Slashdot Mirror
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."
There are actually two high-precision codes. P-Code and Y-Code. P-Code resides at a lower frequency than Y-Code.
So in a ultra-high precision military receiver, the gear can acquire both signals from a single satellite. Since both signals were sent at the same time, they should be identical. But due to the atmosphere acting like a prism, the two frequencies will take different length paths to the receiver. This allows the receiver to compute and semi-cancel-out the atmospheric effects.
Why on earth is the DOD so paranoid of the accuracy of commercial GPS recievers? If I launch an ICBM (or even a piddly small conventional missile), does it really matter if I fly through the front door of the targeted embasy, or the front window on the left? When talking about these things, I think that someone forgot to take into consideration the relative proportions of the situation. just my 0.02.
Also, the military service (PPS) has two frequencies to work with, while civilians can only decode one. This gives at least 5 meters of better accuracy.
See this article for more details.
The cure of the ills of Democracy is more Democracy.
Erlang Developer and podcaster
The better survey grade recievers also use the two different frequency carrier waves to get faster position solutions.
There was a suggestion to start including a third GPS frequency for civilian use on the newer sattelites to improve accuracy. Imagine a few extra carefully selected frequencies built into the system, a single low cost reciever could easily get sub meter precision.
Bleh!
Sub-cm accuracy is useful to know if you're traversing a mined area, too. I wonder if sappers (combat engineers) are using this level of accuracy for mine laying and pathfinding.Two things I'm hopeful about for easier minefield clearing: (1)We'll see a future where emplaced mines can "chirp" their locations (hmm, what if the OPFOR can hack the chirps though?) and (2) we'll have accurate mine-placement maps so little kids won't become amputees 15 years after the fact.
"How many light bulbs does it take to change a person?" --BMcC-->
"comparing the BlackJack to an ordinary GPS receiver is like comparing a home camcorder to a professional studio camera"
This isn't such a good analogy because home camcorders now have exceptional quality and often surpass the performance of professional gear that is just a few years old...
----
Stop worrying about the risks of nuclear power and start worrying about the risks of not using nuclear power.
First of all, if by "Scrambling", you meant "Selective availability", that was turned OFF this year.
Second of all, the Us Department of Commerce requires that an "exportable" GPS receiver cannot be used to guide missiles. At Trimble, we turn off all GPS output when the receiver is traveling faster then 1000 knots, or when it is above 18,000 meters.
Third of all, you can't get better then meter accuracy when you're operating without a base-station or some kind of correction mechanism in place. RTK Fixed, which gives Milimeter accuracy, needs about a minute or so to initalize with the base station to remove ambiguities, and has to be within 10 Km's to start initalizing.
And finally, the antennas that you would need for good reception of a satellite signal would tend to throw off the trajectory of a missile!
Jason Eager
(Works on MS860 GPS Receiver firmware at Trimble).
The difference is important if you are attacking hardened targets (silos, bunkers, bridges) with conventional warheads.
Mea navis aericumbens anguillis abundat
They integrate thousands of measurements at time.
This way they observe slight non-earthquake movements
of mountains, volcanoes, water soaked land, ocean currents, etc.
For example, the north LA mountains continued to
move for months after the Northridge quake, this determined by GPS.
One scientists "noise" is a another's signal.
Note they are using reflected GPS to measure ocean
heights. Normally multi-pathed GPS is considered
error noise. People have also used slight GPS
signal delays caused by ionospheric charge to
map the daily thickness and hieght of the ionosphere
in an economic manner. Normally this charge causes
GPS position in accuracies of a few meters.
100M is generally "good enough" for the average joe in his boat, who uses GPS as an aid to visual navigation. Hell it is even good enough to assist in flying an aircraft.
:-).
Yes, but I wouldn't want to be flying with you when your GPS receiver says you are less than 100M above the ground....
Also GPS vertical accuracy is less than its horizontal precision due to satellite geometry; 100M horizontal accuracy translates into about 140M (100M * root 2) vertical accuracy.
Normally the signal degradation can be significantly improved by using one of the many forms of differential GPS, where a receiver in a known local location transmits error correction data to your unit. IIRC, such units are accurate to less than 1M.
Donte Alistair Anderson Roberts - hi son!
Karma: Chameleon
In an otherwise extremely informative comment, Russ Steffen stated:
Many receivers will say they have 5 or 12 channels - what they mean is that they can track 5 or 12 satellites simultaneously.
This comment is not quite true; systems with only 5 channels can track as many satellites as you like by using one or more of the channels on a time sharing basis. Normally a 5 channel receiver will use 4 channels to track the 'best' constellation of four satellites on a continous basis, whilst the other channel is used to multiplex between the other satellites to reduce seek time in the event it has to switch one of the best ones out of the constellation it's using. Other time multiplexing algorithms are also used.
Donte Alistair Anderson Roberts - hi son!
Karma: Chameleon
Beeline, Integranautics, Modular Mining and Trimble Navigation already have self-navigating farm tractors and mining trucks.
These systems will to my knowledge measure their positions to within 10 cm horizontally and within 30 cm vertically. This is done using differential GPS (DGPS).
Selective availability has been turned off. Using only instantaneous measurements from a constellation the expected accuracy is on the order of 10 m today.
Differential GPS nulls out atmospheric errors by 'knowing' a fixed position and sending CMR (corrective measurement records) to other systems nearby. In dynamic situations (the tractor is moving) you use RTK (real-time kinematic) data to update the system.
Most of the self-navigating systems have inertial measurement as a patch method while the tractors are under trees or bridges.
This works and you can see it here .
Notice that many more such calculations are done each second by "flight simulator" software, and such software has been running on desktop PCs since the time when 40MHz was a fast machine.
Making each plane broadcast its info in a reliable way and allowing it to be used is what the FAA is presently studying. Its simplest use is for collision avoidance -- pointing out a problem to the human pilot who can decide what to do. Automatic flight control is more complicated at several levels (ATC interface, quality assurance, geographic and airspace restrictions, regulatory, and avionics interfaces).
Try the CHAMP satellite site and the CHAMP Systems page has a link to the GPS system. The reflected GPS signals are used for additional GPS altitude info.
Maybe they'll be able to find out if the Bermuda Triangle really does have quirks...
LOAD "SIG",8,1
LOADING...
READY.
RUN
Close only counts in horseshoes and thermonuclear warfare. When you're dealing with unhardened civilian targets and nuclear weapons, it doesn't matter if you're 5 metres or 500 metres off. Inertial guidance is probably good enough, let alone current civilian GPS.
"Gee, good thing that ICBM didn't have GPS. If it had been any closer, it would have hit and killed us... Why am I glowing?"
"Gosh, I'm glad that Anthrax bomb landed in Ned's backyard instead of mine. I wonder which way the wind is blowing..."
GPS actually would be useful for terrorist cruise missiles, but nobody else (other than our allies) really has the technology for that. On the other hand, good GPS could allow for us to finally realize the fantasy of flying cars, since it would be good enough for automatic landing...
I think you're confusing commerical with a commerical unit that's fed with a differential signal (DGPS). This utilizes a known fixed location to offest the error in the GPS signal, however, it requires a fixed known station. This can get pretty good - better than 3 meters. The commercial units you get from Garmin or whatever are good to 10-15 meters, although the altitude measurements are almost worthless in most cases. So, 10-15M is on par with 22 yards (that's maybe a little optimistic).
But, 10M is pretty good!
..don't panic
I thought they already did, what was all that news about a while back?
They removed the "SA" or selective availability coding on the C/A code. The SA code was the intentional in-accuracy in the GPS clocks that made the C/A position solutions less accurate. Without SA, GPS is much more accurate than it was, but still nowhere near the level of P (Precise) code (which is inherently 10 times more accurate, since it has 10 times more bandwidth) There are a bunch of different ways to make GPS more accurate, like looking at the phase of the incoming signal, or using differential, or just averaging, though differential or averaging don't work as well when the reciever is moving at satellite speeds.
I'm not sure that equipping all aircraft with a GPS system could qualify them for IFR (Instrument Flight Rules, which means that in theory, you could fly the aircraft without looking out the window).
If you stuck a GPS system into a Cessna 150, for example, you'd still need to equip it with a radar, ILS and a number of other devices to have it classified for IFR. On some of these planes, there's either no space or no money to do this.
Also, the GPS alone was to be used to avoid collsions, the data received/sent would have to be processed in real-time and then sent either to the controls or to the pilot, before the collision occured. In a one-on- one situation, this is simple. Around Heathrow, things get a little complicated. I'm not sure that a central/distributed system could actually solve this fast enough.
The bottom line is that it won't be the savior of aviation. It will only be a tool that will help to navigate VFR planes.
bart
Most altimeters based on barometers don't have a precision of 1". You're lucky if they can determine your altitude within 10 ft.
Calculating the distance from the satellites (by multiplying the speed of light by the time it took to receive the signal) would actually be more accurate. Last I heard, GPS satellites actually compensate for the time it takes to send or receive a signal up to six digits after a second (0.000001).
bart
I thought they already did, what was all that news about a while back?
No, the DOD didn't turn off the decryption on the military GPS band. All that was turned off was the noise added to the civilian band.
Tetris on drugs, NES music, and GNOME vs. KDE Bingo.
Will I retire or break 10K?
While this WAS true for a long time, the U.S. government turned off Selective Availability in May of 2000, making it possible for civilian users to get what used to be strictly military-grade positioning.
Of course, they might turn it back on in case of a "strategic conflict", as they'd say.
Is there a link to the article or is this it?
Until a link gets put up, here's Special Study Group 2.161 Report Report - Probing the Atmosphere by GPS that seems to talk about the Blackjack system.
May I correctly assume that this is strictly military-based, with no hope of ever reaching civilian hands? This seems far too neeto and spiffy for there to be any hope of the federal government letting it fall into the irresponsible hands of those who pay them...
~moofbong
If 'con' is the opposite of 'pro', what is the opposite of 'progress'?
The funny part about this is that during the Gulf War, the DOD apparently turned SA off, probably because a lot of the units deployed in the Gulf were using commercial-grade GPS receivers rather than the more expensive, less easily-available military-grade receivers. So when they were facing an actual enemy that theoretically might use GPS against them, they had to turn SA off anyway so they didn't get lost in the desert.
-
-
Give me liberty or give me something of equal or lesser value from your glossy 32-page catalog.
I agree, that's why if you take another look at the parent post:
I could very well be mistaken but I thought commercial aircraft already have IFR equipment? My question was therefore whether there were any practical applications of this BlackJack GPS technology to augment the existing IFR instruments in commercial aircraft. The expense would likely be prohibitive in the small puddle-jumper sized craft, but in the larger jets (e.g. Boeing 7x7) I would expect that to be less of an issue.
Also, the GPS alone was to be used to avoid collsions, the data received/sent would have to be processed in real-time and then sent either to the controls or to the pilot, before the collision occured. In a one-on- one situation, this is simple. Around Heathrow, things get a little complicated. I'm not sure that a central/distributed system could actually solve this fast enough.
Good point about greater congestion around Heathrow, but aren't they managing to do this right now? Samples taken every few seconds (e.g. each radar sweep) would be sufficient to plot each plane's trajectory. Further, it's not like there's a cloud of planes swarming around the airport... I don't recall the exact elevations, but (pulling numbers out of the air) some subset of the flights might be given an elevation of 15,000 feet while others might be given an elevation of 10,000 feet, etc. So, it becomes less of a problem of solving all the trajectories in 3-dimensions to solving some finite number of trajectories in 2-dimensions.
Then again, if we need to measure how far one plane is from another using inches... methinks they may be in one BIG heap of trouble already!
One of the newer technologies is SAR, synthetic aperture radar, which is mainly used to image windspeed over a body of water. Basically the way SAR works is this: The satellite transmits a beam towards the ocean surface. If the surface is smooth, it will bounce off the surface and away from the satellite. When the wind blows, the surface becomes rough, and some of the beam is bounced back at the satellite. The more that gets bounced back, the more the wind is blowing. It's a little more complicated than that, but...
As for GPS, apparently you can use GPS signals to monitor different things like this, based on how the signal is changed when it bounces off of a body of water. GPS can also be used to figure out how much moisture is in the air, etc. based on how the signal is changed from when the satellite sends to when you recieve it.
For more on SAR and Remote Sensing in general, check out some of the links on my page.
Sheepdot: Open Source good, Closed Source baaaaaaad!
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
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.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
First one I found, on NASA's site.
End of lesson. You may press the button.