did some work for a civilian company that uses L1/L2 for dual-frequency real-time kinematic positioning in agriculture, earthmoving, mining, and construction. They and their competitors claim RTK positioning performance of 1 cm in the horizontal plane, and 1.5 cm in the vertical, so that would seem to indicate that the channels are not degraded for regular folk.

17

I believe this is wrong and Selective Availability (SA) is a matter if encryption and interpretation of information broadcast in the same frequency not of channels transmitted on different frequencies.

18

Just to correct a few misapprehensions in this thread… I work with dual frequency GPS kit every day.

There are indeed two frequencies, the L1 and L2 frequencies. It is wrong, however to describe L1 as civilian and L2 as military. I shan’t go into the technicalities of what is a very complicated area, unless anyone asks, but:

Civilians can and do use both frequencies. One big advantage to using both is that one can ascertain ionospheric effects on the signals by comparing the two frequencies against each other. Dual frequency receivers are MUCH more expensive though, and confined largely to military, scientific and survey use.

I think some posters have confused the encrypted military P-code with the L2 frequency signal. The P-code is transmitted on both L1 and L2 frequencies.

Selective Availability (the deliberate clock-dithering and/or erroneous ephemeris data) has been off for some years now.

If anyone wants more info, just ask… I’ll fill in anything I can.

19

I love a good conspiracy theory as a bit of a mental exercise, BUT - since this is the first project we’ve ever done on a base, I was just wondering if security issues might have an impact on our work. Clearly the answer is no, so off my proposal will go.

Thanks for all the replies. As usual, fellow Dopers have been most informative. This is why I love this place. :slight_smile:

20

Please go on! The beauty of this place is that there are experts in most areas, and most of them (you included) are good at explaining in easy-to-understand (if not necessarily layman) terms. Please try to bore us with what you know about ionospheric distortion etc. …