U.S. May Reduce Non-Military GPS Accuracy 772
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."
This was *exactly* why we here in Europe... (Score:2, Insightful)
Re:This was *exactly* why we here in Europe... (Score:5, Informative)
it hasn't been scratched.
Re:This was *exactly* why we here in Europe... (Score:3, Funny)
Re:This was *exactly* why we here in Europe... (Score:4, Insightful)
All dumbs jokes aside, EU really really needs to get a GPS system of our own. We should of course let the US use it, and use the original GPS when appropriate (for example extra accurcy or if one fail etc). It is fairly dumb to give away so much power to a foreign military.
Re:This was *exactly* why we here in Europe... (Score:3, Funny)
Re:This was *exactly* why we here in Europe... (Score:3, Insightful)
Re:This was *exactly* why we here in Europe... (Score:3, Informative)
Well, there's always GLONASS [www.rssi.ru].
[sigh] Poor Russian space program.
Re:This was *exactly* why we here in Europe... (Score:3, Insightful)
I am from Europe myself but I am not sure I agree. The reason Galileo is not going anywhere quick is that is enormously expensive (just as GPS was).
Is it really worth the money and the effort to send up an entire system so that coverage can be ensured during the say 2% of time when the GPS signals are distorted for military reasons? I can see a any number of scientific/ infrastructure projects that are much more worthwhile. Of cour
Re:This was *exactly* why we here in Europe... (Score:3, Interesting)
Nope. Remember the quote from The Firm: "Remember - I'm smarter than you."? The guys who designed GPS were better mathematicans and better planners than just about everyone else. This possibility is covered in the design such that SA really does work. Until you get into post-processing, etc.
sPh
Re:This was *exactly* why we here in Europe... (Score:3, Informative)
You have a gps at a fixed location (like a building) logging where it is (which would wander from the introduced errors)
If you were to plot the latitude and longitude, it would be circle like, with the center being a good bet on where your gps receiver is located. Knowing that, you could then broadcast the correction over radio as a differential. I remember that the University of Rhode Island's research vessel used it back in the bad old d
Re:This was *exactly* why we here in Europe... (Score:3, Insightful)
sPh
Re:This was *exactly* why we here in Europe... (Score:3, Informative)
I think you must have been mistaken - IIRC the US tried to persuade Europe to dump the project, basically because it will be accurate to around 45cm (guaranteed to withing 100cm), whereas GPS can often be several dozen metres out (and h
Re:This was *exactly* why we here in Europe... (Score:5, Interesting)
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.
Army's stuff (Score:5, Insightful)
Re:Army's stuff (Score:3, Interesting)
Re:Army's stuff (Score:4, Informative)
IIRC, differential GPS is where you correct for clock error by using a fixed point with a very accurate latitude/longitude measurement as one of your "sattelites". However, let's say the GPS sattelites decide to coordinatedly broadcast the signal that according to the receiver's internal database hey would a few nanoseconds in the future - it would throw off all correction measures, since they all depend on all your sattelites (including your ground station "sattelite") to be using the same clock, and that that clock matches up with the database.
Re:Army's stuff (Score:5, Informative)
It then sends a DGPS stream out, and any GPS receiver capable of receiving that stream can remove the satellite error for satellites they share with the DGPS transmitter.
However, typical low end DGPS will only reduce the error (when SA is turned ON) to 10 meters or so. The receivers used by surveyers with DGPS can go to the centimeter level, longitudinally and latitudinally. Altitude is a different matter...
Garmin is using a system similar to DGPS called WAAS which also helps reduce the error.
The encoded GPS signal the military uses along with high end receivers will, IIRC, go down to the meter without any DGPS. The reason they can't get any better than to the meter is that the atmospheric effects on the signal can't easily be corrected for in real time.
A decent tutorial can be found here [gisdevelopment.net]
-Adam
Re:Army's stuff (Score:5, Informative)
Something I still practice when out sailing (or mountaineering etc) just in case my GPS packs up
troc
Re:Army's stuff (Score:5, Interesting)
I know the US is actively looking to find ways to deny potential enemies access to 3rd party satellite services. If the EU launces their own GPS system, they would be expected to play ball with the US and turn it off in areas where the US military is operating. If they don't the satellites could be considered a military device (since they would be used by a military), and they would probably be jammed at the least, and if that doesn't work then they could be targetted. I'm guessing most corporations that own satellites would just play ball - those satellites cost big money and I doubt their insurance protects against US anti-satellite weapons. Government-owned satellites might be a different story - depending on whether the foreign government wants to make the political move of standing up to the US.
Keep in mind that providing targetting data to a military is hardly a neutral stance. If the US provided military GPS receivers to Chechen rebels, you can bet the Russians would be ticked.
Like Galileo? (Score:5, Insightful)
Rumor has it... (Score:3, Interesting)
Well, the second time the US turned SA off was for that reason.
This article is really amusing because of the fact that the government actually turned SA OFF for the last Gulf War, as there was a shortage of military GPS receivers and soldiers were ordering civilian units mail-order.
Re:Rumor has it... (Score:3, Informative)
Re:Army's stuff (Score:5, Informative)
Re:Army's stuff (Score:5, Insightful)
And the people (through their elected representives) gave the money to the Army for military use.
Re:it *is* our stuff (Score:5, Interesting)
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?"
Re:it *is* our stuff (Score:3, Insightful)
Well, no.. its more like you split the cost of your car and its maintainence with 200 million other people. And also you elected a driver and split the cost of this as well. You're not allowed to drive the car, but you can be one of the mechanics but you have to give up some of your rights as a passenger. You also have some input as to where the driver takes you, but this is split between all t
It'll cost you $1000-2000, Yankees (Score:3, Interesting)
It is worth noting that this is a localised effect (Score:5, Interesting)
Interestingly we were also told that it is not usually done before 24h from action. Anyone want to go a $10 bet with me on that? ;)
Re:It is worth noting that this is a localised eff (Score:5, Insightful)
Comment removed (Score:5, Informative)
Re: (Score:3, Interesting)
Comment removed (Score:4, Informative)
Re:buy a cluestick (Score:3, Insightful)
But, if you're putting yourself in a situation where you could die because SA is turned back on, you need to rethink your nautical habits.
Personally, I wouldn't want to be in a line of business where I have to be navigating narrow waterways at night such that I would need to trust a GPS receiver to keep me on the water. Even if I did have to navigate such a waterway at night, I'd rely mostly on my current tec
Comment removed (Score:5, Informative)
Re: (Score:3, Informative)
They didn't do it in gulf war 1 (Score:5, Insightful)
Look at the second paragraph from the bottom. They were about to reduce the usability of civilian GPS systems during the first gulf war. Turned out that many military personell was carrying their own, civilian, equipment. It wasn't standard issue back then yet.
So they left the resolution cranked up to max, their own soldiers would be most hampered by a downgrade...
Reinout
Why don't you stop taking your stupid pills? (Score:5, Informative)
Do you think that in less than three years people have forgotten about SA? Yes, that's right, SA was only turned off on May 1, 2000 - It hasn't even been three years.
Not a single plane in commercial use today was designed after the SA turnoff - No one designs a commercial jetliner in under 3 years. Every plane that uses GPS has been designed with the assumption that SA can be turned back on any time. In fact, they're designed with many other backup navigation systems, GPS is just a nice convenience but it's the system LEAST relied upon by airplanes.
Delivery people? 100 meters is good enough for these people. At least it had better be - Anyone relying on their GPS rather than having their eyes on the road should have their license revoked immediately. I don't need a UPS driver rear-ending me because he was staring at his GPS.
Face it, 100 meters is more than good enough for most people. For those who need "some navigation", 100 meters is good enough. For those who need more precision - They had better not be relying on SA being off, if they are they're dumb.
Note that survey-grade receivers can achieve millimeter accuracy even with SA turned on. (Thanks to reference stations with known locations that produce data which can be used to measure SA errors and correct for them in postprocessing, among other expensive tricks.)
If the military things turning on SA is a good idea and will help them in the war, SA will be turned on. (Note: There IS a chance that the military could decide that leaving SA off is a good idea, but civilian needs will not factor into the decision at all. During the first Gulf War, military-grade GPS receivers were in short supply and many soldiers phoned home to order civilian units. Military receivers were also unusually large at that time. As a result of this short supply, SA was actually turned OFF for the last Gulf War. Chances are that a lack of military-grade receivers is NOT an issue this time around, and dual-frequency receivers are a fraction of the size they used to be.)
Re:It is worth noting that this is a localised eff (Score:5, Interesting)
Re:It is worth noting that this is a localised eff (Score:5, Interesting)
In a desert with no useful GPS, you have to stick to roads. The bombers know where the roads go. This is how we absoletely hammered the Iraqi army last time (traffic jams for the Iraqis bugging out of Kuwait on the main highway).
Re:It is worth noting that this is a localised eff (Score:5, Funny)
As the GPS network can be degraded or upgraded region by region.
It's OK, I've got the multi-region hack for my GPS decoder ...
Rich.
Re:It is worth noting that this is a localised eff (Score:5, Informative)
What about Gallileo (if it was operative) (Score:3, Interesting)
Re:What about Gallileo (if it was operative) (Score:3, Interesting)
What about last time? (Score:3, Insightful)
You can always have a radio broadcasting the offsets from a known location to compensate.
Re:What about last time? (Score:5, Interesting)
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.
doubt if it will make any real difference (Score:5, Interesting)
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.
Re:doubt if it will make any real difference (Score:3, Interesting)
That's an awfully big assumption. Consider the terrain in southern Iraq. A few tens of metres (or less) is the difference between fording a river with your tank, and getting bogged down in marshland and having to sit and wait for a recovery vehicle, and all the while vulnerable to air attack. I
Re:doubt if it will make any real difference (Score:5, Insightful)
Yes, but did you need these tolerences ? For infantry navigation, you usually don't (Except for some extremely bad terrain types, an infantry officer/noncom who loses himself in an 100mX100m sized square won't do any good anyhow
For precise indirect fire, you do, but then again, my guess is that the Iraqi artillery (both light and heavy) positions are already very well measured.
The US army doctrine probably requires these accuracies simply because they can get it relatively cheaply. Nothing wrong with that; in fact that's the correct thing to do. But that does not mean an army can't fight well with less accurate equipment.
What about tankers, ships and harbors? (Score:3, Insightful)
Re:What about tankers, ships and harbors? (Score:5, Informative)
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".
There is no evidence of the claim (Score:5, Insightful)
Here's what the military says on this topic (Score:5, Insightful)
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.
Re:this is reliable info (Score:5, Funny)
What about planes?? (Score:5, Interesting)
What about other critical systems like police, ambulance, fire brigades and so on??
Re:What about planes?? (Score:5, Informative)
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:What about planes?? (Score:5, Informative)
Civilian planes will still use navigational radio beacons. This is one of the first things they teach you when you go for a private pilot's license. (First step for a non-military commercial pilot's license. Military licensing is probably similar.)
Re:What about planes?? (Score:5, Informative)
Your information is a little dated. GPS is most definitely approved for navigational use. Indeed, many NDB approaches have already been replaced with GPS approaches, and new GPS approaches are being certified all the time.
My aircraft has a Garmin 540 GPS Nav/Com installed, which is certified for instrument approaches. All that having been said, as another noted, any competent pilot knows how to fly using a number of instruments, with as much redundancy as possible. Dialing in VOR (a radio navigational aid) and using DME (distance measuring equipment), monitoring a moving map GPS, and even having a VFR-only LORAN all dialed up and operational at the same time provides invaluable cross-checking, should one instrument or another fail.
I've had my DME fail (but had GPS and even the LORAN availabe as a cross reference, in addition to triangulating two separate VORs), I've had my DG fail (but had the compass and, again, the GPS to cross-check with), and once I even had my compass fail (a seal went bad and the kerosine leaked out, so, while the compass still worked, it was far too wobbly in any but the smoothest conditions to be of much use). Once again, the GPS and working DG were sufficient to navigate on to the next decent sized airport, where I got it fixed. As for my NDB
Pilotage (using visual references like lakes, landmarks, etc.), radio navigation, and competency with a GPS are all skills that are taught a civilian pilot (assuming said equipment is available). For an instrument rating, if the instrument is in the panel, you will be tested on it. This definitely includes a moving map GPS, if your aircraft is equipped with one, and flying a GPS approach if it is IFR certified.
Thats one reason why europe should build own GPS (Score:5, Interesting)
In other news today... (Score:5, Funny)
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.
Re:In other news today... (Score:4, Funny)
This did actually happen in Germany: some idiot drove his car into a river because the navigation system displayed a bridge, but actually there was only a ferry.
See pictures of his car here [freefall.de] (scroll down).
The last paragraph of the text says: "Please note: A GPS system cannot be a substitute for the driver's attention! In december 1998, a driver trusted his navigation system which suggested to continue driving straight ahead. A few seconds later, his car got wet because his road database didn't know anything about the ferry across the Havel (a river near Berlin). This is not a joke! The TV channel RTL had a report."
STDMA (Score:3, Interesting)
It is in use in marine navigation. See also HERE [marinelog.com] and for a tech overview HERE [transpondertech.se].
Apparently, the US has tried to suppress the system as it may well replace GPS because of better performance and other reasons; one can imagine wartime control may be of importance here.
A Regional Blackout More Likely (Score:5, Informative)
Alternative Global Satellite Positioning (Score:5, Informative)
Galileo, which is planned to be completed in 2008, is the EU's alternative. It uses dual frequencies, and may increase accuracy to only a meter. Unforunatly, not all of the 30 sattelites are deployed, and the recievers don't seem to have been built.
GLONASS
GLobal Orbiting NAvigation Satellite System
this is the Russian system, which has a 10 / 20 meter accuracy for it's military signals, and 100 meter accuracy for it's degraded civilian signals.
If Iraqi is going to use something else, it would probably be GLONASS as it is fully operational.
Where are the GPS+GLONASS receivers? (Score:3, Interesting)
I'm assuming that somewhere there are cheap civilian GLONASS receivers-- if they cost about the same as cheap 12-channel
Air Force Space Command GPS Status Page (Score:3, Informative)
Alternatives (Score:5, Insightful)
Apparently the Pentagon sees no compelling reason [www.useu.be] 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 [cnn.com]
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.
Re:Alternatives (Score:3, Interesting)
The interference sounds like a very serious issue as well.
Also, the US didn't annoucne anything--we still have no idea if the signals will be degraded or not. This all come
onstar? (Score:5, Funny)
'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.'
Re:onstar? (Score:5, Funny)
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."
Still doesn't really matter with DGPS (Score:5, Insightful)
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.
WAAS (Score:3, Informative)
Kinda of hard to knock those out...
Also, terrestrial DGPS is not likely to be targeted by a missle for use in the USA.
DGPS isn't too expensive - Almost any civilian GPS receiver can apply the corrections if they are supplied, and receivers for the Coast Guard broadcasts are only $150-200 I believe. (And have been homebrewed for less.) If you have some form of wireless internet connection, do
At least its temporary, not a big deal. (Score:5, Insightful)
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?
GPS jamming (Score:5, Insightful)
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?
Re:GPS jamming (Score:4, Interesting)
On the contrary. The Iraqis' biggest weakness in the first Gulf War was their inability to navigate through the open desert. There's very little in the way of navigation aids out there, so it doesn't matter how well you know the country.
GPS is their ticket off of the roads, allowing them to do what we did-- go right through the unposted desert. My question is how much this signal will be degraded, and whether it will seriously hinder efforts at desert navigation.
Re:GPS jamming (Score:3, Insightful)
Actually, given the satellite photos, reconaissance aircraft and special forces, the US/UK probably know Iraq better than most of Iraq's Generals by now. Look at who's in charge on either side: the Allies have professional soldiers with decades of experience on the ground in wars, peacekeeping, exercises etc all over
The last Gulf War, it was the other way around... (Score:4, Informative)
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.
But (Score:3, Funny)
Try the USCG Navigation Center (Score:3, Informative)
http://www.navcen.uscg.gov/gps/default.htm [uscg.gov]
This was discussed a few weeks back (Score:5, Informative)
Look at
http://www.igeb.gov/sa.shtml
They say it will NEVER be turned back on
also see
http://www.navcen.uscg.gov/
GlobalSecurity.org has an interesting file on this (Score:3, Informative)
GlobalSecurity.org [globalsecurity.org] has posted an interesting FAQ on the war and GPS. It's located here [globalsecurity.org] . It was written by Richard B. Langley from the Dept. of Geodesy and Geomatics Engineering at the University of New Brunswick. It's a good read and answers a lot of questions about GPS and general and possible routes the military can use.
--Nyght--
SA will never be turned back on (Score:3, Informative)
But before SA was turned off, the Air Force had to develop a capability called "Selective Deniability" that would allow it to alter the accuracy of GPS signals over designated theater of operations. I seriously doubt that SA will be re-enabled systemwide.
Someone on a listserv I belog to send the URL of this PDF dated 13 March, 2003 that adddresses some of those questions. The URL is http://www.globalsecurity.org/military/library/re
What about WAAS? (Score:3, Informative)
AFAIK there is no provision for reducing the accuracy of WAAS without just turning it off. The FCC would really like to use WAAS to enable planes to do instrument landings at airports without ILS. Of course the FAA can just turn it off anytime...
WAAS works great though. I've left my GPS on auto-detailed track mode, and I've inadvertantly created a highly accurate map of my campus just by walking around with my GPS in my pocket
Re:What about WAAS? (Score:3, Informative)
As for leaving it on to help in the U.S., the system is not yet certified for aviation.
Here's [garmin.com] a page that says basically the same things.
Conspiracy theorists should NOT submit to /. (Score:5, Informative)
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
Precision Ag (Score:3, Interesting)
Before more people have heart attacks... (Score:3, Informative)
After all, when's the last time you've seen a GPS receiver with a dish antenna? Ground-based signals can logically affect them just as easily as sky-based.
There's a BIG problem with this.... (Score:3, Interesting)
Now correct me if I'm mistaken here, but wouldn't deliberately decreasing the accuracy of GPS signals essentially create a further handicap for this person who uses the equipment in his day to day life to function more like a "normal" person. Somehow that just seems wrong to me.
And on the up-side, it's great news for people who don't want the rental car companies tracking where they drive.
Checking the official resources... (Score:5, Insightful)
The official sites to monitor if you're worried:
www.igeb.gov [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 [uscg.gov] 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.
This would really hose us at my job (Score:3, Insightful)
GPS has become so embedded in our society, that this move just isn't viable anymore, IMO.
Is anyone else in this same situation?
Re:Sanity checks.. (Score:5, Funny)
Have you tried navigating by the stars during the day lately? The blue room can be a big scary place.
Re:Sanity checks.. (Score:3, Insightful)
Columbus thought he landed in India.
Re:Sanity checks.. (Score:5, Interesting)
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?
Re:Sanity checks.. (Score:5, Informative)
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.
Re:A question (Score:5, Informative)
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:A question (Score:3, Interesting)
position within 9 meters. 4-channel GPS units are things of the past (or used sometimes when size
matters more than accuracy).
Also, there are a "new" addition to the GPS system called WAAS (wide-area-augmentation-system) which
uses differente sattelites (this ones being geo-stationary) to send differentials information about
the GPS's sattellites signals. This way, a civilian WAAS-enabled unit like mine (garmin
Using the P code (Score:5, Informative)
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
Re:Not Globaly (Score:3, Informative)
When you start to think that they guy who dropped guided missiles on Israel for the sheer joy of maybe killing some jews now has a UAV, and look at how low-tech crop spraying equipment is, then reflect that he had an anthrax weaponization program, maybe it would be better for people to have to actually use their eyes for the last third of a kilometer than to make this something you could steer a dron
Re:So the US sold the GPS equipment as well? (Score:5, Funny)
Reporter: So how do you know that the Iraqis have Weapons of Mass Destruction.
American: We kept the reciepts!
boom boom.
Re:NTP (Score:4, Informative)
Re:Let's cut off our noses (Score:3, Insightful)
This article [newscientist.com] says that the DOD has better ways to achieve this end, so you can stop crying. But, if degrading the signal worldwide were the only way to degrade it for the Iraqi military, they would be correct to do so.