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GPS Civilian Signal Degradation Turned Off 252

Posted by Hemos from the good-news-for-location-buffs dept.
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.
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GPS Civilian Signal Degradation Turned Off More

GPS Civilian Signal Degradation Turned Off

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  • 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.

  • Don't expect too much. (Score:5)

    by hey! ( 33014 ) on Monday May 01, 2000 @08:26AM (#1098892) Homepage Journal
    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.

  • 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?
    --
  • 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....
  • 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 [uscg.mil], 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 [redhorsetech.com] for information from a company that does just that, and then leases equipment that can use their signals.

    -Michael

  • 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.
  • These limitations are built into civillian receivers using the "P" signals. AFAIK which is not much, this does not exist in regular C/A recievers, however, they do have the same types of limitation inherent in the coarse signal. The DoD can grant per case permission to use the "P" signals in devices, but they will only work with the above limits. Plus there is also the "Y" signals which is addedd to the "P" for anti-spoffing capablity.

    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.

  • IIRC, you need 4 satellites to determine your position, because you are calculating your position in 3 dimensions.

  • Whoosh (Score:2)

    by p3d0 ( 42270 )
    That's the sound of the joke flying over your head.
    --
    Patrick Doyle
  • 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.

  • 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...

    ---
  • 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.
  • 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 [garmin.com]" 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.)
  • 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!
  • 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.

  • 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.
  • Not many people here seem to realize it, but out on the water extra GPS accuracy could well save lives, particularly in "man overboard" situations. It's quite disconcerting to be looking at a GPS chartplotter and watching your GPS plot your position as 100 yards or so east of a reef, when you know perfectly well you're west of it, and you see the little "SA" graphic flashing off in the corner of the GPS set.

    Thank you, whomever was responsible for this decision.

  • 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.
  • I thought they actually made the dither _worse_, to disturb the Iraqi GPS receivers?

    /* Steinar */
  • 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 [slashdot.org], 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 [batnet.com] 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.
  • 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.
  • 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.

  • Possible New Uses (Score:4)

    by jamesl ( 106902 ) on Monday May 01, 2000 @08:39AM (#1098951)
    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.


  • The URL in the announcement was a moving-target pointer to the White House Press Releases,
    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. cgi?date=2&briefing=5 , which at least tonight points to the real site
    www.igeb.gov [igeb.gov]
  • Selective availability only encouraged the development of other technologies for high accuracy positioning, so it was counterproductive to the goals of the military. By turning off selective availability, the military is doing what makes the most economic sense: getting people to rely on a resource that they control completely. In case of a conflict, SA can be turned on very quickly again.

    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 [faa.gov] 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 [www.rssi.ru] 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. [wsrcc.com]
  • Finally, I'll be able to acess the GPS for my missile targetting system to function nominally. It's been really hard to pinpoint the neighbor's cows - it's required a good deal of additional, manual aiming at the last moment. Now it'll be just click-n-steak...

    -------
    CAIMLAS

  • There already is competition... (Score:4)

    by Croatian Sensation ( 27341 ) on Monday May 01, 2000 @12:37PM (#1098968)
    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.

  • 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
  • I'm just requoting what someone said in an earlier post.. but they mentioned it was due to long range missiles performing mid-course corrections. this is the critical time for course corrections, and a small error in the missile's knowledge of it's position will mean it misses the target by a WHOLE LOT. The higher and faster it goes, the more so this is true.
    Who knows. a 1 meter error for an ICBM right before it starts it's descent might mean hundreds of miles of difference...
  • Over at one of the military gps status sites (over here [af.mil]) 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
  • 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...

    No, because to be useful, you have to broadcast the correction. Broadcasts can be scrambled or spoofed.
  • 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.

  • Re:"National Defense Concerns"? (Score:2)

    by Anonymous Coward
    There are other limitations to civilian receivers. If I remember correctly, they are programmed to stop working at speeds over Mach 2.0 (the Concorde being the fastest commercial jet) and at heights over 80,000 feet. For example, when they want to use GPS on amateur rockets (liquid fueled versus model rocketry's solid fuel), special permission is needed to enable the receiver.
  • Some recievers (like my Delorme Earthmate) can do some compensation for SA, but I think it's only good for 2d (lat and long) location. There was many a time where my receiver told me I was anywhere between -100 ft and +300 ft while walking a few yards. I'll check tonight and if there's any interst, report my findings...
  • "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!" :-)

  • already exists! (Score:3)

    by CausticPuppy ( 82139 ) on Monday May 01, 2000 @08:55AM (#1098983)
    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.
    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 /.-ers.
    But I don't know how I ever got along without it. I'm still amazed at what this little box can do.

  • Not quite.. (Score:3)

    by Fastolfe ( 1470 ) on Monday May 01, 2000 @08:56AM (#1098984)
    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.
  • Kickass! I can now find my car!


    - Stop praying for someone to save you, and save yourself.-
    KMFDM

  • So I tried it... (Score:3)

    by Polo ( 30659 ) on Monday May 01, 2000 @08:32PM (#1098989) Homepage
    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.

  • 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

  • Excellent! (Score:3)

    by Greyfox ( 87712 ) on Monday May 01, 2000 @01:12PM (#1099004) Homepage Journal
    Now Saddam can deliver that nuclear payload right to Bill Gates' doorstep rather than 50 feet to the left where it would have done no good at all!

    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.

  • Hey-

    There's some more info on the announcment at the International GPS Executive Board [igeb.gov]. 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 [igeb.gov]

    To quote:

    ...consider a football stadium. With SA activated, you really only know if you are on the field or in the stands at that football stadium; with SA switched off, you know which yard marker you are standing on.
    -Dr. Dennis G. Milbert
    Chief Geodesist


    Anyways, have fun out there.
    Jason

  • Paranoia: Cell-based location vs. GPS (Score:5)

    by billstewart ( 78916 ) on Monday May 01, 2000 @01:35PM (#1099009) Journal
    Actually the proposals were to use the cell system for location, rather than GPS, for a 125-meter (125-foot?) radius. Most digital cell technology can get the accuracy if you wrap enough coordination and processing betwen the cell sites (you might locate _them_ with DGPS, for instance.) GPS is a power hog, so you don't want to use it if you can avoid it; most GPS receivers get about 24 hours per set of batteries, while digital cell phones can last a week.


    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.....

  • 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.

  • 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.

  • You are right. Sorry, guess I read that wrong. Well, cool. Time to go out and buy me a GPS system...

    kwsNI
  • 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.

  • About GPS and signal degredation (Score:5)

    by Kartoffel ( 30238 ) on Monday May 01, 2000 @09:03AM (#1099017)
    There are two GPS signals: the regular C/A-signal and the encrypted P-signal.

    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.

  • The error randomly rotates, so the solution you mention would not work. Some GPSs would try to compensate by taking several readings from several satelites and average them out over several minutes (if one satelite says you are within this thirty foot radius and the other one says that you are within *this* thirty foot radius, you obviously must be in the space where they overlap (GPS actually works with more than just two satellites, which makes this process more exact, but still not perfect).

    It will be interesting to see how such GPSs deal with the loss of the error. Will they still try to compensate?

    I imagine it will not make too much difference. Now both satellites (or however many you get reports from) will report about the same position,and the GPS will average those.
  • As regards Desert Storm, I heard that in fact the degrading was switched off for the duration because:
    1. The Iraqi forces were known to have very few GPS units, and
    2. The US forces ran out of military units but had large supplies of civi units, so they turned it off to allow those units to be used by the men-on-the-ground for finding where the hell they were in all that sand (and darkness, since so much was at night).

    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

  • Nope, not Geosynch. A cut & paste from an About GPS [garmin.com] page reveals:

    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

  • Its dangerous to build anything that relies on GPS to the point where if the service failed, you could be seriously injured. So if that telephone pole suddenly dissapears you don't want to go crashing into it. As the press release said, they still reserve the right to turn back on SA, and completly turn off the signal for certain regions.

    And btw they already have a way of seeing the puck in hockey, where have you been?

  • Re: Broadcasting (Score:3)

    by hey! ( 33014 ) on Monday May 01, 2000 @10:06AM (#1099023) Homepage Journal
    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.
  • I believe that this works by combining two GPSs into one and cross-referencing the data -- the average is likely to be closer to your actual location than either device will independently indicate.

    Thankfully, this is no longer necessary.

    -Waldo
  • Wow, the Whitehouse didn't just put out a press relase...they did something too. SA is off!

    Heres a graph [mehaffey.com] 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

  • All GPS satellites have carried an EMP sensor on them that can locate nuclear detonations. More info can be found here [fas.org]

  • Re:About GPS and signal degredation (Score:4)

    by Eil ( 82413 ) on Monday May 01, 2000 @02:32PM (#1099040) Homepage Journal
    People who read these ./ threads frequently might recognize me as the guy who works on military aircraft avionics. Therefore, I have some knowledge in this area, so I'm just going to try and add my knowledge to this topic.

    The GPS reciever that I'm familiar with is the one in the Enhanced Navigation System in the MH-53J Pavelow III helicopter. It can use both "civilian" and "military" GPS signal format. The unencrypted singal has an error radius of about 100 meters. With the proper encryption keys (our manuals don't state what kind of encryption or what the key format is, as we're not the ones who enter them), the GPS signal receieved is accurate to a radius of 15 meters. You need to have at least 3 satellite signals captured by the GPS receiver to get present position, while you'll have the *most* accurate coordinates with 5 satellites captured. The GPS recievers most people think of can fit in the palm of your hand, but on aircraft, they are about as long and wide as a PC keyboard and around 10" high. AFAIK, GPS works on the principle of triangulation of radio signals... i.e. measuring the angles of incoming radio signals. The encryption keys are changed something like every 48 hours or every week.

    I'm deeply interested in learning more about this, but I'm afraid they just don't tell us any more than we need to know. :P Maybe I'll get one of my ComNav friends to help me out. I will take a look tomorrow at some of our tech manuals and see if there's anything I missed.
  • 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

    "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 /. a few monthes ago.
    "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. "

  • Re:current hardware _will_ work (Score:3)

    by Anonymous Coward on Monday May 01, 2000 @09:10AM (#1099043)
    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

  • USSR Equivilant = GLONASS (Score:5)

    by DJGreg ( 28663 ) on Monday May 01, 2000 @09:10AM (#1099044)

    The former USSR already has an equivilant to American GPS. The GLONASS [www.rssi.ru] 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.. :)

  • 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 [nasa.gov]. 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
  • If I remember correctly and am not just completely full of it:

    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. )

  • Age of transparency (Score:5)

    by Aravaipa ( 30801 ) on Monday May 01, 2000 @09:18AM (#1099048)
    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.
  • What worries me is that the US Military said they'd turn OFF the dither in times of war.

    I know that Cuban kid caused a bit of a political stink, but this seems like a bit of an over-reaction.

  • 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
  • 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
  • Yes, it's reliable enough to fly an aircraft by... a Stanford team demonstrated [stanford.edu] this with a model aircraft circa 1996. And later, they encored by driving a tractor [integrinautics.com] around...


    engineers never lie; we just approximate the truth.

  • Graph showing SA being switched off (Score:3)

    by jdesbonnet ( 22 ) on Tuesday May 02, 2000 @12:42AM (#1099075)
    See

    http://www.wombat.ie/gps/saoff.gif

    for a GNUPlot graph of SA being
    switch off.

  • but how does it affect national security when Joe Citizen has a GPS unit that is accurate to 1 cm

    It depends what country he's "Joe Citizen" of, and whether he's aiming missiles at American troops. GPS is global so this move aids everyone, including America's list of "today's enimies".

    TWW

  • 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.

  • Yes. I routinely drive around with a laptop in the car running Street Atlas and my GPS feeding positions into it. Much amusement. Yeah, it's Windows.

    You can get the data for yourself from the government, or by looking for APRS maps (a ham radio application). The format of APRS is published, so the data should be transportable. Start with TAPR [tapr.org] - they have a very good set of APRS files.

    The GPS units themselves output the data in a format called NMEA. You can get some info here [nmea.org], though I don't know if the GPS spec is published for free through this site. I have seen it on line, though, so it wouldn't be hard to find. Another good site would be here [vancouver-webpages.com].


    ...phil

  • As a pilot, I often get NOTAMS (Notices To Airmen) warning of GPS jamming tests. The tests occur over a region, and appear to use ground-based transmitters.

    I'd guess that the US government will retain the ability to dither the signal.

  • There are several digitized maps available from the US Census and US Geological Survey. Typically they aren't available for download, but you can order them on CD's (a complete vector based map of the US is around 3GB). Bruce Perens put the TIGER map database online on his site at http://www.perens.com/FreeSoftware [perens.com]

    So, it does exist. You just have to look.

    Some guy named Chris

  • The error randomly rotates, so the solution you mention would not work.

    Actually, it works almost exactly that way. It's call DGPS (for Differential GPS). For example, checkout a tutorial here [tapr.org].

    In fact, DGPS can give better accuracy than even military non-DGPS receivers because it can correct for ionospheric delays. Interesting stuff.

  • Re:Great. (no upgrade needed) (Score:3)

    by Anonymous Coward on Monday May 01, 2000 @08:15AM (#1099099)
    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....

  • current hardware _will_ work (Score:3)

    by jone_stone ( 124040 ) on Monday May 01, 2000 @08:17AM (#1099102) Homepage
    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

  • Thank god! (Score:4)

    by KFury ( 19522 ) on Monday May 01, 2000 @08:17AM (#1099103) Homepage
    Now maybe my GPS-controlled car will stop driving off the side of the road!

    Kevin Fox

  • At Lastski! (Score:4)

    by Anonymous Coward on Monday May 01, 2000 @08:18AM (#1099105)
    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.

  • Altitude meters are based on air pressure, which varies (see any local news weather report). And, while in some circumstances you may be on the ground, you probably don't know exactly how much above sea level the ground you're on is. So, you still need to calculate your position in 3 dimensions.

    In order to solve the complete location equations, you need 4 inputs to handle the 4 dimensions (7 dimension if you want to get really technical - 3 trivia points for somebody who can name the other three dimensions). If you have an accurate clock (it needs to be really accurate, since each nanosecond of time uncertainty translates into about a foot of position uncertainty), you can forgo getting the time from the satellites, reducing the number of satellites you need by one. If you can pin your altitude directly, that's another variable resolved. Under that circumstance, yes, you could get away with two satellites. However:

    • Highly accurate clocks are expensive and fragile. It's probably a whole lot cheaper to use the satellite clocks.
    • There are lots of satellites available. My Garmin 12XL usually reports seeing 9 at a time.
    • The amount of math you have to do is pretty much the same, meaning you still need the same amount of CPU horsepower. (note bene: the Garmin 12XL has the equivalent of a 386 in it).

    Under that circumstance, why struggle reducing the number of satellites used?


    ...phil

  • This is fantastic! (Score:3)

    by Jinker ( 133372 ) on Monday May 01, 2000 @08:19AM (#1099109) Homepage
    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.

    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

  • Re:"National Defense Concerns"? (Score:3)

    by Anonymous Coward on Monday May 01, 2000 @08:21AM (#1099112)
    It's only that accurate if SA, or Selective Availability, is turned off. Most of the handheld units are poor locators, and really only good for placing you within a block or so of your real location. The original intent of SA was to prevent the Soviets from placing GPS receivers on their nukes, and using it as cheap, foolproof guidance. SA can fuzz your location from 100-400 meters, which was supposed to be enough that a hardened missile silo could withstand a few megaton hit. Those of us who were using GPS when the Gulf War started saw all our units fail. The Feds basically turned civilian reception off for a few days. Rather annoying. And the ultimate back door. They'll make it more accurate now (since industry can make more money with higher accuracy), but during the next conflict, say, during the China vs. Taiwan-US-Britain war, you'll see GPS turned off again.
  • The error you're seeing is more probably EPE - estimated position error. It's related to the geometry of the satellites being used in the position calculation. Some geometries (satellites more lined up) lend themselves to larger uncertanties in the position calculations, and the EPE figure shows you those uncertainties.

    SA was turned off at 0000Z last night (actually a little earlier, I think - apparently around 2340Z). I went out a little later and noticed a distinct lowering in the wandering of the least significant digits of my location. When I get the opportunity, I intend on firing up some SAWatch and do some extended viewing of the position uncertainty.


    ...phil

  • That was more or less the maximum error. The calculated position would be within 100 meters of the actual position 95% of the time, according to the published specification. It would also be within 30 meters 50% of the time.


    ...phil
  • Those of us who were using GPS when the Gulf War started saw all our units fail. The Feds basically turned civilian reception off for a few days.

    Hmm. I'm not going to contradict you, since I have no firsthand knowledge; however, this is pretty much the opposite of the story I'd been told. As explained to me, there were not yet enough GPS receivers with the military-grade signal decryption available for use in the Gulf War, so DoD purchased a bunch of commercial-grade receivers and turned off Selective Availability so the ground forces could get military-grade resolution and pull off the "left hook".


    Christopher A. Bohn
  • (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 [mehaffey.com] is an awesome starting point. Tons of links and info.
    GPS and NMEA [vancouver-webpages.com]- 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 [uscg.mil] has some good updates and status of gps.
    Garmin [garmin.com]- 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

  • Re:Not much of a choice (Score:2)

    by Anonymous Coward
    There's an entire engineering science devoted to this problem called "remote telemetry". Hit your local university library to learn more.

    Most of the USGS content was done with arial photography and *some* on-site surveying, and predates GPS by many years.

    As far as "Terraserver" supplying this content (in the preceeding post), that's laughable. For a nominal fee, the US Government will happily sell you an accurate map of any non-military location that you would like to nuke.
  • ...personally jam [thegeek.org] 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...
  • 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.
  • Sure, but you want your side to have cheap and good navigation, but the other side not to have it. How do you jam the other side's GPS signal without doing it to your own?

    As far as the jammability of GPS signals, I don't know. The terrestial DGPS signal is certainly going to be readily jammable; I'm not sure whether the sats are just as easily jammed. I don't know about that, it's not my field.
  • Close. The civilian signal has an intentional "drift," and the degree and sign of the drift is encrypted and transmitted out of band, basically. There are some good web pages on how this all works, which I can't find at the moment, unfortunately.
    * mild mannered physics grad student by day *

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