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."
Ouch (Score:2, Informative)
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, Informative)
Well, there's always GLONASS [www.rssi.ru].
[sigh] Poor Russian space program.
A Regional Blackout More Likely (Score:5, Informative)
Alternative (Score:2, Informative)
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: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".
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 drone with...
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: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
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.
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.)
Air Force Space Command GPS Status Page (Score:3, Informative)
Re:It is worth noting that this is a localised eff (Score:5, Informative)
Re:It is worth noting that this is a localised eff (Score:2, Informative)
In short, land use stuff will probably have black box, non-hackable but military-grade kit. In addidtion whey will use differential GPS which combined with military-grade access in the first place will give you accuracy down to a few cm.
So in short - don't worry!
Re:Army's stuff (Score:2, Informative)
You are correct in saying that the diferential station knows where it is to high accuracy. It doesn't act as another GPS station though, it sends out corrections to the received signals from the GPS satellites. Hence it would indeed correct for SA in its coverage area.
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.
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
GPS a carrier for other systems (Score:1, Informative)
Try the USCG Navigation Center (Score:3, Informative)
http://www.navcen.uscg.gov/gps/default.htm [uscg.gov]
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:NTP (Score:4, Informative)
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/
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.
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--
Re:Army's stuff (Score:5, Informative)
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
Comment removed (Score:5, Informative)
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:Army's stuff (Score:1, Informative)
Read more on
http://www.navcen.uscg.gov/faq/dgpsfaq.htm
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
Half the story, one day late! (Score:2, Informative)
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
Re:Army's stuff (Score:2, Informative)
From source rather than a German Auto Club (Score:2, Informative)
The Interagency GPS Executive Board (IGEB) was established in 1996 by a Presidential directive to manage the Global Positioning System (GPS) and its U.S. Government augmentations. The IGEB is a senior-level policy making body co-chaired by the Department of Defense and the Department of Transportation.
The IGEB have a statement on their website about Selective Availability [igeb.gov].
Of course they could change this policy, but for the moment it looks like SA will not be turned on.
Re:This was *exactly* why we here in Europe... (Score:2, Informative)
Re:Is this worldwide or only in Europe/Middle East (Score:1, Informative)
The majority (25+) pass overhead a twice in 24.x hrs (I think it is a 12hr orbit, not positive.)
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:Rumor has it... (Score:3, Informative)
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.
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.
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 a search for dgps-ip - Essentially RTCM correction data that can be obtained by connecting to a port on a dgpsip server.
Sure... (Score:2, Informative)
It'll be interested to see if they actually turn SA back on or not. And if they do, will it be world-wide or can they really selectively turn regions off? Of course, as you want to hit 3+ satellites at a time, that will still effect a lot more that Iraqi. For example, right now Salman Pak (Iraqi Bio-war site) [annotatedearth.com]can see:
GPSBIIA-16(PRN01) GPSBIIA-25(PRN03) GPSBIIR-03(PRN11) GPSBIIR-06(PRN14) GPSBIIR-04(PRN20) GPSBIIA-12(PRN25)Turning SA on these 6 Satellites would screw up GPS in Iraqi - and a large portion of that hemisphere.
Comment removed (Score:5, Informative)
That's absolute crap (Score:2, Informative)
First of all, differential GPS operates with differences of a signal phases, not the timestamps.
Second, DGPS was the one and only high-precision ("tenths of millimeter") method available before Us army decided to remove all artificial noice from the signal in 2000. It is a static method though and requires two nodes to sit tight on their spots for at least few minutes to accumulate enough redundant data.
Thirdly, there is a kinetic methods that apply to a processing of noise GPS signals by moving objects. I dont remember all math behind it, but it works out into automatic cancellation of phase measurements error and gives a decent location and speed accuracy even with S/A on. Not suitable for high speed objects (such as missles), but more than enough for driving around.
Comment removed (Score:4, Informative)
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 has even known to be several hundred km out!), and the US gov doesn't want European civilians having better tech than their military. The project was debated but they finally agreed to go ahead with it anyway and tell the US gov to get lost (ok, more politely, but that was the basic effect).
Comment removed (Score:3, Informative)
Re:Local Transponders? (Score:2, Informative)
Before people point out that ILS does that without depending on GPS at all: True, but the paths that ILS can guide airplanes along are not very flexible. Difficult-to-reach airports like those situated in valleys gain a lot of alternative approach paths by using the GPS-based system.
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: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 days of SA.
Re:What about planes?? (Score:2, Informative)
Larger aircraft use intertial navigation systems in preference to GPS based. INS is dependant on the aeroplane being in a known location before flight, and usually can place itself to a few metres after flying half way around the globe. It's also not prone to the obscure jumps and hops that can happen with GPS.