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."

Army's stuff

[SlamMan (221834)]

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.

Like Galileo?

[gotan (60103)]

Well, it seems the US government isn't too comfortable [state.gov] with that and tries (german link) [heise.de] to make (german link) [heise.de] the EU abandon that project. Naturally the EU doesn't like [guardian.co.uk] depending on a US-monopoly for such an important system.

Re:Army's stuff

[moon_monkey (323491)]

Well, you can believe what some German automobile club says, or you can talk to the Pentagon - According to the story on New Scientst [newscientist.com] 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.

Re:Army's stuff

[EzInKy (115248)]

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.

Re:it *is* our stuff

[LazySlacker (212444)]

Isn't that a bit like saying. It isn't your car - the money used to buy it was given to you by your employer.

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:Army's stuff

[stienman (51024)]

This is incorrect. A DGPS transmitter knows its own location, and can therefore determine the error of each satellite in its view.

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

[troc (3606)]

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

[Rich0 (548339)]

I don't think the army is too worried about DGPS. It would solve the civian problems of accessing accurate GPS data, but not the military problems. Why? Because the DGPS station would need to have a transmitter, and it would have to remain at a very precisely fixed geographic location, and transmitters don't last long in war - especially if they are in fixed locations.

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.

It is worth noting that this is a localised effect

[James_Duncan8181 (588316)]

As the GPS network can be degraded or upgraded region by region. I used to be in military service and one of our tips for civilians was that they could always tell when the shit was really about to hit the fan because the GPS accuracy will change massively when compared to a know coordinate point.

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

[Chanc_Gorkon (94133)]

I was going to say that I thought they had figure out how to scramble or turn on SA where they wanted to. Do the editors check even the story? The newsline was frankfurt, not here in the good old US of A! The old SA is not going to be reinstated. Too many of our homeland thing depend on it. Planes, delivery persons and anyone else who needs to know where they are. Things are much different then during the gulf war. Many people depend on GPS's for at least some navigation.

GPS and boating.

[fmaxwell (249001)]

Things are much different then during the gulf war. Many people depend on GPS's for at least some navigation.

You are absolutely correct. I am a boater and GPS enabled me to find channels and return to port late at night after effecting repairs at sea and after being forced to return to port at slow speeds due to rough seas. GPS is important enough to me that I always have two units on my boat, a console-mounted unit and a handheld which I keep for backup in case the console unit fails. Most boaters now rely on GPS extensively and, while 100 meter accuracy on the open seas is often acceptable, it would cause havoc on inland lakes and waterways as well as at coastal ports.

I know that this may sound melodramatic, but turning S/A back on will result in deaths and injuries. I'm not saying that death or injury would be common, but it would happen, whether because someone hit an obstruction or because they ran aground in inclement weather. While many smarmy, self-impressed types will, no doubt, claim that these people relied on GPS too heavily, don't listen to them. These are probably the same people who would freak if their depth finder went out, couldn't fix their engine if it failed, and think that a sextant is a marital aid. Marine radar, sonar depth finders, VHF radios, and accurate GPS have all increased safety on the water. Take any of them away, and deaths and injuries will go up.

Re:GPS and boating.

[fmaxwell (249001)]

Aparently you are new to boating.

I've been a boater for over 20 years and have owned numerous boats. It also appears that I know more about navigation in general and GPS in particular than you do.

Not only should you be using differential GPS if you need perfect accuracy

DGPS is not "perfect accuracy." In fact, it is signifigantly worse than WAAS-enhanced GPS, which is what I use. DGPS has an accuracy of five meters while WAAS has an accuracy of less than three meters. You also are apparently ignorant of the fact that DGPS coverage is quite limited and significantly worse than WAAS. For those outside of the U.S., other governments are developing systems analogous to WAAS. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS).

but it wasn't too long ago that GPS _always_ had a 100m accuracy.

In 1994, I bought my first GPS, a Garmin GPS-45, so I am well aware of the SA-imposed inaccuracies. But it was only guaranteed to be within 100 meters 95% of the time. The other 5% of the time, it could be off by more than that.

In fact it wasn't too long ago that GPS wasn't there.

And I used conventionals charts with binoculars and compasses and I was less safe as a result.

If you are on the water and the lack of GPS is a threat to your life then you should hit the books an learn some real navigation.

If you are out on the water and think that accurate, easily-determined positional information does not contribute to your safety, then you need to sell your boat and stick to Carnival Cruise Lines.

Every improvement in navigational equipment, from the astrolabe to the cross-staff to the quadrant to the sextant to Loran to GPS has increased safety and saved lives. If you revert to a less accurate and slower system, you are at greater risk of injury or death. Imagine having a fire onboard and your boat sinking. Would you rather have a GPS or a compass and paper charts to determine your position as you radio for help? Suppose a passenger fell in the water at night. Would you rather hit the "man overboard" button on your GPS or try to determine your position with a sextant, chronometer, astronomical almanac, and charts?

Of course it is important to know how to navigate without a GPS and to have the equipment on board to do so, but to pretend that older, slower, less accurate means of navigation are just as safe, especially in an emergency, is foolhardy.

They didn't do it in gulf war 1

[Reinout (4282)]

Civilian GPS in gulf war 1 [pbs.org]

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?

[Andy Dodd (701)]

News for you buddy: The government can and will turn SA back on any time they want to. It's always been clear that this is the case and anyone who relies on SA being turned off for a critical application is just plain stupid and they deserve what they get if it's turned back on.

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

[Chanc_Gorkon (94133)]

Yep. GPS is pretty old hat now and the military can totally jam it to others in some instances (so I have heard). I think that this is a stupid idiot spouting off. of course I could be an idiot for spouting off about totally jamming GPS in Iraq(what makes you think they could not? I mean I have no proof either way). GPS has a far reaching effect now. Much more now then it did even 2 years ago. Surveyors use it now as well as Amateur Radio operators with APRS and lots of other people use it. Buses use it in my town. Besides, GPS is not needed necessarily to guide a missile. What did they do before? Same thing that's been already done. Having a GPS just increases your chances of hitting the target you want. You could still hit it without GPS.

Re:It is worth noting that this is a localised eff

[James_Duncan8181 (588316)]

The nice thing about conrolling it is that you can offset it at will. So you could have a (stupidly designed) nuke in the air and offset GPS by 20 miles and watch your command bunker avoid a direct strike. Also you could completely ruin the GPS in this way system for desert use. This is why is is good to suddenly kill it, since you have got the tank crews used to using it.

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

[Richard W.M. Jones (591125)]

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

[Chanc_Gorkon (94133)]

Well, in a word....yes. The DOD was just being nice by letting the public use it. They did not have to make it available to the public. That said, there are alot of other systems that rely on GPS and they just can turn on SA over night.

doubt if it will make any real difference

[guybarr (447727)]

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

[guybarr (447727)]

In the USian Army we required 100m accuracy for Armor, 10m accuracy for Infantry/dismounted folks. All of the instruction had these tolerances.

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.

There is no evidence of the claim

[MyNameIsFred (543994)]

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.

Here's what the military says on this topic

[north.coaster (136450)]

Here's [newscientist.com] what the US military has to say about this.

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

[realnowhereman (263389)]

Of course - I forgot that the American chiefs of staff have a strict policy of inform and consult with the German auto club.

What about planes??

[borgdows (599861)]

Civilian planes use GPS, don't they?
What about other critical systems like police, ambulance, fire brigades and so on??

Re:What about planes??

[mwillems (266506)]

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:What about planes??

[flonker (526111)]

Last I heard, GPS is not approved for navigational use by the FAA. Meaning, you can use it, but you need to have alternate systems, and can't rely on it.

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??

[FreeUser (11483)]

Last I heard, GPS is not approved for navigational use by the FAA. Meaning, you can use it, but you need to have alternate systems, and can't rely on it.

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 ... I had the finicky thing pulled out to make room for my GPS Nav/Com ... an additional glide slope, moving map positional awareness, and nav/com more than made up for the loss of AM Radio reception and the ability to navigate using an ever decreasing number of NDB stations. Of course, in South Dakota a number of AWOS and ASOS stations broadcast on NDB frequencies, but then that is what UNICOM or Flight Service are good for, in a pinch.

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

[Neuronerd (594981)]

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 [satcoms.org.uk] nicely explains the concept. More is available here [eu.int]. 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.

In other news today...

[Noryungi (70322)]

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.

A Regional Blackout More Likely

[avdi (66548)]

It's more likely that localized blackout or jamming in the Iraq region will be used, rather than a global downgrade. See here [newscientist.com] for more.

Alternative Global Satellite Positioning

[Gryftir (161058)]

There are two alternatives to the GPS system.

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.

Alternatives

[Bozovision (107228)]

And this is EXACTLY why the EU wants their own alternative civilian version of GPS [bbc.co.uk], and why the US has argued against it [cnn.com]. Suprise!

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.

onstar?

[Joe the Lesser (533425)]

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.'

Re:onstar?

[Lxy (80823)]

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."

Still doesn't really matter with DGPS

[Drunken_Jackass (325938)]

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.

At least its temporary, not a big deal.

[nurb432 (527695)]

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?

GPS jamming

[g4dget (579145)]

I suspect the Iraqis have little need of GPS--their military probably knows their country pretty well and they don't have much in the way of smart weapons.

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?

This was discussed a few weeks back

[CharlieG (34950)]

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/

Conspiracy theorists should NOT submit to /.

[EriktheGreen (660160)]

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

Checking the official resources...

[jvaigl (649268)]

It's an interesting discussion, but doesn't look like it's going to happen. The article they're referring to is just some German auto club that says the thing maybe it could happen when the war starts. Hardly authoritative.

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.

Re:Sanity checks..

[zCyl (14362)]

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.

Re:Sanity checks..

[James_Duncan8181 (588316)]

For your 2 questions:

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..

[jpellino (202698)]

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.

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/ am mn.gif

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

[pteron (11383)]

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

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

[Amroarer (645110)]

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.

Re:What about last time?

[Nogami_Saeko (466595)]

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.

Re:A question

[Apro+im (241275)]

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.

Using the P code

[Andy Dodd (701)]

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.

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/g12ri n.htm and http://www.nottingham.ac.uk/iessg/gringo/

Re:What about tankers, ships and harbors?

[James_Duncan8181 (588316)]

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:So the US sold the GPS equipment as well?

[mgblst (80109)]

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.