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Blackjack: Ultra-Accurate GPS Measurement
Conrad_Bombora writes: "NASA's Blackjack Global Positioning System (GPS) receiver, flying on the Argentine satellite SAC-C provides a new way to study Earth's gravity field and atmosphere. The Blackjack looks at how the radio signals from the constellation of GPS satellites are distorted or delayed along their way. While a typical GPS receiver can determine its position to about 22 yards, the BlackJack can pinpoint the position of its host satellite continuously an accuracy of about one inch, and can be used for a variety of Earth studies." The paragraph I find most interesting says "the BlackJacks are also equipped with small down-looking antennas to attempt to receive GPS signals that reflect off the oceans."
Actually, when I still worked at a major GPS manufacturer, there was a lot of evidence both simulated and theoretical that showed that with the advances in frequency standards and digital filtering that C/A code was only slightly less accurate than P code. What P code really bought you was 2 frequencies which let you do direct measurment of ionospheric interference. The ionosphere acts like a speedbump to the GPS signal, delaying it by a tiny fraction of a second depending on how charged up it is at the time. Single channel GPSs use a mathematical model to predict isosphere conditions. Dual channel receivers can measure this directly because the two bands are effected differently - like light through a prism. Unfortunately, they only put C/A code on one frequency so outside of the military we're stuck with one frequency. It turns out that after satellite geometry and SA, the ionosphere is the next biggest source of error. When we turned off SA and the ionosphere errors in a simulator, the C/A-only fixes were within centimeters of the P-code fixes.
I just realized that I used channel and freqency in an sloppy manner and I'm too lazy to got back and correct it all. GPS has two frequencies, called L1 and L2 they are at approx 1.2 and 1.5GHz and all the satellites broadcast on those two frequencies. Many receivers will say they have 5 or 12 channels - what they mean is that they can track 5 or 12 satellites simultaneously.
Throw in an altimeter/barometer and transceiver, and you have the ultimate tracking device. Sure sure, now we can all bitch and moan about evil gov't uses of this... but why not more interesting things, such as putting one on each player in a paintball game? You could then uplink all of the data to a UT/Quake3 server, and people could watch the paintball matches over the internet. Make it lazer tag for even more data aquisition...
--Cycon
Course, i would actually expect to see something like this in the head of a missle, for remote steering...
Your Brain + EEG + LEGO Robots = Brainstorms
Apropos GPS accuracy degradation...
does it really matter if I fly through the front door of the targeted embasy, or the front window on the left?
Nope it doesn't make much difference which window of the Chinese embassy you fly your bomb through, it'll still be the wrong target!
Back on topic though, whilst it doesn't make much difference when hitting an embassy, 22 yards does make a difference if you are trying to kill a hard target such an ICBM silo or a nuclear-resistant bunker. When attacking such targets, you need pinpoint accuracy.
In practice it probably doesn't matter too much, even in the Gulf War (1991) the US was playing around with using an optical recognition system in conjunction with GPS. In those circumstances, the GPS gets you in the proximity of the target and the optical recognition system can drive you through the keyhole.....
Donte Alistair Anderson Roberts - hi son!
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The really accurate GPS requires the use of both GPS signals- commercial handheld recievers use just the C/A (Coarse acquisition) code, which was there just to give an approximate fix to feed to the P(Y) code engine, which is where the true military accuracy comes from. I suspect that these satellites use the encrypted P code (the Y code) that is reserved for government use. This must be a non-run of the mill receiver, since commercial recievers have altitude and velocity restrictions (50,000 ft and 999 mph, I think, but I'm not really sure at all). Since they have gone to all that effort, I'm sure they took the extra care to get a full military grade, dual channel, P code receiver.
Your normal handheld receiver uses the C/A code, which only uses 1 MHz of bandwidth, which limits the possible accuracy of the position solution. the P(Y) code is 10 MHz wide, and broadcast on two different frequencies. I don't think that the technology used in these satellites will have much effect on commercial receivers, until the DOD removes the encryption on GPS, which they are *very* adamant about not doing.
Excellent. Now we can find out exactly where Bill Gates is for the orbital cream pie drop.
End of lesson. You may press the button.
The receivers use a combination of C/A code (the coarse code that inexpensive receivers use to get you within about 10M) and processing of the GPS carrier wave itself to perform measurements to that accuracy.
Five to Ten years ago you would set up two of these recievers recording GPS measurements and let each run for 15 miniutes or more, then process the data sets against each other to determine the relative positions of the two antennas. Then advances in computing the position for each epoch of satellite data recieved allowed one reciever to be mobile during data collection, only stopping to increase accuracy for each unknown point. Combine this with a radio transmitting the stationary receiver satellite data and a mobile processor powerful enough to do all the fun matrix math involved and you have a Real Time (within a second or so) Kinematic (moving) Survey Grade GPS system. Costs you about $40,000 or so.
One reason these aren't useful to Saddam is the fact that high dynamic situations (like an ICBM) break the entire system, from the C/A solution to the carrier wave processing.
If you want to know more, here is one article that goes beyond the basics of GPS positioning.
Oh, and to respond to some of the people on /., the military USED to introduce error (called Selective Availability) into the C/A code, reducing the accuracy of the measurements from a single GPS reciever from about 10m to 75m or more. That introduction of error has been turned off, though it can be turned back on in case of a national emergency. The military also has an encrypted transmission from GPS satellites called P code, it achieves a higher level of accuracy (with military recievers that can decrypt it) than the C/A code does without SA. Using two recievers (or a reciever and a differential correction signal such as that from the Coast Guard) narrows the error down to around 1m, the differrence is made up by errors introduced by the ionosphere, other atmospheric variables and the internal accuracy of the reciever clock itself. FWIW, GPS recievers are being used to measure atmospheric water vapor content, to aid in weather models and prediction.
Bleh!
The official ratings given for civilian GPS accuracy are very conservative.
My own experiments with plotting several thousand fixes on the same location showed that 99.9% of the time they fell within a circle of 6m. Other people trying this experiment with longer periods reported that the points fall within 2.5m of the mean 50% of the time, and 7m 95% of the time. With the USCG differential beacon they get within 1.6m 50% of the time and 4.2m 95% of the time.
I've worked with units from SatLoc and Racal that have sub 2m and sub meter accuracy from their own private satellite differential broadcast. The subscription fees are high though. You can also set up your own differential broadcast station for not too much money and get highly repeatable fixes if you aren't within the range of the USCG broadcasts.
Supposedly you can take these survey grade units and get to within 1cm by averaging out the fixes over 24 hours. It does take time though. The reason my own experiments seemed better than the people who took more fixes is that if you average over a short time, say ten or fifteen minutes, the position probably won't change much; every so often the fix slews to a new area a couple meters away and stays put around there for some time. This may have something to do with satellites coming up over the horizon. In any case, a long term averaging is needed to improve precision to what is being reported in the article.
By the way my military acquatences say that it's hard even for them to get their hands on the real good GPS stuff -- they end up buying commercial equipment. We've had some military guys look at the Racal unit despite having a subscription fee of several thousand dollars.
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First one I found, on NASA's site.
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