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GPS Civilian Signal Degradation Turned Off
Brian Demsky wrote to us regarding the release from the White House stating that the United States government will no longer purposely degrade civilian-use GPS signals. This mean more accurate data for people working with GPS, as "national defense concerns" had kept civilian results less accurate for years.
The satelites are grouped together, with only a few being visible at any one time from any one point on earth. If you turn SA on for a set of the transmitters, then any receiver that is relying on two or more of these now bogus signals, will become confused. The amount of SA is variable, from a few feet up to several miles. Note that the satelites used by a reciever in Dallas, TX are different than those used by a receiver in New York. Or even New Orleans.
This space for rent.
SA coming off is a good thing. However your results without SA will depend on a number of different things.
First and foremost, the quality if the maps you are using. GPS is significantly more accurate than many survey maps, which turn out to be extremely wrong about absolute position in some cases, particularly remote coastal areas. This can be fixed by re-registering your maps using a GIS. Another issue with electronic maps is the scale at which the streets were digitized; If they are digitized off 1:100,000 maps, as many are, you will have significant mistmatches. Sometimes the quality of matching to GPS signals on the same map may vary by feature type (major highway, street etc.) because each feature set was digitized separately, by different people, sometimes at different scales.
I've gone out with DGPS on the fly (both Racal and Satloc, both with sub 2m accuracy), and have been absolutely spot on for some kinds of streets and systematically 100m or more off.
In the city, you may have both coverage and multipath distortion issues that will limit your accuracy for a single reading. Receiver quality counts for quite a bit. Survey users will benefit the most because they can average a number of readings.
Finally, the clocks in some cheap GPS receivers are jittery (kind of like cheap Ethernet cards used to be), and may limit their performance even without SA. After all, they didn't have to be that good with SA turned on.
That said, it's great that SA is coming off.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
Terraserver is completely different from GPS. Terraserver has satellite images and arieal photography. GPS tells you where you are, now with great precision. As far as I can tell, they are not competing services. What is the connection?
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Looks ike they actually did turn SA off!
Last night my EPE (estimated position error) was between 30-50M. This morning it is toggling between 1 and 2M.
Cool!
After reading this article, I raced home to get my GPS receiver, and I'm just after testing it out in the car park.
Using the position averaging feature of my receiver for a short (10-20 sec) period of time, I got a "Figure of merit" of 6 metres. I'm not too sure what, exactly, the Figure Of Merit means, but my guess is that I should not trust the GPS to give my instantaneous position to better than 10 metres or so.
But it's a lot better than it used to be - averaging over 2-3 hours used to give me a figure of merit of 10-12 metres if I was lucky
I was pondering about the same....maybe they are using EST, after all it's the US:) Or the calendar date change, so another 10:30 hours or so....
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Thanks
What you describe is call Differential GPS (or DGPS), but it's only useful over (I'd guestimate) a few tens of square miles.
You're wrong. It's available over a large percentage of the US. The Coast Guard has set up plenty of DGPS transceivers along navigable waterways, for use by boats. Looking at the maps at the Coast Guard's website, it looks like the range of each is around 70 miles, about 15,000 square miles each (with overlap between them).
It only works when the two receivers can receive roughly the same GPS satellites.
Meaning that it's useful as long as the two are within 30 or 40 degrees of each other, well beyond the range of the weak radio signal. At any time, there are typically at least 7 satellites above the horizon, and can be as many as 12. If I'm 60 miles away from the transceiver, we'll probably both see all of the same satellites. Even if we don't, if we can each see 4 satellites in common (definitely the case within 70 miles) then we're able to use the dgps signal.
So it's useless in the middle of the ocean, for example, or over (I'd guess) most of the world.
Congrats, your third and final statement is correct. However, it is possible to put dgps equipment in place around harbors and other places of interest for not too much money, and some private companies even do this. You can look here for information from a company that does just that, and then leases equipment that can use their signals.
-Michael
Do you have ESP?
Its quite easy. They know exactly where the satellite was, and they know exactly what direction is pointing. Therefore, they know the exact grid co-ordinates of stuff it took pictures off.
Hhhhmm...
;-)
IIRC there was a US proposition too put a GPS locator into mobile phones 'so the emergency services could get to you quicker'. Now with SA off and accurate GPS in your phone they can track you to the nearest meter (sorry, yard
Maybe you live in interesting times
The 100M error attributed to SA is the maximum error that can be affected. There is also a large amount of error in the C/A signal itself without the P signal, so consumer stuff will still be ~100 times less acurate than military. The whole point is that with SA you could be right on, you just wouldn't know it since the error moves around a lot.
As far as terrorists go, the Soviet GLONASS system is apparantly almost as accurate as GPS with no encryption.
Si vis pacem, para bellum
The only thing more annoying than a Libertarian is an (un|mis)informed Libertarian
IIRC, you need 4 satellites to determine your position, because you are calculating your position in 3 dimensions.
-Dead Lesbian Witches! Think about it!
Depends on how accurate the NTP gets....GPS has a secondary mission of being within 100ns of UTC time. SA cancellation does not affect this mission. Hope it helped.
Based on experience, I would say that the GPS errors were *less* than 30m. How much less, I don't know. Oh yes, the type of terrain that one is mapping also controls this error estimate. For instance if it was totally flat, terrain matching wouldn't tell you anything.
All this doesn't mean much now as the GPS system is much different today than it was 10 years ago; for instance, no more prototype satellites, full constellation, DGPS. But back then it was all pretty new and exciting.
My point was that the reason the gov't was disturbing the signal was for "security" reasons -- they didn't want the ability to pinpoint locations that accurately to be in the hands of ordinary citizens. However, that ability is now available (or will be in the very near future) with other satellite services (of which Terraserver was one -- possibly poor -- example). It just struck me that the march of technology allowing this sort of service to be available to just about anyone was rendering the sort of government intrusion this demonstrates to be more and more obsolete.
If that makes any sense.
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- Sean
It's a fine line between trolling and karma-whoring... and I think I just crossed it.
- Sean
That's the sound of the joke flying over your head.
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Patrick Doyle
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
I read in another report it will bring the accuracy down from 100m of the fuzzed to 10m of the accurate
SA is just the very basic way to skew the position, which is done by the sats. The error stream is added with a known equation, then the military models also know this equation and subtract it off to get the real position. In war times this error can be increased DRAMATICALLY, many many times the error from what it is now. The military spec also can use land based transmitters so the accuracy can be increased to very very fine amounts, I have read of uses in some special construction projects, where the government has allowed the very accuate models to be used were able to measure changes down to millimeters.
The problem has always been that if GPS is too accuate it is relatively easy to take a rocket (an unsteerable projectile) and make it a very accurate balistic missle. In fact the national organization for rocket builders now has their new members list checked by the NSA or the FBI, because with GPS and a few servos you can make a missle that would be fairly accurate. You may not be able to pick out what window you want to hit but you can hit the building.
I don't believe National Security plays a role in many parts of our lives, but GPS is something that is too powerful not to be well regulated.
Yes! Now I can finally build a nice and precise robot for doing all my "dirty work". I will build droves of them... my evil army of the night. Or not.
:-)
But this is really cool that they took that gosh darn "jitter" off. Maybe now I will get one of those fancy in car navigation systems... or build one...
Anyone have any good info on hardware needed to put together a decent in car navigation system? maybe even some text-to-speech. Fancy facny.
--Ryan
"Don't nargin your MEX files!!!"
As I understand it, they just introduced a random error into the signal. To get around it, you just put a receiver at a known location. Then, (received signal - known location) = error signal.
Then you just compensate for the error. Seems like sort of a pointless exercise all around...
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Thats the point, landing planes also use local transmitters, which are reliable. But if you completly rely on the satallites, you may be happily dissapointed, as was stated that the government reserves the right to turn off the signal for certain regions.
You and I know that, but how many others really do? I think that we will see an increase in sale of not novice users, but people who really think that they need an upgrade.
Romanes eunt domus? People called Romanes, they go the 'ouse? It says Romans go home. No it doesn't. What's Latin fo
Not to mention working out where your PC/TV/Palm/AIBO is if it/they wonder off without you.
Or maybe your high-security laptop?
I can't say that I don't give a fuck. I've just run out of fuck to give.
I guess I don't have to mention that the DoD has been promising to review Selective Availability for years now, always hinting that it "may" be shut down RSN. Good to know it's finally going to be turned off.
..
It can be selectively degraded, I guess, for large geographical areas, although that involves mapping out satellite ground tracks and dithering the clocks on a real-time basis, or possibly very big anti-pseudolites a la DGPS. Wouldn't be cheap, and considering we're about the #1 terrorist target, probably not practical. If DoD has a better way to do it, I'd be very curious
73 de N5VB (ex-KD5BIV) AR SK
No, you may keep using it. The
degradation is/was introduced in the
signal, not in the receptor device.
The signal was degraded by adding a random
walk component with mean your current position and
certain sigma which they now set to zero.
There was a way before to eliminate the
degradation, for which you needed two
GPS receivers, one stationary, another (yours)
mobile. So, this will eliminate the need of the
stationary one and cut costs and make your life
easier. Otherwise, whoever wanted a precise
GPS signal could have gotten it anyway, at the
price of a second GPS receiver.
Best,
I've been wanting to get a GPS receiver for my mountain biking (a.k.a. Louisiana swamp biking) adventures. Without Selective Availability (the degredation mentioned), a GPS receiver will be a lot more useful. To quote from Garmin's "About GPS" page, "Under SA, GPS accuracy can be degraded to a maximum of 100 meters (328 feet). Of course, they don't typically degrade GPS accuracy to that level, but errors of 30 meters or more are not unusual."
Obviously, having an error of about 3 meters will make finding a trail a lot easier (30 meters, being about a third of an american football field, is quite a lot of ground to cover in thick underbrush with a bike on your back while you hunt for a trail).
I can hardly wait to save up enough money to pick up my very own GPS receiver. (I am a very happy camper now.)
True, but most altimeters are calibrated on atmospheric pressure, which will alter considerably depending on your position wrt weather systems etc. Equally, Radar Altimeters are only a part of a solution, because the earth's crust has a tendancy to flex under the tectonic forces (great application for GPS, incidentally!) - if you are military, you don't want to use your radalt unless you really need it, because they use high intensity rapid sweep(IIRC) radar systems - which light up your position like a microwave xmas tree.
The requirement to add location information to cell phones does not mean that cell phones will have GPS systems on board. The existing cell system can be used to locate the caller with reasonable accuracy by triangulating from signal strength information in adjacent cells: For each cell that records a signal from the phone (the same data that is used to determine when to switch cells), draw a circle at the estimated distance represented by that signal strength (calibrated occaisionally), the location is in the region where the circles overlap.
Everything for this to work is already in place and has been used to locate people (one the most notable cases was the woman trapped in a car by a snow storm a couple of years ago). Adding GPS to the system would not gain much and raise the would cost of phones.
Just a guess, but I'm thinking the birds simply stop transmitting when they fly over a part of the world to which the gov't wants to deny GPS service
Are they in a geosynchronous orbit? Weather satellites are, for example, but I'm not sure about GPS satellites. If they are, they're fixed over one part of the globe, which would rule this out.
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I pledge allegiance to the flag...
of the Corporate States of America...
I checked my little Garmin receiver this morning, and I am getting an accuracy estimate of 23 feet. That estimate has never been below 53 feet, and I rarely saw that level of accuracy. It has been no higher than 35 feet all morning. Hooray!
I believe the GPS satellites are organized in geo-staionary orbits spaced somewhat evenly around the planet. I think the minimum is that you can "see" at least 4 satellite from any point o the surface of the Earth (and you only need 3 for triangulation). Do selectively reduce a region's accuracy then would be to re-enable SA on the satellite visible from that area. Can anyone confirm or deny this?
I don't have any URLs handy to back this up, but when I worked for Hennepin County (Minnesota) last summer, I understood from some people that companies figured out ways to compensate for the built-in signal degradation a while ago.
This is probably just an acknowledgment of the fact that you can get good quality signals (if you're prepared to pay for them) already on the commercial market.
I don't see what the national security problems were with the GPS system being off by 100m or so. To me, it seems that if I created a missle, and it landed within 100m of my prefered target, it would take out my target. Did the government finally come to this conclusion, too, or am I missing a point?
Doh!
Altitude is not known, as that varies wherever you are on Earth.... cept in a boat on the ocean I suppose. Aircraft altimeters are +/- 75 ft to meet spec, but are usually kept within 20 ft or so. The accuracy of an altimeter usually goes down as high altitudes are attained. Civilian altimeters are not necessarily more accurate than military - there are many types for many applications, but I digress. Back to GPS: One airplane I fly has an FAA approved GPS for instrument approaches. This connects to the aircraft's altimeter as part of an extravagant backup system - if the GPS measured altitude starts going haywire compared to the altimeter encoder's output, it signals the pilot that nav information isn't accurate enough for the instrument approach. I've seen this happen on about 1 out of every 10 approaches shot - I'm very curious to see how turning off SA will effect this. Also, when the GPS doens't work quite right for the approach, its not a big deal - we can always switch to a more archaic (and often more accurate) approach system designed 30+ years ago!!! Heck its even easier to use (this is coming from the mouth of a compute engineer) So much for new technology.
It is 10 meters like the differential GPS.
Well, yes, but as there are only two possible positions for a three-signal reading and one of them is way out in space, it's easy to eliminate one possibility. Not many 50,000 mile high mountains on earth, last I checked ;) You do often need four signals to get a good reading with commercial models, though, because the clocks are crap.
Having spent several years in the military as a combat targeting person, here is the skinny... The ICBM missile will burn for less than 3 minutes which is not enough time for the missile to leave the atmoshere. There after it is like a baseball thrown into the air. If accurately thrown then it will come down from the higher atmoshere right on target.
Adjustments for the Coreasis effect... that is the point of impact is NOT aimed for... for the Earth is rotating under the warhead. The point of impact arrives under the warhead just as the warhead arrives. Depending whether a ground burst (dirty) or an air burst (destructive) is desired this point of impact is calculalted slightly different... allowing for the slight Earth rotation..
It is similar to shooting a flying duck. You aim well ahead of the actual target knowing that by the time the pellets arrive that the duck will also be arriving.
Most guidance is inertial (gryos) so the need for GPS is not required. Believe me... the missiles are VERY accurate.
And in the end, the love you take is equal to the love you make
I've seen several comments with these kinds of fears.
What you need to consider though is who is going to attack you and why?
- If its a foreign country that wants to send a lot of big missiles intent on causing a lot of strategic damage, they could probably also create their own navigation system. The Soviets have a nav system called GLONASS, which I've heard is actually better than the US's GPS system.
- If its a terrorist group, they don't need acurate missiles. In Oklahoma they simply parked a vehicle outside the building they were targetting.
Thank you, whomever was responsible for this decision.
Any sufficiently advanced technology is indistinguishable from a rigged demo
--Andy Finkel (J. Klass?)
I could be wrong, but they were probably using the system where they compare the civilian signals and the military signals to cancel out atmosphereic interference etc. and record the civilian position over a long period (e.g. a day) to get an average postion.
This is handy when you want to measure the movement of faults over a few years in a remote location. The equipment is damn expensive though, so we undergraduates don't get to play with it.
The only crack involved is the one in the ground.
(Disclaimer: I am not a Geologist. Yet.)
-Dead Lesbian Witches! Think about it!
So is this going to cause my stratum 1 GPS linked NTP server to step time at midnight, or just slew? I'm thinking it'll just slew the clock.
Temkin
So now y'all can get from the liquor store to the Kwik-E-Mart faster than ever before...
The conclusion of your syllogism, I said lightly, is fallacious, being based on licensed premises
The satellite is moving at roughly 7700 meters per second, in orbit around the not-quite spherical Earth, with a non-uniform gravitational field. Very small errors in measuring the attitude and position of the spacecraft, or the time the image was captured, would produce large errors in the maps. Not to mention any distortions in the optics and imaging sensor or thermal effects on the spacecraft.
Well, it's about time. My question is, will existing GPS hardware work or will I have to "upgrade"?
kwsNI
That's 3.3 football fields. How would a GPS be of any use anywhere except on a four-lane highway? Glad I never bothered to buy one.
You wouldn't be able to tell the difference between the street you're on and the next two!
Gamingmuseum.com: Give your 3D accelerator a rest.
Actually, the error of a sum is not the sum of the errors. The square of the error is the sum of the squares of the individual errors. According to your figs, with SA jitter, the error is 30.6 m; without the jitter, the standard error is 5.8 m.
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I find my way to Tipparrarri!!!
This post encoded with ROT26. If you can read it, you've violated the DMCA. Handcuffs please, sergeant.
This may sound kind of silly, but how does it affect national security when Joe Citizen has a GPS unit that is accurate to 1 cm (or whatever it is)? While some extremist groups *might* be tempted to do this, chances are JC isn't going to embed it in any munitions systems....
=================================
I pledge allegiance to the flag...
of the Corporate States of America...
Keep in mind that the US government doesn't care how much something costs. "A billion here and a billion there, and pretty soon you're talking about real money." -- (US Senator Everett Dirksen, Illinois)
Anyway, the point I'm making is that the US government is much more interested in political power, and the abilty to effectively shut down tranportation facilities is a real potent tool. And it looks like it is in the interests of the US government (!= interests of the average citizens of the US) to keep this technlogy in the hands of the US military.
That and the technology to build a real competitor to the GPS satellite system is now available. Consider the costs over doing something like the Iridium, and putting up a competitor to the GPS satellites would be relatively trivial. Other than the fact that the GPS satellite uses a hyper accurate clock, it isn't all that much more complicated than the original Sputnik satellite put up by the USSR. It just transmits the current time and its position (with some accuracy references.) Ground stations are necessary to control the satellites mainly to make sure the clocks are accurate.
With the US government eliminating the intentional errors, they have dried up the market for all potential early users of a GPS alternative. I always thought it was a stupid thing to do anyway... deliberately engineer something to be less accurate than it could be. And introduce bugs and other problems in trying to cope with those intentional errors.
Well... that trashes one 007 plot =)
Good news, though.
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script-fu: hash bang slash bin bash
[ approaching AI ]
While I'm sure the US military reserves the right to reinstate the dither in times of war, it means that the bulk of the time we now have ultra cheap, worldwide navigation.
The funny thing about that is, the dither was a peace-time-only hobble.
The stated purpose of the scrambling was to prevent other countries from targeting their missiles really well by it, but during the Gulf War, when there were actually other countries tossing missiles at our stuff, the military turned the dither off.
completely ridiculous idea, completely ridiculous implementation. glad to see it go.
-Eldurbarn
My interpretation of that statement in the press-release was that in 1996, they planned to eliminate the use of SA in GPS satellites by 2006 at the latest -- we are 6 years ahead of schedule.
It'll be interesting to see. 20m RMS is about right (I think its 17.8 m horizontal, 27.7m vertical), but as I pointed out in a different post, your results will vary depending on the quality of your GPS receiver (clock stability and noise rejection).
High quality DGPS systems are currently available with sub 2m and sub 1m accuracy in the field. I believe the achieve this through additional geographic referece points.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
Elevator position sensors. Eliminates all those pesky micro switches, limit switches, relays and wires.
Three Dimensional Pointing Device. GPS receiver on finger tip. Feeds back to computer via infrared. Great for 3D Games. Just wave your arm around, point and shoot.
Collision Avoidance System for Cars. I know exactlly where I am, where you are, where the poles and bridges are. My on-board computer knows where the brake pedal is. No more big bangs.
Where did I leave my keys? Now you'll know. What happened to the dog (cat, gerbil, hamster, guinnea pig)? Now you'll know.
And finally, a solution for trying to follow the puck on televised hockey matches -- not to mention the ball in golf. The possibilities are endless.
Actually, the article says:
Today, I am pleased to announce that the United States will stop the intentional degradation of the Global Positioning System (GPS) signals available to the public beginning at midnight tonight.
The part you are referring to simply states that in 1996, they made it their *goal* to shut off SA by 2006. And for once, the government has beaten a deadline-- by more than 50%!!
The part you are referring to:
My March 1996 Presidential Decision Directive included in the goals for GPS to: encourage acceptance and integration of GPS into peaceful civil, commercial and scientific applications worldwide; and to encourage private
sector investment in and use of U.S. GPS technologies and services. To
meet these goals, I committed the U.S. to discontinuing the use of SA by 2006 with an annual assessment of its continued use beginning this year.
Well, if you think about it, it should make DGPS work even better, since DGPS was telling your receiver how 'off' the signals its receiving are. This should give DGPS an even better accuracy than it had before.
Seth
$5 / month hosted VPS on linux = awesome!
Actually, the reason Europe wants it's own GPS network is because of the Selective Availability.
I hope that this decision is not going to stop those plans, as the USA will undoubtedly turn SA back on whenever someone in the white house has a bad dream.
OK. I get in trouble with SOMEONE everytime I do this, so let me don the Kevlar...
1. There are 2 GPS bands, referred to as frequencies, centered at 1542.75 MHz (L1) and 1227.60 MHz (L2). Coarse Acquisition and P-code are available on L1 and P-(Y)-code on L2. Each band is 10 MHz wide, and the signals are categorized as direct sequence spread spectrum.
2. Coarse Acquisition (C/A) code was originally envisioned as a means of acquiring the Clock to allow for P-code acquisition. It was then noted that as an unclassified signal with the core navigation information, it could be used for civil applications with an accuracy degradation.
3. P-code was encrypted (became P(Y)-code about 7 years ago, but the exact date escapes me right now.
4. receivers with P-code capability are embargoed by the Munitions Export regulations. Go figure.
Standard Positioning Service utilizes L1 C/A code-phase, navigation and ephemeris data to determine a receiver's antenna position in a cartesian system that is rooughly Earth Centerd, Earth Fixed, and is based on the GRS80 ellipsoid. It uses ephemeris data (orbital parameters that can be propagated to a given time and used to calculate a satellite's instantaneous position) and navigation message data that amounts to precise timing information. The receiver's clock circuitry is phase-locked to a reference satellite's clock. The delay in signal timing between transmission by the satellite and reception at the receiver is correlated to the speed of light and a vector is established to each satellite in view. With _4_ satellites in the solution matrix, X, Y, and Z can be resolved in the matrix. If an altitude is known, it can be transformed to an appropriate offset value set (X,Y,Z) and 3 satellites can approximately establish the remaining parameters. Similarly, if TIME is already known, 3 satellites will allow calculation of all the remaining parameters at a given epoch, but really accurate clocks are harder to come by, than a reasonably disciplined quartz clock phase-locked to a satellite cesium or rhubidium standard. SPS provides an autonomous positional accuracy of approximately 29m RMS at 2 sigma and 56m vertical 2-sigma, RMS.
Precise positioning System uses a similar approach, but uses the P(Y)-code, which incorporates a higher chipping rate by an order of magnitude. Thus, the inherent accuracy at a given epoch is improved by a degree of granularity. PPS is stated to provide 14m 2d RMS horizontal accuracies autonomously and 33m vertically.
Engineers at Rockwell tell me that SpaceCom have been incorporating a few more bits of precision in the P-Code satellite ephemeris data, and that some receivers (Rockwell military hardware) now can routinely achieve 5m horizontal and 7m vertical accuracies autonomously.
As a rule of thumb, differential correction, via either real-time of post-processing data, affords a 10x improvement in accuracy providing all the games are appropriately played: The DGPS base is seeing all the same satellites as the rover, epochs are synchronized, there are no significant disturbances in troposphere or ionosphere between base and rover. Thus, SPS should see 2-3m 2d (hor) RMS accuracies with DGPS and 5-6m vertical accuracies, while PPS should see 0.5-1.5m 2d and 1-3m vertical accuracies. There are notable times when the error budget aligns in your favor and these accuracies improve.
Surveyors utilize the RF carrier wave characteristics to perform their magic, and also use the two seperate frequencies to calculate ionospheric propagation effects. Ionospheric modelling is not possible without at least 2 different frequencies, and diversity in spectrum accentuates the deltas. use of a reasonable model allows for a further reduction of the error budget.
In survey GPS observation, a baseline, or series of baselines, is eatablished between the point of interest and 'n' well-surveyed control monuments. Using the method of least-squares, a network of these points is adjusted for best error distuibution and the result ascertained. A satisfactory number of observations must be obtained, usually requiring observations at 30 sec epochs for 30 min to as long as 72 hours, depending on the application. A process known as integer ambiguity resolution may als be employed to establish on an epoch-by-epoch basis the integer number of carrier wave cycles between each satellite and the reciver; the fractional portion (phase angle) of the received signal is easy to measure. Resolving the integer ambiguity allows for a much finer estimate of satellite-to-receiver distance (vector magnitude) and thus a better solution of the position matrix.
If there are questions, follow up by e-mail and I'll either repost answers here, if there's a general call, or respond by private e-mail
Regards,
Gerry Creager
Texas A&M University
Never ascribe to malice that which can adequately be explained by tenure.
As a followup, it got better today. Today
my accuracy is around 14 feet with 5-6 satellites.
I wonder if SA was really off last night.
Not bad!
The URL in the announcement was a moving-target pointer to the White House Press Releases,. cgi?date=2&briefing=5 , which at least tonight points to the real site
so today's 0th press release is something about www.americasteens.gov, a Federal program to prevent the corruption of our kids' precious bodily fluids or something. If you dredge the pointers to previous days, you get a probably-moving-target pointer
http://www.whitehouse.gov/library/PressReleases
www.igeb.gov
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
Businesses figured this one out a long time ago: by giving away things for "free", they suppress the emergence of potential competition.
Turning off SA has another consequence: in conjuction with the upcoming requirement for putting GPS into cell phones, it means that any cell phone user can be pinpointed very accurately now. I strongly doubt that that kind of requirement was put it for the reason given, to be able to locate people in case they call 911.
While widespread availability of accurate GPS has many useful advantages, it's important also to think through the dependencies and privacy implications it creates.
The real big issue is to get GPS useable for the FAA's WAAS system. Right now other countries are not taking it serious because of Selective unAvailability (S/A or SA).
The other issue is the Europeans keep trying to get into the GPS frequency range. As long as its military system there isn't much opposition. If its a fully civil system used for aviation then its much more likely that GPS will maintain its exclusive use of the frequency.
Basics of how GPS works:
Sats send out the time from their atomic clocks.
The receiver figures out what time it is and difference between its time and the received time.
It calculates its position based on the time difference between the sats and their position.
It gets a better idea of the real time and keeps updating its time/position. The internal oscolators can be within 10 ns of "real time" even on the cheap GPS receivers.
To keep the bad guys from using GPS against the US, the sats will delay their time transmissions by some pseudo random time.
The device to do this has never worked on the older sats and is broken on others (I think prn #1, #20, #6)
As far as if the US military receivers are any better than the cheap handhelds, I'm not sure they are better. Trimble has had the best receivers in the world for some time and their best are not the military systems (but use the encrypted signal to help do some phase calculations). There have been reports that the better marine units were giving better position reports than the military units the last time S/A was turned off.
Also the Russian system GLONASS has a number of problems and may never get any more sats launched. They currently have 10 listed as working and another 9 listed as unusable and there should be 28 sats total. There had been rumors that Sweeden was going to by it.
You can do DGPS over the internet too.
I've got a $150 GPS reciever that estimates its own error in the horizontal plane. At rest it usually estimated that it was about 35ft off - and the vertical error is something like 3 times that. I never really tried to verify this rigorously.
The error goes up if you are moving rapidly, or if there isn't a good view of the sky - such as in a canyon or in a big city. But for hiking, I think it would be plenty useful as it is/was before they turn/turned off SA.
Still, I'm pretty psyched that my toy just became more useful. It's mildly tempting to stay up and see if the estimated error drops right at midnight (I assume that Clinton meant Eastern Daylight Time), but I don't really care that much.
"The simplest solution is to ignore your dead children."
Great news, but it reminds me of something about accuracy..
If 99.9% Were Good Enough
From InSight, Syncrude Canada Ltd, Communications Division
If 99.9% Were Good Enough, then...
Two million documents will be lost by the IRS this year.
811,000 faulty rolls of 35mm film will be loaded this year.
22,000 checks will be deducted from the wrong bank accounts in the next 60 minutes
1,314 phone calls will be misplaced by telecommunication services every minute.
12 babies will be given to the wrong parents each day.
268,500 defective tires will be shipped this year.
14,208 defective PCs will be shipped this year.
103,260 income tax returns will be processed incorrectly this year.
2,488,200 books will be shipped in the next 12 months with the wrong cover.
5,517,200 cases of soft drinks produced in the next 12 months will be flatter than a bad tire.
Two plane landings daily at O'Hare International Airport in Chicago will be unsafe.
3,065 copies of tomorrow's Wall Street Journal will be missing one of the three sections.
18,322 pieces of mail will be mishandled in the next hour.
291 pacemaker operations will be performed incorrectly this year.
880,000 credit cards in circulation will turn out to have incorrect card holder information on their magnetic strips.
$9,690 will be spent today, tomorrow, next Thursday, and every day in the future on defective, often unsafe sporting equipment.
55 malfunctioning automatic teller machines will be installed in the next 12 months.
20,000 incorrect drug prescriptions will be written in the next 12 months.
114,500 mismatched pairs of shoes will be shipped this year.
$761,900 will be spent in the next 12 months on tapes and CDs that won't play.
107 incorrect medical procedures will be performed by the end of the day today.
315 entries in Webster's Third New International Dictionary of the English Language will turn out to be misspelled.
Integrals? Cross products? It's not even that hard... it's just 0's, 1's, and nand's.
--
You don't need military hardware to get sub-metre accuracy. There are commercial systems available which rely on the phase of the signals from the satellites. It uses a series of FM signals to specialized portable receivers and you can get sub-centimetre accuracy horizontally and accuracy to about 1 centimetre in the vertical. It's already in use all over the country (Canada and the US), and I've seen it used for surveying in boreholes and the resurveying of benchmarks.
Just cuz you ain't paranoid, doesn't mean they're not after you.
-------
CAIMLAS
~/ssh slashdot.org ssh: connect to host slashdot.org port 22: too many beers
The Soviet military and space agency built a GPS system at exactly the same time as the Americans. The system is still functional, and many of the GPS receivers from European firms actually accepts both of the signals, and can use either on for positioning.
The Russian system gives you many more satelites in the arctic regions due to the large area of Russia that's located in the arctic.
Just cuz you ain't paranoid, doesn't mean they're not after you.
Looks like a good time to get a receiver. Anyone have an opionion on the best sports/recreation receivers in the $400 range, and/or the best place to get them?
--
grappler
Vidi, Vici, Veni
i don't really know anything, but i seem to remember reading somewhere (slashdot? discover magazine?) about a bunch of seizmologists who found a way to get ultraaccurate (sub-meter) GPS readings by using some really wierd workaround to the system. It wasn't the "differential" one everyone else has mentioned where you average values, i think. I think it had something to do with the strength of the signal. But i can't quite remember.
Anyone know what i'm referring to and want to fill in the huge gaps in the story? Am i just on crack?
Irritable, left-wing and possibly humorous bumper stickers and t-shirts
I meant that to be "sub $400 range" - I forgot that less-than signs don't necessarily show up :-)
--
grappler
Vidi, Vici, Veni
Over at one of the military gps status sites (over here) they mention the mission being global nav, time, and nuclear detection.
Nuclear detection?
Anyone have any other info on this? I'm mostly just curious, I didn't think those satellites also had that feature. Or are they simply the standard relays for another detection facility?
Just wondering,
Jason
I'm glad they're allowing it to resolve down to a finer point, although the privacy implications of this are horrific. When I was coding (in perl) some scripts for Nextel, I saw some of the frequency chunks they dropped out for mil use only in the spectrum, and it was quite significant - nice bands with chunks missing every so often. Pretty easy to tell where the signal was.
However, the old GPS was useful in terms of driving moderate to long distances - you could save the GPS point for a route change or your house, or select an intersection, and then tell if you overshot it by a block, but it wouldn't let you encode "turn left in 6 meters" or anything useful for fine navigation.
Of course, that's without buying the commercial "fixed" GPS service some carriers offered for a premium that would let you get back to the correct locations.
Now I can post the GPS location of my Theme Camp at Burning Man and people can find exactly where it is!
Will in Seattle
So I think they can still change the accuracy for a specific region, without neccessarily changing it global.
Uhh...no.
The release says it is being turned off at midnight tonight.
The release also said that they government had a mandate to remove SA by 2006. This means they had until 2006 to do it. It means that after 2006, they CAN'T use it. Currently, they will turn it off, and see how things go. They can still turn it back on if they want.
Not quite. Even though SA will be turned off, civilian GPS recievers still can't be used for guidance packages. The units shut down if they experience a high acceleration (> 3 g's or so IIRC) or a high velocity (Somewhere around mach 1). This prevents them from being used in a missle type application.
This
"Oops! I didn't mean to hit that building full of civilians. Who asked you to degrade that GPS anyway? We were trying to hit that building!" :-)
This was a smart move by the US military. With the increased GPS accuracy there is nothing to drive the development and deployment of an alternative by some other hostile country. The Russian Glonass effort will wither. In the event of war the US airforce still preserves the exclusive capability of steering a 500 lb bomb to some poor chump's bedroom window!
I have a little Garmin GPS III+ that has a little moving map display, "here you are" arrow, routes, waypoints, altitude, and it's about to get a whole lot more accurate.
/.-ers.
Having a web-pad or some other PDA (handspring!) with GPS capability would also be very cool, and it would have more memory than my Garmin (which has enough memory for about 10 MapSource counties with street-level detail, and the entire US with highway detail).
Of course, the Garmin isn't open source, and thus would be utterly useless to some
But I don't know how I ever got along without it. I'm still amazed at what this little box can do.
-CausticPuppy "Of all the people I know, you're certainly one of them." -Somebody I don't know
GPS satellites orbit in the area between low earth orbit and geosynchronous orbit. This means they move across the sky relatively slowly (geosynchronous satellites have no apparent motion, while low earth orbiting satellites are only usable in the sky for 10-15 minutes). This altitude means receivers can pick up the signals easier (geosynchronous satellites require heftier antennas or a parabolic reflector) and their slower relative velocities make them easier to lock on to and position over longer periods of time.
As far as the actual question, the other comments pretty much said it best. A satellite can be temporarily disabled or introduce error into the signal only as it's within range of the region in question. Even if you're 1/8th of the way around the world and end up locking on to this satellite as it starts introducing error into the signal, there are still several other satellites in range of your position that your receiver could lock on to instead, compensating for the error. This means the actual affected area can be isolated as much as they need to.
i was out side (of corse) and had 5 sats locked on my e-map and i had a 15 foot accuracy i have never gottenbelow 70 feet or so its awsome! i am finally truly happy with my gps! thank you but no thank you government! i measured the football field at school and it was 20 feet off now thats awsome!
I read in one of the AOPA's (pilot's organization) that the FAA was having difficulty implementing WAAS which is wide area augmentation service for GPS. It is sort of a instrument landing system using GPS but GPS needed to be augmented near the airports because of SA. Perhaps this helps clean up the WAAS implementation?
Aircraft carries can survive Nuclear blast.
It will push the ship about 100 meters underwater, and(assumning the ship is locked down, it will resurface. However, the people won't survive.
Now I'm talker a blast greater the 1500 meters here. Obviously direct hit would vaporize it.It also depends on tonage of the yield.
Yes I know scuds aren't guided, I was trying to use it a a missle size reference, not a scud capability example. I guess I choose a poor example.
Yes our cruise missle do, my point was, someone could build a missle that was fairly effective using the GPS system. No it wouldn't have pinpoint acurecy, but it could be effective for terrorist activity.Again, I must of chose a poor example. I apologize for that.
On a personal note, It was very hard not to flame you for comparing me to those idiots that think the moon landing was fake. I hate those people.
peace.
The Kruger Dunning explains most post on
Kickass! I can now find my car!
- Stop praying for someone to save you, and save yourself.-
KMFDM
- Think for yourself, question authority.-
I have a Garmin E-Map GPS receiver. I used it on
the commute to work, and waited until 9:00pm Pacific
to drive home. (then I found it was 8:00 eastern
when they turned it off). The GPS receiver has
a satellite status page that shows Lat/Long,
altitute, speed and accuracy. On the way to work
the altitude typically said stuff like -200 feet,
-100 feet and so forth. The accuracy readout
varied between 57 and 150 feet.
Coming back from work, the altitute was around
200 feet and only varied by 1-2 feet. The accuracy
readout always read around 88 feet though. It
didn't fluctuate as much as before. I looked at the
track of my trip to work, and it tended to wander
back and forth and to the side of the road. On
the trip home it was always centered right on the
road and in the correct lane.
Don't know how the receiver figures accuracy, but
it seemed like it might be more stable. I just
expected to see 'Accuracy: 3 feet' or something.
I believe the EU (which feels it needs to have a home grown alternative to every single thing developed in the US) and vice-versa...
--- Can i borrow your Clue-Stick(tm)? I need to go beat a few people with it...
Latitude and longitude are the only two values that we're concerned about in our GPS systems. Height comes from other sources (radar altimiter, doppler, corrected static pressure systems, etc). Meanwhile, time is entered into the whole Enhanced Navigation System after you switch it on. Then you just wait a few minutes for the GPS to capture a few satellites. I'll look into it, but I don't imagine that the original time input by the flight crew or technicians is corrected by GPS's time. Exact time is relatively unimportant for most aircraft systems.
The russians DID put up a gps like system http://www.maxwell.af.mil/own/space/glonas.html :P
Though it is very buggy and prone to outages
and only certain parts of the world are covered
those crazy ruskies!!
Military aircraft actually have several sources of altitude. The most common way they do it is to have a static pressure line from the outside of the aircraft that leads into a computer of some sort that corrects for things like the venturi effect. That computer would then combine and average the static pressure signal from the outside with something like a radar atimiter, acceleromters, or doppler radar. And if any one of these systems were to break, the reading might be a bit less accurate, but you'd still have a reading. If the computer that controls the averaging of signals were to break, you'd still have access to the original direct static line fed directly into your altimiter. Which would be good enough to backup purposes, since you'd then have a maximum margin of error somewhere around 500 feet.
I beleieve the type of accuracy you are refering to either rely on your having access to a ground stations whisch send out another timing signal and your reciever's being in place for a certain amount of time. (ie. you will not get this resolution at anything near realtime.)
Yes, radar altitude is usually only used in landings or to check ground proximity in the event that the pilot suspects his alititude signal is haywire.
there already is a competitor http://www.maxwell.af.mil/own/space/glonas.html
the russians have had one for years, since about '85
No, it's really 15 meters. I read it in my military techical manual at work.
But like you mentioned, it is quite possible to get an extremely accurate position when you figure in other things like land-based signals and that radius-tracking stuff.
That could be part of it, but another major part of it is how an ICBM would affect a hardened missle silo. One programmed to hit a silo dead on and carrying a civilian GPS might hit 100m off-center... This may cause some local devestation, but it wouldn't destroy the ICBM contained within. The US, with the ability to 'see through' the SA, would be able to target *your* silos perfectly. So they could safely withstand your pre-emptive strike, and effectively retaliate with their own. At least, that's what I hear... 8-P
ANonymous Coward just posted a good explanation in #81
I'm not a moderator today, so I'll quote it, but you can go moderate the original up if you'd like.
--------------
from Trimble Navigation, "Differential GPS Explained":
Summary of GPS Error Sources:
Per Satellite Accuracy Standard GPS
Satellite Clocks 1.5 m
Orbit Errors 2.5 m
Ionosphere 5.0 m
Troposphere 0.5 m
Receiver Noise 0.3 m
Multipath 0.6 m
SA 30 m
Typical Accuracy 50 m
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
Now I'm going to go off topic for a second to speculate that if the DOJ breaks MS up, Billy Borg will buy Cuba (I bet Fidel would sell if you offered him, say, $5 billion,) evict everyone, and erect a Giant Flying Windows Logo visible from Space.
I'm trying to teach myself to set people on fire with my mind... Is it hot in here?
Great news. Now if the FAA will follow suit and allow GPS approaches at *any* airfield, more pilots will get IFR qualified, and we'll hopefully see fewer GA crashes.
"You done taken a wrong turn."
-Bill McKinney, in Deliverance
There's some more info on the announcment at the International GPS Executive Board. There are some announcments from the secretary of commerce, so I think that's where some of the motivation came from.
There's also a great representation of the difference in accuracy with and without sa on. take a look at:
http://www.igeb.gov/sa/diagram.shtml
To quote:
Anyways, have fun out there.
Jason
This doesn't mean that *you*, the cell-phone user can locate where you are - it just means that the phone company can, so that 911 can locate you (if you're not paranoid, and for some reason believe the official explanations), or so anybody with a badge can locate you (if you *are* paranoid), or so any 2600-script-kiddie or at least any good social engineer can locate you.
If you do want to check out the paranoia options, spend a while thinking about the requirements that the 911 center be able to locate you any time your phone is on without you acknowledging it, and the lack of requirements for a standard locatee user interface.....
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
I suppose, though, if the gov't gave everyone a free TV with an embedded non-disableable (without making the TV functions die too) listening spy device and I suppose they'll gladly install it in their home.
What is the connection?
With TerraServer you can construct VERY accurate maps, with GPS you can determine where you are on the map with VERY good accuracy.
Basically together they are the 21st Century equivalent of a compass and a hand drawn map. One without the other may be useful, but you really need both to get from point A to point B efficiently.
Work for Change & GET PAID!
Well, yes, you do have to broadcast the result. But if John Q. Terrorist needs an accurate signal, he can get it. Rebroadcasting the correction is relatively trivial, technically. If you build/control the transmitter, you could be quite certain that you were getting the right information.
---
I've worked with 2 GPS systems. Both were diferential GPS.
The first was for the Arieal Robotics Competition held in the summer of 1996 at Epcot Center Orlando Florida. I was with Oakland Unversity. We built a helecoptor that used diff. GPS with resolution of 1 cm, plus or minus 1 cm of acuracy, giving us within 3 cm of acuracy. We put an antenna in the nose and the tail. This gave us enough resoultion for bearing (I cannot remember if we went with this solution or if we used the honneywell compas, we were limited on the number of channels the transever had.) Anyway using the diferential GPS we had resolution of 3 cm.
The second project was for the automatic docking of ships. I did not have the exact specs. of the GPS but to be able to dock a ship you need more resolution than 10 meters.
I have no idea waht the resolution of the consumer GPS recevers will have. It basically depends on the number of Sats. the unit will use.
If at first you don't succeed, skydiving is not for you.
Is there any sort of a project that has
1) free map data from all over the world and
2) a program to display that data, preferably in Java?
I only know about commercial products, even for the Psion 5mx (and someone mentioned the Palm), but no free ones. I guess it's hard to get the map data digitized without 'stealing' it from proprietary software.
The C/A signal is easier to aquire and less precise. Standard positioning services data is accurate to within about 100 meters. IIRC, this is the signal that is degraded. They randomly skew the data so that your results are slightly off.
The encrypted P-signal uses more than just the standard GPS frequency. It provides precise positioning services is for military use only. Receiving and decoding the P-signal requires special hardware.
What I suspect this press release means is that the standard positioning services will no longer be intentionally degraded. The press release also mentions that they could begin re-degrading the signal at any time. It's even possible to deny GPS coverage on any arbitrary region of the globe, with minimal effects elsewhere.
I remember at the time hearing about soldiers sending off mail-order for civilian units for their personal use.
Also, given the orbital physics of the GPS's themselves, I would think any regional control would be very course grained.
TWW
"Encyclopedia" is to "Wikipedia" what "Library" is to "Some people at a bus stop"
The global positioning system is a satellite-based navigation system consisting of a network of 24 orbiting satellites that are eleven thousand nautical miles in space and in six different orbital paths.
The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours. If you do the math, that's about 1.8 miles per second
Well, yes, you do have to broadcast the result. But if John Q. Terrorist needs an accurate signal, he can get it. Rebroadcasting the correction is relatively trivial, technically. If you build/control the transmitter, you could be quite certain that you were getting the right information.
Even easier, you can buy commercial DGPS transmitters fairly cheaply, or if your target is near the coast (as 90% of the US population are) then you can simply use the USCG DGPS broadcasts! IMHO, SA has never made any sense at all in terms of blocking terrorists. If it were that useful, then we'd have seen a GPS bomb by now.
SA is probably more about military applications, which I don't know much about. Probably the DoD had reasons to be concerned, but either decided that they could be addressed or that the role of SA was going to be obsoleted by widely available technologies.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
Now, wouldn't that be really great when platoon A opens fire on the wrong hill 50' from where they should have
Or bombs a Chinese embassy.
Oh, wait....
Wow, the Whitehouse didn't just put out a press relase...they did something too. SA is off!
Heres a graph of the improved accuracy.
>I always thought it was a stupid thing to do
>anyway... deliberately engineer something to
>be less accurate than it could be.
You have no future at Microsoft.
--Jeff
Yes, they were selling jamming kit in Russia a few years ago for not many dollars.
Did anyone see a real bright light?
Don't worry about the Ukraine, they returned all their nukes to Russia... and I doubt they had nukes stationed there headed for the States anyway, as that's about as far away as you can get within the former USSR as you could get.
Also, in reply to another reply, I don't believe that any ship could survive a nuke detonation at 100m, even a small one. If the blast itself doesn't melt or evaporate the thing, the radiation will kill everything and everyone, not to mention the "tidal wave" that follows.
- da Lawn
't used to be LawnMOWER, really...
A slasdotter wrote
>As I understand it, they just introduced a
>random error into the signal. To get around it,
>you just put a receiver at a known location.
>Then, (received signal - known location) = error
>signal.
True, and I recall Ericsson made a system a few years ago that did just that.
But with the new Millennium Copyright Act, this would be illegal right?
The DCSS-crack is being proscuted, because the copy protection system was defeated. And sites providing instructions and code about how to break the protection are prosecuted as well. The issue is not that anyone tried to sell copyrighted material .
So if the GPS signal is protected with an error then it is illegal to try to defeat that protection?
/Patrix, Sweden
As it seems to be the limiting factor...
was just a little too lazy to read... :-)
2. If the GPS is inaccurate, it is useless in cities with the above applications
With Differential GPS (see other posts), and inertial navigation systems, measuring speed and direction, then no it isn't. You just use GPS to update errors in the INS, and vice versa, using a technique known as kalman filtering, and you can get pretty good accuracies. There is currently a project on at my university, to see if neural networks can be used... it is actually my project, so feel free to ask any questions....
http://liftoff.msfc.nasa.gov/RealTime/JTrack/3d/JT rack3d.html and http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nm c?78-020A
/. a few monthes ago.
"GPS employs 24 spacecraft in 20,200 km circular orbits inclined at 55 degrees. These vehicles are placed in 6 orbit planes with four operational satellites in each plane. "
Check the Java applet out that lets you track the orbit of the sats in realtime...it was posted up here on
"The first eleven spacecraft (GPS Block 1) were used to demonstrate the feasibility of the GPS system. They were 3-axis stabilized, nadir pointing using reaction wheels. Dual solar arrays supplied over 400 W. They had S-band (SGLS) communications for control and telemetry and UHF cross-link between spacecraft. They were manufactured by Rockwell Space Systems, were 5.3 m across with solar panels deployed, and had a design life expectancy of 5 years. Unlike the later operational satellites, GPS Block 1 spacecraft were inclined at 63 degrees. "
from Trimble Navigation, "Differential GPS Explained":
Summary of GPS Error Sources:
Per Satellite Accuracy Standard GPS
Satellite Clocks 1.5 m
Orbit Errors 2.5 m
Ionosphere 5.0 m
Troposphere 0.5 m
Receiver Noise 0.3 m
Multipath 0.6 m
SA 30 m
Typical Accuracy 50 m
The former USSR already has an equivilant to American GPS. The GLONASS constellation has been operational since about 1986.
The GIS company I work for has receivers that use both GPS and GLONASS for most of our survey work. When used in differential mode, these things are capable of sub-meter accuracy. (usually less than 0.5 meters error)
Still, turning off SA is great news.. :)
Yes, one day I may actually learn to spell...
They actually orbit quite a bit higher than LEO (in the area between LEO and geosynchronous). NASA has a great visualization tool at http://liftoff.msfc.n asa.gov/RealTime/Jtrack/3d/JTrack3d.html. I mainly use this for amateur radio satellites, but GPS satellites are in the catalog, and you can see where they are relative to most others.
At a given location, there are realistically 4-6 GPS satellites providing a solid signal. If you move one or two hundred miles away, 1 or 2 of those satellites will be different. If you limited your intentional errors to those initial satellites, you now have like a 20% accuracy increase. Move a few hundred miles away from that and you're further out of the error zone. A smart receiver could possibly figure out which satellites were giving the better signals and ignore the ones that were giving errors. You're right that error can't really be introduced with pinpoint accuracy, but the area where signal quality is 100% affected by intentional area can be made relatively small. Of course at least 1/4 - 1/2 of the world will be affected in some way, no matter how selective you try and get your satellites.
I imagine this ability (to switch off or introduce tremendous error) has been in the satellites since the beginning. The process of switching it on and off in real-time, as satellites pass over black-out areas, may be almost entirely automated. Just feed a set of coordinates to all of the GPS satellites, and have them figure out for themselves when to activate selective availability. *shrug*..
Actually, you will only get ~10 meter accuracy with a non-military receiver (that is, about 10x better than now). Millitary receivers use a second encrypted frequency to receive higher precision satellite orbital elements that circumvent SA anyway. I believe they DO get about 1 meter accuracy but I'm not too sure. Ian
The reason that pinpoint accuracy is important when looking at long range ballistic trajectories is for mid course corrections. When an ICBM leaves the Ukraine headed for Florida, it has to travel through a whole lot of poorly modelled atmosphere. Accordingly, at various points along the track, the missile has to make small adjustments to compensate for un-modelled forces in order to hit its target area. These mid-course corrections are very sensitive to small errors, so position inaccuracy at these critical points can lead to large errors in the missles final destination. Given this, a loss of precision of a few hundred meters means you hit Havana instead of Disney-Land. It does not mean that you hit Epcot instead of Space Mountain.
(ps. I like Disneyland and am not in any way condoning launching anything at in. )
I see a trend here. Last year pictures of a North Korean missle launch site were commissioned and taken by a private satellite, revealing a rather unimpressive facility not nearly matching the threat advertised by the Pentagon. Then it was pictures of Area 51 by a Russian satellite released to the public. Now we have the GPS degradation turned off.
Let's be honest here. It was only a matter of time. The longer restrictions were in place, the more likely it is that private or non-U.S. satellites would walk right on by, rendering the degradation irrelevant. Just like the restrictions on what satellite pictures U.S. satellites are allowed to take, they become pointless as more private and foreign instruments proliferate.
This is a good thing, IMO. It's becoming harder for governments to hide information from their citizens. When you read some of the now released transcripts on how the CIA manipulated the media in Iran and Vietnam and countless other hotspots how can you not have positive feelings on these developments.
I would like to nominate David Brin for a Slashdot interview, given all of the recent transparency and privacy stories that have come up lately.
I know that in a coal mine that my brother in law worked at, they had Loran navigation for the big trucks. On foggy days they could operate when they couldn't see the ground. Imagine driving up and down a mountain in a 300 tonne truck without seeing the road.
Just imagine all the damage the US could cause if someone had a bad dream and they decided to turn the GPS system off? What would happen if they figured out some way to only let some geographical areas get high-precision GPS? What something happened to the GPS sattelites, say some sort of technical problem? This sort of dependency is just plain dangerous.
TerraServer has a lot of very out of date maps though. Very up to date satellite photos are still rather pricey. And let's not discuss video.
TS could get to be more useful, but it's not as useful as it should be....
-- This and all my posts are in the public domain. I am a lawyer. I am not your lawyer, and this is not legal advice.
I know that Cuban kid caused a bit of a political stink, but this seems like a bit of an over-reaction.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
In addition to everything else, I suspect they use regional jamming. There have been a number of Notice to Airmen and Notice to Seamen posted in the past couple of years saying that GPS will be unreliable in fairly restricted areas, like off the coast of Maine.
...phil
...phil
"For a list of the ways which technology has failed to improve our quality of life, press 3."
This is great for land surveyors. I used to be a civil engineer and had gps been around it would have made for easier laying out of grids etc.. The non -military one wasn't accurate enough.
I think surveyors had a work around to get the higher accuracy (they set up a base station at a known point and used a remote sensor. the difference between the 2 gpses would be accurate as the error would be the same between both insturments).
I've seen laser levels contol dozer blade hieghts (you set up a spinning laser and a device on the dozer blade to read the laser and tell the operator to cut or fill more..) Now if they could get the hight of the dozer and a computer with a topo map and gps......
Just think of GSM (worldwide, 'xpt the US), tte Metric system (same there), PAL vs. NSTC (dunno how big PAL is accutally - could be wrong here)
-henrik
It would seem to me that they had to do something like this pretty soon. What with all the TerraServer's and the like popping up, there wouldn't have been much point to it. It's one thing if there's no other way of getting such good/accurate information, but now there are other ways. And more and more seem to be showing up every day. So any "national security" concerns (or whatever) are by now pretty much bunk.
--
It's a fine line between trolling and karma-whoring... and I think I just crossed it.
- Sean
It's a fine line between trolling and karma-whoring... and I think I just crossed it.
- Sean
I think this is a clever move. With the increased precision even more devices will rely on GPS.
Maybe missiles of other countries will become as precise as the US ones. But this doesn't really matter if you can threaten destroy the infrastructure of a whole country by scrambling GPS-signals for that area.
This will mean that
civilian users of GPS will be able to pinpoint locations up to ten times
more accurately than they do now."
Can I look forward to Linux powered cars? Lawnmowers?
Ok guys, so instead of waiting an longer I went to sleep and got up in the morning(live in London) and went on the roof of the house I am living in, all those tall building don't do GPS to well as it is line of sight. I took out my Garmin III+ and switched it on: 6 satelites 6m EPE 9 satellites 4m EPE / 1.2 DOP This is fantastic! Older readings of my system with 9 satellites had 15m PEP if you were really really lucky and did not move! some more stats: 4 sats 2 good and 2 real bad: 19m EPE/ DOP 4.2 this was something like 50m This is also very interesting as you often have/had problems when being in the mountains or dense forrest and did not have the "ideal" line of sight, therefore living only on 3-4 sats. I mean now we are talking, with an accuracy of 4m you can do very good navigation(maybe not the ship docking stuff, even though I recon if you add some other sensors for the last meters this should not be an issue either) for instrument approached in planes it will also work, AFAIK, I mean it even worked before cause I used it a lot for that when there are no documented apporach patterns(as is often the case with small airports or airstrips), and as long as the runway is not to narrow and the plane too big(mum can I take the Jumbo today...No stay with your C172...buhuu). So next time when you take your C172 into JFK, no worries to ATC you got GPS.... Another VERY nice by-result is, that the alitude will and is way more accurate. We alwayws said that the alitude error is 3 times the PEP error, so if you were about 25m where youwere supposed to be long/lat you were about 75m where you were supposed to be when talking for da 3rd dimension which is kind of fuzzy...this will help a lot of mountaineers... enjoy it all!!!! DOP is an internal measure of the Garmins, that is directly correlated to the EPE(=Estimated Point of Error), which is uses for juding the signal quality and reception.
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There are critical technologies that other countries need and they just don't want to rely neither on the US nor in any other countries to provide them.
GPS is not a civilian technology, it was designed by the militars and it was seen that it could have a civilian use afterward, but even now with the SA open we, outside the US, cannot rely on GPS to do some tasks, as it is as easy to start using encrypted codes again.
I agree with you in that this has been done to stop others from building its own competitive GPS networks, but as it happens with encryption, it won't stop nothing, because the main issue is that no one wants to rely on third countries on this kind of technologies, even on civilian applications: you have heard of UMTS, the third-generation mobile system, it will use CDMA modulation where time accuracy is critical, the system needed a precise global clock accesible to all the mobile phones, GPS was proposed as it is in fact an extremely good global clock, but it was rejected and UMTS will use it's own time singal.
Why? because UMTS will become worldwide in a matter of years, and do you imagine the whole world (except the US) without UMTS communications because the GPS networks is purposely down? Again, the key issue is that no one wants to be dependant on others, even friend, alliate countries.
I don't know but this seems kind of strange...adiplomat driving around with a military grade GPS to get the coordinates for bomb runs, in a city? I don't think so. A city has 2571 fix points which are well known and can therefore be used to help the sat maps, you don't need any diplomats running around. Besides the GPS was ONLY used in Kosovo b/c the weather was that bad and infrared missiles and land shape recognition systems did not work. Additionally AFAIK the GPS did not do the homing in on the last meters but was used for the long distance navigation, anyone has a comment on that, that's at least what I was told by some ppl that were down there....could be wrong
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Without the degradation, think about how easy it will be to hack together a cheapy, and accurate, missile guidance system. I'm quite surprised the Clinton admin did this considering how paranoid they are normally. It kind of makes me wonder though... This being the same administration that is hell bent on decresing personal privacy to the government while increasing it in regards to business (they're maintaining their monopoly).
Yes, it's reliable enough to fly an aircraft by... a Stanford team demonstrated this with a model aircraft circa 1996. And later, they encored by driving a tractor around...
engineers never lie; we just approximate the truth.
what kind of accuracy do you need for harbour approaches? As a pilot you want to hit the runwayand I don';t see a prob anymore with 4m EPE but with a ship that goes real slowly and 4m accuracy, what is the problem?
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This is great... too bad it is such a struggle to get any of the information and resources accumulated with our tax dollars. Clearly there are sometimes national security interests to protect, but the degree to which the government slaps on the "classified" stamp is pathetic. Most of the concerns on /. about the govt.'s relationship to information center on the flow from the people to the govt - so seldom do we see examples of the equally important issue of openning up the government to scrutiny. It is vital to a democracy that we not only have access to resources such as these, but have access to ALL government information, so that we the people can hold the state accountable for all of its actions.
From the release: Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened.
Anyone have any idea how they do this, exactly?
Interesting point, but I don't think that this was their primary objective:) More that the Europeans are disincentivized of shooting up their own system, which AFAIK are going to decide soon upon. So this was just in time. Considering the Russian competitor, I have heard that they already had some serious problems with their satelites, the money shortage story again...anyone has more news on that, naturally they don't have anything on this on their page:) Eagerly waiting to try out my III+ without SA...
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See
http://www.wombat.ie/gps/saoff.gif
for a GNUPlot graph of SA being
switch off.
Now we need to take a webpad or equivilant, and add moving map displays to it.
Have a map with a little you are here arrow.
Is all the data necessary for this availaable? We have dem and landuse data...maps and roads. Tie it into mapquest and you would have an opensource navigation system for your car!
Binder
If you have a legitimate use for it, you can get a waiver for one (and only one) of the limits. Amateur rocket experimenters have used this to waive the altitude restriction so the receiver starts reporting data again when it goes subsonic while the rocket is coasting near its apogee (highest altitude.) Then they can prove how high it went.
The current amateur rocketry altitude record is 72,000 feet, set in May 1999 by JP Aerospace, a non-profit amateur rocket group from Sacramento, CA. It was measured by GPS. The launch was also taped and aired by CNN.
The GPS satellites are not in geosynchronous orbits. They are in 12-hour oribits, inclined about 55 degrees.
If you think about it, a GPS receiver wants to see 4 satellites that are as widely spaced as possible to minimize geometric distortion. If they were in geostaionary oribits, they would all be clustered on the southern (or northern) horizon.
Now, there was a plan by the FAA to put a single geostationary GPS "helper" satellite called WAAS (Wide Area Augmentation System) to help make GPS accurate and reliable enough for aviation purposes so that they could start the phase-out of VOR. The theory behind it is that if a GPS recveiver could get one undegraded signal, that it knew was undegraded, it could give that one a higher weight in the navigation calculation. The result would be a much better fix even with SA turned on. Last I heard that project was behind schedule and over budget and they hadn't even started building the satellite yet.
No, it means they promised not to use it on a routine basis after 2006. They still CAN use it, and you can bet if we get in a war and somebody's using GPS to guide their missles at the US's most strategic targets (The Geek Compound, yes?) then they'll turn it right back on. (And I'm glad they've got that capability. I really, really hate for my tax dollars to be used to build a satilite which gets used to blow me up.)
Well the above comment may seem funny, but it is true. If you've ever driven and watched your GPS location on the map, it sometimes looks like you are driving off the road!
Now if we can just get rid of all those digital watermarks, where map makers insert deliberate errors into the map to make it easier to catch unauthorized *cough* *cough* copies....
I'm not a journalist, but I play one on slashdot
This is awesome news. I use gps nearly every weekend, and it's always bothered me (just a little) knowing that it wasn't as accurate as it could be.
Here are some good links to gps info:
US Coast Guard Nav Center Great status info and general policy stuff.
GPS Info Website The best central source of info on gps that I've found so far. Tons of links and content.
GPS and NMEA Site Another good source of info, including hooking up a gps to other equipment (hand computers, autopilots, etc).
Garmin A well know maker of gps units, their site also has some decent info in general on gps.
Anyways, have fun out there.
Jason
I'd guess that the US government will retain the ability to dither the signal.
It's Linux, damnit! Pay no attention to renaming attempts by self-aggrandizing blowhards.
"My decision to discontinue SA was based upon a recommendation by theSecretary of Defense in coordination with the Departments of State, Transportation, Commerce, the Director of Central Intelligence, and other Executive Branch departments and Agencies."
Now the way I see it all these people would have to be involved witht eh degradation orders, so if they wanted it gone how could it have even gotten past them?? ITs just not making sense to me
Theres one problem with reflecting your reality, sometimes your reality starts to reflect you.
We're forgetting another major reason that SA is being turned off! And that is: Our own troops are using NON-MILITARY GPS recievers! Yup! Now, wouldn't that be really great when platoon A opens fire on the wrong hill 50' from where they should have... I don't know what official roles these have, but plenty of our own troops use non-military GPS units during training and other maneuvers.
}#q NO CARRIER
Or 20 years behind schedule. Ronald Reagan (2 presidents before Clinton for all you 13 year olds) asked congree to turn off SA when he was president. The DoD "claimed" at the time that they would have to orbit new satellites to do so. Bush (not that one) did the same. This doc from 1996 outlines the plan for the president to evaluate SA each year and turn it off by 2006. Bear in mind, however, that GPS is currently funded through 2006.
Si vis pacem, para bellum
The only thing more annoying than a Libertarian is an (un|mis)informed Libertarian
So now, any whacko can go down to Circuit City, pick up a Sony Super Duper Accurate GPS system, hook it up to a homemade missle(made from plans he found on the internet) and aim it directly at MY cubicle.
In the past, at least I had the comfort of knowing that there was a good chance that the entire office would be taken out..but with this kind of precision possible, I've lost all peace of mind.
D
The first, last, and only tech news site on the net
No one will need to upgrade their hardware. SA, or Selective Availability, is really just a way of statistically fuzzing the signal to make your post-reception calculations wrong.
The other interesting tidbit from Desert Storm was that because the GPS satellite constellation wasn't complete at the time, there were 2 approximately 20-minute periods per day where not enough sats. were above the horizon to allow navigation in the Gulf region.
Guess which 2 x 20 minute periods per day a given Coalition Army unit could be relied upon to be stationary?
What I find most telling about this state of affairs, though, is that even the "supplier" of navigation, who should be most intimately aware of it's limitations etc, is taken in by the technology to the exclusion of a more common sense approach. I think there's a lesson there for all potential users.
Just like it says in the manual, folks: Never rely on your GPS alone for navigation.
--
henley, who will be very nervous about flying when they finally turn off ILS in favour of (D)GPS for landing aircraft....
--
I'd rather have a bottle in front of me than a frontal lobotomy
Current GPS hardware will work as usual, but instead of the ~50-foot precision, it'll be much more precise. The press release suggests that the improved system will be ~10x more accurate, which would seem to indicate ~5-foot (1-2 meter) precision. Anyone have any more specific numbers?
-David
Now maybe my GPS-controlled car will stop driving off the side of the road!
Kevin Fox
Kevin Fox
PCS is GSM by another name, so actually there is GSM in the US. However, GSM is not very suitable for sparsely populated areas, i.e. most of the US.
The UK supposedly went metric in 1901, but has been changing so slowly that the US seems to be catching up now!
I think NTSC is older than PAL, so you can't really blame the US for having their own standard.
Now secret missle in basement is accurate enough to destroyt moose and squirrel once and for all!
I suspect this was done to try to head off anyone building a competitive GPS network. I believe the EU (which feels it needs to have a home grown alternative to every single thing developed in the US) has been planning a military precision GPS competitor system for a while now. Maybe the US gov't doesn't want that or other systems to actually happen.
It's not quite reliable enough for automatic landing of aircraft (differential GPS with ground based transmitters is necessary for that), but is good enough for MANY other applications.
I know that in a coal mine that my brother in law worked at, they had Loran navigation for the big trucks. On foggy days they could operate when they couldn't see the ground. Imagine driving up and down a mountain in a 300 tonne truck without seeing the road.
I think it's almost time for me to buy a GPS for my bike. Or maybe one that plugs in to my Palm Vx.
This is too damn cool.
I only wish it wasn't another example of a military spin-off technology. It could have been developed in the civilian arena with a little governmental support. I can understand how bitter it is for the USAF to swallow the entire cost of the constellation of satellites, only to see Radio Shack selling $100 units at good profits.
Greg
Well, according to the release, it's supposed to happen by 2006. Who knows what will be put into place before then.
kwsNI
Tracking one satellite with a GPS receiver will give you the correct time. Two will give you location. Three gives you altitude. More satellites improve accuracy. I have tracked as many as 11.
Someone finally told the president that most of the military never used the 128 bit encryption key used to give more precise location measurements. The only people who used it were the cruise missile guys. Yes, with the proper key a GPS can fly a missile into a 1m air duct. now everyone can fly a cruise missile into a 1 meter air duct. Yay. Since the average "civil use" does not require that kind of accuracy, I wonder why bother shutting down GPS encryption?
Since it was only a 128 bit key, perhaps it has already been cracked by anyone who might be considered a bad guy. I never assume the US would do anything for anything other than completely selfish reasons. Nice PR move to "out" your compromised security system worth billions of dollars for the good of mankind. Much better than to have some 13 year old kid hack your encryption algorithm and post it on the web.
Now if only the DVD consortium would realize (along with all you PGP using folks) that the goal of encrypting information "until evil no longer lurks in the hearts of men" is just a piece of fiction. Great fiction mind you (Cryptonomicon~Neal Stephenson).
Anyway, Use a map. Use a compass. Don't trust that 10 digit grid coordinate. A GPS tracking two satellites will put you 2 km inside Iraq when you should be 10 km inside Kuwait. Been there done that bought the T-shirt... ran like hell.
If voting were effective, it would be illegal by now.
ok a quick primer... The civilian channel(unencrypted) of GPS is accurate to about 30m if SA is turned off. From the very beginning of the NAVSTAR(GPS) progam the USCG saw the benefits of the program and wanted to be able to use it in a maritime setting but the DOD was adamant about being able to spoof if if nessessary so the DOT and USCG developed a way to defeat the spoofing in a localized setting (just around the coastal and inland water ways of the US) by converting their old LORAN stations (an antiquated HF doppler navigation system that was be phased out because of GPS anyway) and use them as an error correction stations. Because the precise location of these stations were known what the DOT and USCG did was simple put a reciever in the stations and then compared that to the recieved signal from GPS and then transmitted the correcting code via the communications gear already installed in the stations. The catch? it only works inside the territorial waters of the US and you have to have a special reciever to get both signals. and OH BTW DGPS is even more accurate than GPS even with the SA turned off.to see the USCG powerpoint brief on this click here (be forwarned it's large)
"I don't code the things you use, I make the code your things use better."®
Differential GPS is the way to go.
With DGPS you don't have to worry about the govt induced error. It also gets rid of errors due to weather and the atmosphere (another significant source of error).
The concept is simple - an independent station with a known, fixed position transmits its GPS readings via radio or the internet (internet, cool!). The mobile stations looks at these readings (including the all important timestamps) and uses the error observed by the fixed station, due to the above mentioned effects, to correct its own GPS readings. Using this method you can get sub-10 meter accuracy!
This best part is you can do this at any time! In other words, if you record your data you can post process it and figure out your "real" position.
There are several available GPS beacons, especially along the coast. (Here in Boston the closest one is in Chatham, off Cape Cod) As long as the GPS station is within a thousand miles or so, you'll get a pretty accurate reading.
So what exactly was the accuracy you had at the time of the gulf war compared your usualy accuracy? What kind of systems fo you use? thanks!
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(Sorry if this is a repost, this didn't post the first time)
This is awesome news. I use gps nearly every weekend, and it's always bothered me a little knowing that it was off a little bit.
Anyways, for more info here are a couple of links to gps sites:
GPS Info Website is an awesome starting point. Tons of links and info.
GPS and NMEA- A good FAQ site on gps in general and also hooking a unit up to other equipment (hand computer, autopilot, etc.).
US Coast Guard Nav Center has some good updates and status of gps.
Garmin- A well known gps maker, their site also has some good general info on gps (not just their units).
Anyways, have fun out there!
Jason
"Also, given the orbital physics of the GPS's themselves, I would think any regional control would be very course grained. "
Would it be possible for the US Military to simply jam receivers in a specific region by transmitting a strong signal on the same channel as the civilian band, while leaving the military band alone?
Have you ever noticed that at trade shows Microsoft is always the one giving away stress balls...
What you describe is call Differential GPS (or DGPS), but it's only useful over (I'd guestimate) a few tens of square miles. It only works when the two receivers can receive roughly the same GPS satellites. So it's useless in the middle of the ocean, for example, or over (I'd guess) most of the world.
// TODO: fix sig
...personally jam all GPS communication (among other things) for about $7500...
Great fun if you live out the sticks and want to sell a lot of maps...
BlackNova Traders
A DGPS unit as such only receives, for use in an inertial navigation system it works in a number of ways, mobile phones, microwaves, uhf....
Just a guess, but I'm thinking the birds simply stop transmitting when they fly over a part of the world to which the gov't wants to deny GPS service.
Admit nothing, deny everything and make counter-accusations.
Finally. So what exactly is the actual accuracy for consumer GPS now that SA has been turned off ? I've heard an average number of about 20 meters (versus 100 with SA turned on). This still doesn't match the accuracy of differential GPS (about 10 meters), or does it ?
I'd think that Netional Security concerns almost require that civillians find GPS to be the "best" positioning solution available. We want as many potential enemies to be using GPS as possible. Imagine if SCUDs used GPS navigation. GPS signals can be turned off / altered to suit US millitary needs. If GPS is not competitive, and potential adversaries use a competing system, we lose the ability to pull the rug out from under them.
Simple. They just turn off the civilian signal (or add back in the signal degradation to whatever degree they desire), but only for the satellites involved in that region.
So if any enemies of the United States decide to do something we don't like, we just disable civilian GPS wherever they're operating, and they can't use it. This is especially useful if we've sold GPS-guided missiles to someone that we liked yesterday.
Now if the Cold War were still on, the USSR would put up their own GPS system, so you could buy devices that would use whichever one was operating in the area.