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U.S. May Reduce Non-Military GPS Accuracy
ward99 writes "The U.S. government may be degrading GPS satellite signals, to cripple Iraqi forces' ability to use those systems
during the war. This could potentially reduce accuracy from ~3 meters to
over ~100 meters. Users depending on GPS systems may want to do sanity checks on any data returned by those systems during the war. The U.S.
will do this by increasing the inaccuracies on the civilian C/A code, turning back on S/A (Selective Availability), by having the satellites deliberately and randomly return inaccurate information on where they are. S/A degrades GPS
accuracy to only 100 meters 95 percent of the time and 300 meters the other 5 percent of the time. This will not effect the military P code."
This may seriously affect my handy-dandy Honda navigation system built into my Odyssey - it already has trouble guessing which road I'm on when the roads are close together - imagine it thinking I'm a block away from where I really am.
According to http://europa.eu.int/comm/dgs/energy_transport/gal ileo/faq/index_en.htm
it hasn't been scratched.
Well, there's always GLONASS.
[sigh] Poor Russian space program.
It's more likely that localized blackout or jamming in the Iraq region will be used, rather than a global downgrade. See here for more.
--
CPAN rules. - Guido van Rossum
It is not so well known that the russians have a system similar to GPS. More information at http://www.rssi.ru/SFCSIC/english.html
Because that would defeat the purpose of the military code.
GPS 101:
Every GPS unit stores internally a fairly accurate clock, a database of every GPS sattelite's individual code and its expected position in the sky for something like the next few weeks. This information is updated by syncing with a sattelite every so often. These codes are long enough that based on what portion of the code the receiver is receiving at a given time from a particular sattelite, it can calculate the time elapsed since the signal left the sattelite (by comparing to what portion of the code the sattelite should be transmitting according to its internal clock).
Using time elapsed, and roughly the speed of light (with minor corrections) for the speed of the wave, it can then calculate distance from the sattelite. Given three sattelites, you narrow down your location to one of two points (the maximum number of points of intersection of two non-congruent spheres. Luckily, one of these points is almost always inside the earth or in outer space, so a fourth sattelite isn't needed for that triangulation.
A fourth sattelite is used, however to make corrections for the GPS receiver's internal clock. That is, the receiver assumes its clock is off of the atomic clock in each sattelite by a constante amount, and therefore a fourth sphere won't intersect either of the points of intersection. However, by correcting for a constant time difference, the points of intersection eventually line up, and that is a fairly good approximation of the unit's location.
This means, by telling the sattelitest to vary the rate of transmission of their own unique code in some random way, the accuracy can be made much lower.
Since the system is based on knowledge of the codes, and only the civilian codes are published, the military codes look like just noise.
So there you have it - if the military doesn't give us the necessary information about the sattelites (information that changes every so often), we have no way of using the military-level accuracy.
Small boats franky should not be on the water if they cannot stay the right side of a clearly marked beacon. This is equivelent to saying "I have no GPS, how will I know what side of the road to drive on".
"To any truly impartial person, it would be obvious that I am right."
Actually, won't work - differential GPS only really corrects the innacuracies inherent to the correctly operating system.
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.
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
---
BDOS ERR ON A:>
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.)
Not really.
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.
Before and after the previous Gulf War, we had S/A "jamming" by the military, resulting in "errors" of about 10 to 100m.
During the last GW however, the US military disabled the jamming, because they were unable to produce military grade GPS receivers. They gave "normal" civilian GPS receivers to officers and disabled the jamming, thus defeating the entire purpose of the S/A system...
This was one of the reasons they turned it off a couple of years ago.
D GPS works, as described, by comparing it's known location with that received from the satellites and transmits - in real time - the correction factor, so the correction factor varies with the changing position give by GPS. It requires the DGPS station to be fairly close to the handset as it needs to be using the same satellites - and therefore to be receiving the same information. So non-systematic errors are equally well fixed, in fact the error is non-systematic, it's simply a less accurate measure of the time given by the atomic clock on each satellite (less decimal places) which leads to a larger "cocked hat" for the handset to be located inside (cocker hat from the old days of triangulation where you drew lines for the bearings of three places and assumed you were inside the small triangle where they intercepted.
;)
Something I still practice when out sailing (or mountaineering etc) just in case my GPS packs up
troc
Troc's dubious podcast and blog: http://www.trocnet.net
GPS was never really meant as a highly accurate positioning system for civilians in the first place. The GPS satellites carry what is called an NDS package or Nuclear Detonation detection System. This package uses various methods, including visible light, EMP, X-ray, and Gamma Ray sensors to locate nuclear detonations. It is this package that uses the GPS positioning most accurately because it is colocated ON the satellites themselves. Along with this, the encrypted military positioning signals are used to guide military craft and munitions to their final destinations. The civilian positioning signal is purposely less accurate than the military signal. This is, after all, a military owned and controlled system. I was in the USAF when GPS Block 2 satellites were first going up, and in a group called AFTAC (Air Force Technical Applications Center) that ran the NDS as part of the 1963 limited test ban treaty monitoring system.
For all your GPS news and status.
http://www.navcen.uscg.gov/gps/default.htm
yes, www.dotcomforwardslash.com is my real URL.
if you have a garmin unit (the one without the goofy cartoon guy planting flags) the EPE (estimated positioning error) is right on the satellite page.
/ am mn.gif
Garmin is a bit generous with the calculation for this number (for a discussion, you could check out gpsy.com) but in a clear area the SA changes it from about 20 ft to about 100 feet.
Here's a graph of when SA got turned off two years ago -
http://www.ngs.noaa.gov/FGCS/info/sans_SA/world
Look for that to reverse.
And they prolly need to turn it off globally - because they think there's a good chance bad people will target things all over the world now that we'll be fighting. Plus the last thing they need right now is a bunch of people making sure the army works and your lexus dongles work.
War is hell. Buy a map. Your GPS will still get you close enough to throw a line to someone if they need rescue.
"Win treats sysadmins better than users. Mac treats users better than sysadmins. Linux treats everyone like sysadmins."
It's essentially the same thing. The accuracy is degraded by offsetting the clock just a little bit. We're talking errors of only miliseconds here so you can still set your watch by GPS, but it's enough to throw in an uncertainty volume into the computed position. Remember that the position is actually calculated by the GPS device through the differences in received timestamps (simple version of how it works).
This has been discussed before.
Look at
http://www.igeb.gov/sa.shtml
They say it will NEVER be turned back on
also see
http://www.navcen.uscg.gov/
-- 73 de KG2V For the Children - RKBA! "You are what you do when it counts" - the Masso
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.
... 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.
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.
The Future of Human Evolution: Autonomy
GlobalSecurity.org has posted an interesting FAQ on the war and GPS. It's located here . 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--
Well, you can believe what some German automobile club says, or you can talk to the Pentagon - According to the story on New Scientst they've promised not to degrade the signal. "We would not create a global problem for transport out of spite for Saddam," says a spokesman at the US Department of Defence.
It has been the policy of the Interagency GPS Board that Selective Availability would never be turned back on, mainly because there are so many civilian users the rely on the more accurate signal since it was turned off. It would be a huge public relations blunder for the government if it did.
p ort/2003/iraq-and-gps_faq.pdf.
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
While it's hard/impossible to obtain a receiver that can directly use the P code, it IS possible for a civilian receiver to use the encrypted P code for additional accuracy without decrypting it.
i n.htm and http://www.nottingham.ac.uk/iessg/gringo/
The civilian C/A codes are only broadcast on one frequency. Both the C/A and P codes are pseudorandom bit sequences designed to have a very high peak in their self-correlation function. (Effectively turning the CW transmitters on the satellites into high-power pulse transmitters as far as SNR requirements at the receiver.) The encrypted P code has a much lower peak in its self-correlation function, but it STILL has a peak.
The C/A code is only broadcast on one frequency, while the P code is broadcast on two frequencies. Why? Because one of the leading sources of error in GPS reception when SA is turned off is the fact that the ionosphere delays the signal. Fortunately, the ionospheric delay is a linear function of the frequency. (I.e. a signal at 1.7 GHz is delayed 1.7/1.2 times as much as a signal at 1.2 GHz). So, a military receiver can measure the delay between the two frequencies, and from that calculate the ionospheric delay.
Now go back to the fact that even the encrypted code has a peak in its self-correlation function. A high-end civilian (usually surveying) receiver can receive the encrypted P-codes and correlate them (since they happen to be identical). Since the self-correlation peak of the encrypted code is much lower, the signal strength must be higher than that for unencrypted codes and the process is SLOW, but it can be done. Receivers capable of this cost $$$$$$. (For example, in the GPS lab at Cornell University, they have only 1-2 dual-frequency receivers, while they have plenty of single-frequency receivers on ISA cards to allow for advanced postprocessing of data.)
As far as SA - Even when SA is on, it's possible to get millimeter accuracy from a civilian receiver, using the same techniques needed to get millimeter accuracy from a civilian receiver with SA off. The most important thing is a "reference receiver" nearby - One whose location is precisely known. This receiver can measure all of the errors generated by the satellites, which can be used later to postprocess the data from a remote receiver and correct it.
In addition to clock dithering, SA puts errors in the satellite ephemerides (The description of their orbits). It's possible to download precise (even better than non-SA) ephemerides from various standards organizations for post processing.
Want to try post-processing yourself? Until recently, the answer was "tough luck" with the exception of expensive receivers and the Delorme Earthmate. Only the Earthmate allowed the user to capture raw pseudorange data (The data needed to obtain a navigation fix) for later processing. Fortunately, some people found out that it was possible to obtain pseudorange data from 12-channel Garmin civilian receivers by using some undocumented commands. See http://mywebpages.comcast.net/dmilbert/softs/g12r
retrorocket.o not found, launch anyway?
Thats realy not a correct description of differential GPS. You have your groundstation at a known location, and it will measure the broadcasted signals from each visable satellite. Since it knows where it sits, it can calculate corrections for each satellite. These corrections are then sent to your DGPS-receiver, effectively erasing the effect of SA (Select Availability).
Read more on
http://www.navcen.uscg.gov/faq/dgpsfaq.htm
For those who aren't familiar with WAAS, it's the Wide Area Augmentation System. It's like DGPS on speed. It's run by the FAA. They have a dozen receiving location scattered around the country, at precisely surveyed locations. They measure the difference between where the GPS satelite says they are, and where they actualy are, and then transmit that information to geostationary satelites, which then beam the info back to earth. In a nutshell you get 3m GPS accuracy.
:)
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
-73, de n1ywb
www.n1ywb.com
As heise already reported yesterday a US spokesman at the EU denied that the US would anything mentioned above.
See (lazy me, cut and paste from a google news post):
l ity.htm/ gps_pdd.htmS _SA_Event_QAs.pdf
http://www.igeb.gov/sa.shtml
http://www.navcen.uscg.gov/gps/selective_availabi
https://www.peterson.af.mil/GPS_Support/documents
http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GP
In short, NO, they won't degrade GPS.
I dunno what's more disappointing, that some lamer submitted this to slashdot, or that more of you supposedly "Educated" geeks don't challenge the idea.
Erik
When the resolution (if that's the right word for it) was upgraded so that civilians could use it for accuracy up to 2-3 meters instead of 15-20ish, they were *very* clear that the resolution would be degraded when there were military actions.
I had a sucky sig.
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.
Of course they could change this policy, but for the moment it looks like SA will not be turned on.
Gimme a break. The Galileo system was conceived and designed as a "euro-prestige" project. Even if the US military decides to degrade GPS today (which nobody from the US military has said that they are going to do - at least not that I've heard), they are launching many more satellites with civilian only circuits which in the future will provide a separate signal from the military one. Galileo is transparently designed to as a duplicate to the US system to demonstrate EU technical competence and as a prop to their space industry. Loyloa de Palacio, EU Transport Commissioner, said, "Europe wishes to be present on the international scene ... in all aspects of cutting-edge technologies." This is hardly the only project in which the euro-govt's are spending big to try to duplicate already existing US tech. The A400M military transport is another example.
they are not geosynchronous. (exception: The WAAS satelites are; but there are only a couple).
The majority (25+) pass overhead a twice in 24.x hrs (I think it is a 12hr orbit, not positive.)
The Wall Street Journal was recently reporting that many soldiers are feeling over weighted with stuff to carry, so they are resorting to chucking all MRE items that aren't particularly high in carbs, and tossing the GI GPS units for civilian ones that use the same batteries as their headsets to reduce the different types of batteries that must be carried in the field. I'd imagine that such soldiers would be rather disappointed if they had to go back. According to the article, the Army is aware that this is happening. This article appeared about a month ago.
I'm a loser baby, so why don't you kill me.
According to this article, nothing needs to be done to the satellites at all. Jammers can be deployed to scramble civilian GPS signals over a localized area.
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.
All I want is a kind word, a warm bed and unlimited power.
Some Garmin receivers already support WAAS, which is a DGPS variant that uses data from geostationary satellites.
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.
retrorocket.o not found, launch anyway?
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) 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.
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".
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.
3.243F6A8885A308D313
havn't scratched our own project (Galileo). The first of the 30 satellites (27 + 3 active spares) will be launched in 2004, with an initial service operational by 2006 and the full system operational by 2008. The links are here (European Commission site), and here ESA site).
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).
Most GPS systems don't use local tranbsponders. Only systems where accuracy is crucial and cannot be dependent upon the US military's whim use local transponders that send very accurate signals. Such configurations are used, for example, on some airports to guide planes to the tar strips. The accuracy of such systems can be measured in centimeters.
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.
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
-Adam
There is dgps, and I'm pretty rusty but it goes something like this.
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.
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.