Slashdot Mirror
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."
Let me understand this, the head of a German Autoclub says the U.S. military MAY, I repeat MAY, degrade GPS accuracy. No evidence. Just pure conjecture. Consider that GPS has woven itself into our lives. How, it arguably supports critical functions. I strongly doubt that they will do this. While I understand the world's fears concerning GPS because it is run by the military, I put this article in with all FUD.
Civilian planes use GPS, don't they?
What about other critical systems like police, ambulance, fire brigades and so on??
According to http://europa.eu.int/comm/dgs/energy_transport/gal ileo/faq/index_en.htm
it hasn't been scratched.
There are plans for building a similar system to GPS in Europe so that we are not too much depending on the american empire. The following page nicely explains the concept. More is available here . This is technically very interesting and should open up new possibilities for navigation. Furthermore being constructed jointly by many partners and nations we can be reasonably sure that it can not be compromised by one weak leader.
Googlefight "Slashdot Troll" against "BSD is dying" 303:229. BSD thus cant die.
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.
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".
"To any truly impartial person, it would be obvious that I am right."
Even if it was, it would still remain under the control of the Russian military, just as GPS is under the control of the Pentagon.
The whole point (well, one of the major ones, anyway) of Galileo was to create a network which wouldn't be under military control, and so could be relied upon not to be switched off at inconvenient moments.
Apparently the Pentagon sees no compelling reason for an alternative to GPS. Oops, that would be before they checked their GPS units round about now. Oh wait, I forgot, they have their fingers on the buttons, perhaps that why they can't see a compelling reason.
Oops look; those pesky photons might interfere with each other
On the other hand, to be fair, the US could have just degraded the signal without announcing it. At least now ships and planes probably won't be piloted into rocks.
Army doesn't own it. People own it. The money spent on the satellites came from people's taxes.
And the people (through their elected representives) gave the money to the Army for military use.
Time is what keeps everything from happening all at once.
I rather like to idea of getting a bunch of people together - going round to military HQ and saying - "can we have our bomb back?"
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
Troc's dubious podcast and blog: http://www.trocnet.net
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.
See (lazy me, cut and paste from a google news post):
l ity.htm/ gps_pdd.htmS _SA_Event_QAs.pdf
http://www.igeb.gov/sa.shtml
http://www.navcen.uscg.gov/gps/selective_availabi
https://www.peterson.af.mil/GPS_Support/documents
http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GP
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
I've got a friend in MN. One winter night he and his buddies decided to have some fun with Onstar. Knowing that the call center is somewhere in Florida or California, they drove onto Mille Lacs lake (a very huge lake for those of you who don't know, it's larger than most counties). They called up Onstar to ask for directions:
Onstar: "hello?"
Friend: "We're lost. Can you help us find our way back?"
O: "Sure, hold on. Hmm.. this must not be working right. It says you're in the middle of a lake!"
F: "I know. We need to find our way back to shore"
O: "????"
F: "We're in Minnesota. There's ice on our lakes here"
O: "???? How are you in the middle of a lake?"
F: "We drove out on the ice"
O: "Why would you do that?"
F: "Ice fishing"
O: "?????"
F: "Can you direct us to the nearest landing?"
O: "Umm.. hang on a second. I need to get my supervisor."
There is no reasonable defense against an idiot with an agenda
:wq
The official sites to monitor if you're worried:
www.igeb.gov: The IGEB is a senior-level policy making body chaired jointly by the Departments of Defense and Transportation. Its membership includes the Departments of State, Commerce, Interior, Agriculture, and Justice, as well as NASA and the Joint Chiefs of Staff.
Right after 9/11/01, they posted (still there) this: "GPS Selective Availability (SA) has not been used since its deactivation by the President on May 1, 2000. At that time, the United States Government stated that it has no intent to ever use SA again. There has been no change in this policy."
http://www.navcen.uscg.gov/gps/default.htm is the official source for notices to civilian GPS users about schedule satellite outages, etc. They have nothing related to S/A being turned back on, and they certainly would if it were going to happen.
We can jam or dither the civilian code over the theater if we need to.