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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."
Don't like it, but it's the army's stuff. They can degrade it that far if they want to. Don't like it? Send up your own GPS satalites.
Mod point free since 2001
My guess is that for high-precision locations, the Iraqis already measured them with high accuracy, while for, say, infantry navigation all you really need is 100m accuracy. (Even less for armored forces, of cource)
And given the air threat, I also doubt their forces will change their localtions too much; if it's camouflaged enough to survive the initial attacks, it will probably stay put.
Working for necessity's mother.
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??
Which sane person would rely on GPS data for something even as trivial as navigation?
Have you tried navigating by the stars during the day lately? The blue room can be a big scary place.
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.
John R. Smith, of Peoria (Ill.) drove his brand-new SUV through the security glass doors of his bank, while following his GPS navigator.
"I was only following the indications of this @!!%!! machine -- and it told me I still needed to travel straight ahead for a hundred meters!", Smith tried to explain as he was taken into custody by the Peoria Police Department for "breaking and entering".
The Peoria Intercontinental Bank representatives were unavailable for comments.
The right to offend is far more important than the right not to be offended. (Rowan Atkinson)
a) A sane person would rely on GPS because they may not be very good with compass and map, or they may have cordinates for something not marked on a map. When driving at speed the margin of error is negligible, and it enables you to navigate through featureless terrain such as desert playa which are impossible to use a map in.
v)Well...I would walk to a point which I knew the exact location of and then do 10 or 15 GPS location checks to see what the margin of error was.
Did you really not know these answers or am I just feeding a troll?
"To any truly impartial person, it would be obvious that I am right."
It's more likely that localized blackout or jamming in the Iraq region will be used, rather than a global downgrade. See here for more.
--
CPAN rules. - Guido van Rossum
There was a big discussion on one of the GPS newsgroups about this very fact - at the time of the last gulf war, civillian GPS units were cheaper, more plentiful, and had more features that the troops wanted/required than the more cumbersome military GPS units.
One of the soldiers was talking about it in the group and basically said the military units were limited to showing long/lat and doing goto-waypoint distance/direction operations. At the same time, civillian units had mapping capabilities, easy to use graphic displays, and were about 1/2 the size.
As other posters have said, it's possible to adjust the SA signal geographically, so they could degrade the signal in the middle east without changing anything in north america. This is the first step that seems logical.
Alternately, they could leave SA off alltogether, and just jam the GPS signal in the area that they are performing operations - the GPS signal is relatively weak and an ECM aircraft could easily block hundreds of miles of GPS reception while flying out of range of ground-based weaponry.
N.
"Nothing strengthens authority so much as silence." - Charles de Gaulle
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.
Don't a number of commercial services like onstar use gps to track vehicles?
'Be calm madam, you are not lost. According to us you are floating off the coast of San Diego. You should be fine once high tide rolls in.'
"I only speak the truth"
Karma: null(Mostly affected by an unassigned variable)
S/A has always been a bit of a farce. It can be circumnavigated (no pun intended) if you use Differential GPS.
Basically, you set one GPS receiver up at a known, surveyed location and program that location into the unit. Then when the receiver trilaterates its position based on the information the satellites provide, it does on-the-fly corrections (You say i'm here, but i know i'm here). It can then use that correction algorithm to correct the positions of other receivers.
Of course doing that part on-the-fly is a bit more difficult (read expensive) because now you have to invest in radio communications back and forth between the two or more receivers - but it's often done. There are even services that have base stations set up across the country that sell a subscription-based service for that purpose.
Most times, survey firms just log the data and correct after-the-fact back in the office from the base station (the differentiator) located in the same area.
All in all, S/A only imposes the error to systems that don't have the capability == money to do DGPS.
There are 01 types of people in this world. Those that understand binary, and me.
I guess we all just pospone that trip to the wilderness to get away from things..
Take a MAP ( remember those things? ) on your next road trip...
After the war the service will return to normal.
Besides, who said we had a right to use GPS anyway?
---- Booth was a patriot ----
GPS is much more important to the US military, which does not have on-the-ground knowledge there. The US should be more worried about the Iraqis jamming GPS signals and other communications.
Of course, so far, it looks like Iraq is pretty feeble militarily. I suspect the war will be over very quickly. Which brings up the question again: why are we going?
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?"
Before and after the previous Gulf War, we had S/A "jamming" by the military, resulting in "errors" of about 10 to 100m.
During the last GW however, the US military disabled the jamming, because they were unable to produce military grade GPS receivers. They gave "normal" civilian GPS receivers to officers and disabled the jamming, thus defeating the entire purpose of the S/A system...
This was one of the reasons they turned it off a couple of years ago.
Well, it seems the US government isn't too comfortable with that and tries (german link) to make (german link) the EU abandon that project. Naturally the EU doesn't like depending on a US-monopoly for such an important system.
"By the way if anyone here is in advertising or marketing... kill yourself." -- Bill Hicks
A joke in The Australian newspaper the other day:
Reporter: So how do you know that the Iraqis have Weapons of Mass Destruction.
American: We kept the reciepts!
boom boom.
The reality is that in the time period since S/A was turned off many businesses have become dependent on the GPS. If S/A were to be turned back on worldwide, then that would provide one more reason to oppose the war. COnsidering the current political climate, both in the US and worldwide, I can't see this happening.
if you have a garmin unit (the one without the goofy cartoon guy planting flags) the EPE (estimated positioning error) is right on the satellite page.
/ am mn.gif
Garmin is a bit generous with the calculation for this number (for a discussion, you could check out gpsy.com) but in a clear area the SA changes it from about 20 ft to about 100 feet.
Here's a graph of when SA got turned off two years ago -
http://www.ngs.noaa.gov/FGCS/info/sans_SA/world
Look for that to reverse.
And they prolly need to turn it off globally - because they think there's a good chance bad people will target things all over the world now that we'll be fighting. Plus the last thing they need right now is a bunch of people making sure the army works and your lexus dongles work.
War is hell. Buy a map. Your GPS will still get you close enough to throw a line to someone if they need rescue.
"Win treats sysadmins better than users. Mac treats users better than sysadmins. Linux treats everyone like sysadmins."
This has been discussed before.
Look at
http://www.igeb.gov/sa.shtml
They say it will NEVER be turned back on
also see
http://www.navcen.uscg.gov/
-- 73 de KG2V For the Children - RKBA! "You are what you do when it counts" - the Masso
Of course - I forgot that the American chiefs of staff have a strict policy of inform and consult with the German auto club.
Carpe Daemon
While it's hard/impossible to obtain a receiver that can directly use the P code, it IS possible for a civilian receiver to use the encrypted P code for additional accuracy without decrypting it.
i n.htm and http://www.nottingham.ac.uk/iessg/gringo/
The civilian C/A codes are only broadcast on one frequency. Both the C/A and P codes are pseudorandom bit sequences designed to have a very high peak in their self-correlation function. (Effectively turning the CW transmitters on the satellites into high-power pulse transmitters as far as SNR requirements at the receiver.) The encrypted P code has a much lower peak in its self-correlation function, but it STILL has a peak.
The C/A code is only broadcast on one frequency, while the P code is broadcast on two frequencies. Why? Because one of the leading sources of error in GPS reception when SA is turned off is the fact that the ionosphere delays the signal. Fortunately, the ionospheric delay is a linear function of the frequency. (I.e. a signal at 1.7 GHz is delayed 1.7/1.2 times as much as a signal at 1.2 GHz). So, a military receiver can measure the delay between the two frequencies, and from that calculate the ionospheric delay.
Now go back to the fact that even the encrypted code has a peak in its self-correlation function. A high-end civilian (usually surveying) receiver can receive the encrypted P-codes and correlate them (since they happen to be identical). Since the self-correlation peak of the encrypted code is much lower, the signal strength must be higher than that for unencrypted codes and the process is SLOW, but it can be done. Receivers capable of this cost $$$$$$. (For example, in the GPS lab at Cornell University, they have only 1-2 dual-frequency receivers, while they have plenty of single-frequency receivers on ISA cards to allow for advanced postprocessing of data.)
As far as SA - Even when SA is on, it's possible to get millimeter accuracy from a civilian receiver, using the same techniques needed to get millimeter accuracy from a civilian receiver with SA off. The most important thing is a "reference receiver" nearby - One whose location is precisely known. This receiver can measure all of the errors generated by the satellites, which can be used later to postprocess the data from a remote receiver and correct it.
In addition to clock dithering, SA puts errors in the satellite ephemerides (The description of their orbits). It's possible to download precise (even better than non-SA) ephemerides from various standards organizations for post processing.
Want to try post-processing yourself? Until recently, the answer was "tough luck" with the exception of expensive receivers and the Delorme Earthmate. Only the Earthmate allowed the user to capture raw pseudorange data (The data needed to obtain a navigation fix) for later processing. Fortunately, some people found out that it was possible to obtain pseudorange data from 12-channel Garmin civilian receivers by using some undocumented commands. See http://mywebpages.comcast.net/dmilbert/softs/g12r
retrorocket.o not found, launch anyway?
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
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.