U.S. May Reduce Non-Military GPS Accuracy

ward99 writes "The U.S. government may be degrading GPS satellite signals, to cripple Iraqi forces' ability to use those systems during the war. This could potentially reduce accuracy from ~3 meters to over ~100 meters. Users depending on GPS systems may want to do sanity checks on any data returned by those systems during the war. The U.S. will do this by increasing the inaccuracies on the civilian C/A code, turning back on S/A (Selective Availability), by having the satellites deliberately and randomly return inaccurate information on where they are. S/A degrades GPS accuracy to only 100 meters 95 percent of the time and 300 meters the other 5 percent of the time. This will not effect the military P code."

Comments

Re:This was *exactly* why we here in Europe...

[pteron]

According to http://europa.eu.int/comm/dgs/energy_transport/gal ileo/faq/index_en.htm
it hasn't been scratched.

Re:A question

[Apro im]

Because that would defeat the purpose of the military code.

GPS 101:

Every GPS unit stores internally a fairly accurate clock, a database of every GPS sattelite's individual code and its expected position in the sky for something like the next few weeks. This information is updated by syncing with a sattelite every so often. These codes are long enough that based on what portion of the code the receiver is receiving at a given time from a particular sattelite, it can calculate the time elapsed since the signal left the sattelite (by comparing to what portion of the code the sattelite should be transmitting according to its internal clock).
Using time elapsed, and roughly the speed of light (with minor corrections) for the speed of the wave, it can then calculate distance from the sattelite. Given three sattelites, you narrow down your location to one of two points (the maximum number of points of intersection of two non-congruent spheres. Luckily, one of these points is almost always inside the earth or in outer space, so a fourth sattelite isn't needed for that triangulation.
A fourth sattelite is used, however to make corrections for the GPS receiver's internal clock. That is, the receiver assumes its clock is off of the atomic clock in each sattelite by a constante amount, and therefore a fourth sphere won't intersect either of the points of intersection. However, by correcting for a constant time difference, the points of intersection eventually line up, and that is a fairly good approximation of the unit's location.
This means, by telling the sattelitest to vary the rate of transmission of their own unique code in some random way, the accuracy can be made much lower.
Since the system is based on knowledge of the codes, and only the civilian codes are published, the military codes look like just noise.

So there you have it - if the military doesn't give us the necessary information about the sattelites (information that changes every so often), we have no way of using the military-level accuracy.

Re:What about tankers, ships and harbors?

[James_Duncan8181]

This is why all harbours have pilot boats that deliver a helmsman who knows that harbour (he is their employee) to guide large tankers etc in.

Small boats franky should not be on the water if they cannot stay the right side of a clearly marked beacon. This is equivelent to saying "I have no GPS, how will I know what side of the road to drive on".

Re:What about planes??

[mwillems]

In small planes (I used to fly them), GPS is auxiliary. A good pilot does not rely on GPS. Precisely becuase it dould be disabled.

And in large aircraft, where GPS is used there are many other systems as backup. And final approach etc is of course never based on GPS. So, do not worry.

Michael

Re:Army's stuff

[troc]

D GPS works, as described, by comparing it's known location with that received from the satellites and transmits - in real time - the correction factor, so the correction factor varies with the changing position give by GPS. It requires the DGPS station to be fairly close to the handset as it needs to be using the same satellites - and therefore to be receiving the same information. So non-systematic errors are equally well fixed, in fact the error is non-systematic, it's simply a less accurate measure of the time given by the atomic clock on each satellite (less decimal places) which leads to a larger "cocked hat" for the handset to be located inside (cocker hat from the old days of triangulation where you drew lines for the bearings of three places and assumed you were inside the small triangle where they intercepted.

Something I still practice when out sailing (or mountaineering etc) just in case my GPS packs up ;)

troc

Re:Army's stuff

[moon_monkey]

Well, you can believe what some German automobile club says, or you can talk to the Pentagon - According to the story on New Scientst they've promised not to degrade the signal. "We would not create a global problem for transport out of spite for Saddam," says a spokesman at the US Department of Defence.

Conspiracy theorists should NOT submit to /.

[EriktheGreen]

See (lazy me, cut and paste from a google news post):

http://www.igeb.gov/sa.shtml
http://www.navcen.uscg.gov/gps/selective_availabil ity.htm
https://www.peterson.af.mil/GPS_Support/documents/ gps_pdd.htm
http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GPS _SA_Event_QAs.pdf

In short, NO, they won't degrade GPS.

I dunno what's more disappointing, that some lamer submitted this to slashdot, or that more of you supposedly "Educated" geeks don't challenge the idea.

Erik