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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."
The GPS measurements are so accurate that if you were to embed GPS receivers into playing cards, it would be possible for players to know whether the house has a blackjack, because the position of each card would be known to within one inch. Hence the name.
Hmm. I thought I covered that when I said "simultaneously." The term "channels" usually refers to the number of code generator/correlator pairs the receiver has and thus represents the maxinum number of SVs that can be tracked or searched for at one time. But yes, it's easy and quite common to have one channel bouncing between all of the visible SVs.
What's really ironic is that the first GPS satellites and receivers were built by "Rockwell" - Rockwell International to be precise.
If you need a more standard measurement than yards, 22 yards is roughly a fifth of a football field.
Pretty please?
"Dogs and cats, living together...it's mass hysteria!"
Now play a bunch of tricks involving using all the recorded signals. Take for instance, the signal (lets say a sine wave) from one statellite recorded by base station (B) and the moving platform (A). The cycle sequence (e.g., sine wave 1, 2, 3,...) will be something like:
B: 0,1,2,3,4,5,6,7
A: 0,1,2,2,3,4,5,5
From this one can infer that (A) is moving faster towards in the general direction of the satellite than (B). Hmmm, this sounds like it will yield a pretty precise tool.
This is a very very simplified description of kinematic GPS, something that has been in use for about a decade.
The real missing link is here. If nothing else, this /. article gave me a few minutes amusement looking for the "missing link". Now where are those damn dirty apes?
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I agree, I hate uniformed authors. I much prefer writers who are casually dressed.
Mod down posts with a "Free Mac Mini/iPod" sig, they're spam!
Ok, I don't get it. How does 1-meter accuracy versus 10 or 20 meters make ground troops suceptable to air assaults? It seems to me that if you pin them down to within 20 meters, that's about all you need to do before the bombs start dropping.
Mod down posts with a "Free Mac Mini/iPod" sig, they're spam!
Err, thing is, US gov't doesn't scramble gps anymore. My cheapo garmin eMap usually gives me aboug 12 foot accuracy, more if I can get more satillites. Besides that.. is 12 feet going to matter a whole hell of a lot when you're talking nuclear weapons? You have to miss your target by a whole heck of a lot when your blast radius is, oh say.. a city?
USCG differential bacon...
Pope
Freedom is Slavery! Ignorance is Strength! Monopolies offer Choice!
It doesn't mean much now, it's built for the future.
Active ground troop tracking.
Nerd rage is the funniest rage.
I think you're confusing the encryption of high-precision signals with the added noise in the civilian signals. They're two different things.
The military's highly accurate signals are still encrypted, but the consumer-level signals aren't being intentionally degraded anymore (but they're still less accurate).
I'd thought that consumer GPS systems were deliberately detuned so they couldn't be used in a military capacity...
Cretin - a powerful and flexible CD reencoder
You need a GPS to find it =)
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ping -f 255.255.255.255 # if only
Automated lawnmowers have been commercially available for 30 years. Most use a buried wire to detect the edge of the lawn. Some mow randomly within those limits, while others follow the wire for the first cut and use grass-edge detectors to spiral inward. Do a web search for "robot lawnmower" to find the Mowbot, Lawn Nibbler, Robomow, and others.
Signals reflected off the oceans would travel further than direct signals. By knowing where the satellite really is, perhaps they estimate the delay and can use the reflected signal as if it is an extra satellite. Whatever they're using the reflected signals for, it's an interesting idea -- unfortunately only something above ground level can use it.
In that case, maybe it is a good comparison. Note that the chap got the two the wrong way round ;)
.|` Clouds cross the black moonlight,
~Tim
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~Tim
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Rushing on down to the circle of the turn
A little more than 21 metres.
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Am I the only one who thinks Microsoft is a misnomer? Perhaps Macrosoft would be a better fit?
I can't vouch for the claims on the accuracy for these receivers, but they work differently in orbit than on the ground. On the ground, you're looking up to three or four GPS satellites for triangulation to determine your position. In orbit, the satellite has to look up, to the sides or down (depending on the altitude) to the GPS satellites. When you can only triangulate with ones to the sides, accurately pinpointing your position in three dimensions is quite a bit harder.
This is something I learned about these receivers during my work with the CHAMP satellite, the other satellite mentioned in the article.
BigFig
a search on google seems to point to this release. Just a random guess.
Think Sharper Image. You will NEVER lose your car keys ever again!
The US government scrambles GPS signals for a reason, you know
Wrong: On 31.05.2000 the so calles SA (Selectivy Avaibilty) was shut down on advice of US President Clinton. It's an amazing accuracy since then. My GPS computes the estimated position error between 4 and 7 Meter...:-)
Second point is: No civilian GPS receiver will work at speeds above ~900 Km/h, just too slow for nukes....
Michael
Opps, forgot this link if someone likes further info:
Michael
Debunking time, in general terms.
In my experience (non-trivial, actually) the current state of autonomous civil (L1, CA-code phase) positioning provides an accruace of about 6 meters 1d RMS (horizontal) and about 8m 1d RMS (vertical). Incorporation of a decent Differential GPS receiver and update system allows for improvements of about 1 order of magnitude (0.6m (H), 0.8m (V)).
The last time I checked, 60 cm was on the order of 24 inches... not 3 inches. Incorporation of WAAS (Wide Area Augmentation System) and eventually LAAS (Local Area Augmentation System) into thepicture will provide similar levels of improvement. but will NOT get you to the 1 cm level, reliably.
Geodetic positioning, where accuracies at or below the centimeter level are desired/required, require differencing the results from paired observations (2 receivers, one on a known monument) over a significant period of time (my current research, as well as that of the National Geodetic Survey (in work) have demonstrated observation times of 4 hours minimum, while I'm recommending a 50% increase in that duration while we're near the peak of the current sun spot cycle, because of the increased, and somewhat variable scintillation of the ionosphere.
Post-processing of the difference data result in a statistical answer for a baseline offset between the two sites. When incorporated with multiple observing sessions, a network of these baselines is developed, and the position of the point of interest is resolved with reference to the known, fixed and well-surveyed monuments, usually using the technique of least squares.
Where was I? Oh, yes... Autonomour hand-held concumer-grade receivers are generally good, now that Selective Availability has been removed, for accuracies on the order of 6-10 meters, with most degradation due to either the inadequacies of the receiver (increased processing noise, inadequate internal clock stability, poor algorithm implementation, failure to comply with IDC-GPS-200) or because of antenna anomalies. If you look at the error budgets for the GPS system, ignoring the problems associated with the now-defunct Selective Availability, a pessimist would conclude that your best hope for accuracy should be on the order of 29 meters (horizontal) for the Coarse Acquisition-Code solution, and 17 meters for the Precise Positioning Service code solution. That the results are better than that are a tribute to the system designers in the 1970s who dreamed this all up and started implementing it, as well as the current stable of designers and implementers (satellites, Military, and receiver manufacturers) who keep up with the system.
Never ascribe to malice that which can adequately be explained by tenure.
As long as there is encryption strong enough to protect access to those satellites we should be okay. Having worked with military grade GPS units in the past, I know just how accurate they are. This latest advancement makes them even more so. I would not presume it is for nuclear weapons protection, but rather the protection of ground troops. Ground troops are very susceptible to air assaults if their exact position can be pinpointed. This is the governments interest in keeping such technology out of the wrong hands.
Russian Russian Russian RussianDollSig DollSig DollSig DollSig
From the article... "With SA discontinued, GPS users should see an improvement in position accuracy from approximately 100 meters to 10-20 meters. Any GPS user requiring better than 10 meter accuracy should continue to use Differential GPS (DGPS) corrections to remove the longer term errors in the system such as orbit and satellite clock errors, atmospherics, multipath and receiver noise. " What this effectively says is that you can now get accuracy of up to 10-20 meters. Military grade equipment can effectively locate a target of 1 square meter(12 digit grid?) and has been able to do so for quite some time. They are NOT and WILL NOT be releasing this ability to the public for general availability any time soon. Such technology has the potential to compromise the security of ground troops to air assaults. The military will never allow this to happen.
Russian Russian Russian RussianDollSig DollSig DollSig DollSig
http://www.jpl.nasa.gov/releases/2000/blackjackgps .html
What is 22yards in metres?
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Finally i'll be able to locate that backpack that i dropped in the Maryanas trench...
I am !amused.
I thought that GPS units (or possibly the satellites themselves) were made to be just that little bit off so someone couldn't just borrow the electronics and hook them up to a missile for precision as they blow something to kingdom come. I'd hate for some terrorist to suddenly feel they had a round-about $100 missile targetting system.
.--bagel--.---------------.
| aim: | bagel is back |
| icq: | 158450 |
( o ) one could say I'm rather baked
That would be a song by "Rockwell", if I remember correctly.
Of course, I could Google it to be sure, but I'm too lazy.
It's interesting to note the real reasons behind Clinton's decision to eliminate SA.
There are two main reasons. The first is that he eliminated SA just before an ITU meeting at which there was an intention to give out to the US Government for using radio bandwidth for such a service and not providing the full benefit of the service for the citizens of Earth. There was the potential that the ITU was going to re-allocate the frequencies that GPS uses if the US government did not open up the service. And the second is that the E.U. is planning and will (eventually) launch it's own GPS system which will be compatible with the U.S. version and which will have no degradation of service whatsoever.
The bottom line is that Joe Public can expect continual improvement in the satellite positioning services until some sort of theoretical limit is reached
Is there any use for this in the consumer market? Like the blurb says, I have noticed GPS readings to be off 30 foot or so within a 15 minute period. This is pretty close considering where the transmitters are. But, will this make navigation systems in cars more reliable.
I don't 30 ft making too much difference in the middle of an ocean, but it does when chosing the right street in a town or city.
Yep, I never spell check.
More incorrect spellings can be found he
I didn't find one useful bit of information at all in that article - the author was uniformed and I disagree strongly with his assumptions. I feel that his background in the material in question was lacking and his conclusions were unsubstantiated. So there -
The strength of BlackJack seems to be the real-time accuracy. Regular GPS has to sit in one spot for a long time to get an accurate measurement, and real-time kinematic (RTK) is only differential as I understand it. Meaning, it is accurate and real-time relative to another GPS receiver but not relative to the globe.
Maybe the NASA people are planning to place small gps transmitters in all the aces and jacks in a casino, then with this system, they will be able to tell if they or their fellow players have black jack.
Look for NASA to have a large increase of funding soon.
Finally Lawnmowers and Farmtractors that will drive themselves!
This will be sweet because you will only need a 10 inch wide lawnmower. So what if it takes 8 hours to cut the entire lawn. I'll be inside on the computer.
:-)
I have my doubts about using this technology to guide cars on the road, but with some visual recognition devices for collision avoidance, who knows?
I have no idea why people keep talking about GPS accuracy measured in yards. GPS accuracy can be measured in millimeters using freely (as in no government restrictions) available civilian technology. I work in the civil engineering field where GPS would be a logical non-military application. If GPS had a measure of accuracy measured in yards it would be nearly useless in our line of work, yet it is routinely used on more and more projects.
Here is a link to the specs to just one of many civilian GPS surveying receivers where you can see that post-processed measurements can be accurate down to 5mm and real-time kinematic measurements down to around 1cm horizontal And 2 cm vertical.
http://www.sokkia.com/Products/Radianspecs.htm
Great, now squirrels can remember EXACTLY where they buried thier nuts....
I used to hate computers, but then a server went down on me.
Sorry, it's here: http://www.spaceflightnow.com/news/n0007/21jplcham p/
I found a related article here about the launch of the german satellite with some info about blackjack: http://www.spaceflightnow.com/news/n0007/21jplcham p/
A more accurate system is currently being produced (and launched, I think), something that will have much better accuracy than GPS. Frankly, GPS sucks when it comes to altitude control, so until this is solved aircraft cannot be guided solely by satellite and definitely cannot make a precision landing relying on GPS.
www.eissq.com/BandP.html Ball and Plate System. Amuse your friends. Crush your enemies.
I was able to locate myself in my chair within 6 inches of my desk. Utilizing a slightly modified digital mirror, I could refine that measurement to within 15.43cm. Damn impressive if I say so myself. Mirrors avaialble at the above address.
Busy aligning my non-linear thoughts.
Would it be useful to have something like this installed in all comercial aircraft? I'm thinking along the lines of better navigation aids to landing in inclement weather, to better collision avoidance systems, and to help avoid hitting the terrain under poor weather conditions.
Another use would be to mount receivers in each of the wingtips, the nose cone, and the tail and to record all this information in the "black box". This would permit a much-more detailed reconstruction of the plane's orientation and flight trajectory when investigating a crash.
Anyone else have some practical applications for these?
until the DOD removes the encryption on GPS
I thought they already did, what was all that news about a while back?
Sneakemail is to spam filters what an ounce of prevention is to a pound of cure.
The military can turn on the GPS scrambling at a moments notice if it finds a missile on flight. More interestingly, what if they delibretly introduce errors to the signals such the they control the trajectory of the missile!
This GPS receiver can pinpoint the satellite it's on to within an inch?
It seems to me that finding the satellite with upward looking radar would pinpoint it to within a few multiples of the radar wavelength....hrm...
Um, I'm not sure you realize this, but it's a lot cheaper and easier (at least in the US) to walk up to aforementioned civilian target and stick a homemade bomb in the mailbox. Why bother with a very expensive missile?
And now, instead of fruitlessly trying to remove myself from a spammer's mailing list, I can just physically locate him and beat him like a redheaded stepchild, no?
Seriously though, I hope you're not serious. A system like that would cause way more shit than it'd be worth.
Without the pad, it's not Dance Dance Revolution, it's Listen
Does a nuclear warhead really hurt less if it goes of 22 yards off target?
Not too many people know that the current GPS systems DO allow better resolution. Up to a few millimeters. I know this for a fact because I use GPS a lot here in the Marine Corps. You do however need a code to input into your GPS receiver. If you don't have the code, then there is a forced error put into the transmission. That's why civilian receivers don't get past 20meter accuracy.
Zarvox
I have a feeling that after the last skirmish, that the noise has not been turned back on.
anyone who actually knows want to comment?
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.
After all, the GPS system was originally developed for the Military and was never meant to be able to be used to navigate your car between tall buildings in a city (Hell, You would more than likely loose the sat signal while trying to use it for this purpose anyway, let alone the bullshit of winning blackjack :-) )
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We did the same thing for the NASA article that we are supposed to be talking about.
a/s/l here. Sorry, adding domain tags to your s
When you clicked on that article to read it, that 80's tune fired up. You know the one..."I always feel like somebody's watching me..."
a/s/l here. Sorry, adding domain tags to your s
The GPS system is actually much more accurate then 22 yards. However the only ones that can harness such accuracy are the military. Noise is intentionally added to the signal to decrease the accuracy for civilian receivers. Military receivers know of and account for the intentional noise and thus remain supprisingly accurate.
This site has an informative (albiet simplified) tutorial on the workings of the GPS system.
Precision GPS isn't really that big a deal anymore.
See my journal, I write things there
I read an article in the New Yorker a few weeks ago that said that the govt puts an error into the gps signal to make it less accurate for all those who don't have the decoder. This is intended to make the US info from gps more accurate than any commercial info, or more importantly, enemy info.
without putting in too much effort to find out my self, how accurate are these two standards? My understanding was that govt decoded GPS is accurate to about a meter, while the commercially available stuff is accurate to about 3 meters. This article says "a typical GPS receiver can determine its position to about 22 yards."
Which one's right?
I am a sig.
"The good thing about Alzheimer's is that you can hide your own Easter eggs."
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- Gov. Jesse Ventura
If you are lost and have a handheld gps and say cant figure out where you need to get unless you are within an inch of it I will come hit you over the head with your piece of junk. I'm sure there are practical "big brother" type uses, but one in my car or on my watch isn't one of them.
It's a shame that the military gets all kinds of groovy toys before any of us normal folk get a chance to play with them.... The shameful thing is that the only guaranteed manner of getting funding for any project is to get the military interested in it.
-- "If evolution is outlawed, only outlaws will evolve." -- Jello Biafra
It's really better to have these terrorist shoot more missile in order to be accurate that their target is down, and its more for tracking part of the device (not for missile, but for privacy) that the government add noise to the signal.
"We are not what you would call... normal"
I always tought that standard gps were modified to keep that "innacurate" level, but that us army didn't had to deal with such trouble, and that they were able to pin-point target withing 3 feet radius... or maybe its only me that watch too much movies.
"We are not what you would call... normal"
I thought they were using a new system in stead of GPS for army uses. the new system could come in handy when I try to find my house after a night in the pub.... :)
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 20 metres, the BlackJack can pinpoint the position of its host satellite continuously an accuracy of about 2.5cm, 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."
Let's see, two military uses for this technology: "...BlackJack can pinpoint the position of its host satellite continuously with an accuracy of 2 to 3 centimeters..." 1. Satellite warfare would be one. That whole "Star Wars" deal the American government is excited about (and committed to). "The BlackJacks are also equipped with small down-looking antennas to attempt to receive GPS signals that reflect off the oceans." 2. Sounds like a great way to pin-point the location of "enemy" submarines too. Get your "launch codes" and get tagged by a space-ear. http://www.jpl.nasa.gov/releases/2000/blackjackgps .html
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.
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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...
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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.
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.
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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 .
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 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.
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.
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Give me liberty or give me something of equal or lesser value from your glossy 32-page catalog.
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...
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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.
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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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