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GPS Civilian Signal Degradation Turned Off
Brian Demsky wrote to us regarding the release from the White House stating that the United States government will no longer purposely degrade civilian-use GPS signals. This mean more accurate data for people working with GPS, as "national defense concerns" had kept civilian results less accurate for years.
SA coming off is a good thing. However your results without SA will depend on a number of different things.
First and foremost, the quality if the maps you are using. GPS is significantly more accurate than many survey maps, which turn out to be extremely wrong about absolute position in some cases, particularly remote coastal areas. This can be fixed by re-registering your maps using a GIS. Another issue with electronic maps is the scale at which the streets were digitized; If they are digitized off 1:100,000 maps, as many are, you will have significant mistmatches. Sometimes the quality of matching to GPS signals on the same map may vary by feature type (major highway, street etc.) because each feature set was digitized separately, by different people, sometimes at different scales.
I've gone out with DGPS on the fly (both Racal and Satloc, both with sub 2m accuracy), and have been absolutely spot on for some kinds of streets and systematically 100m or more off.
In the city, you may have both coverage and multipath distortion issues that will limit your accuracy for a single reading. Receiver quality counts for quite a bit. Survey users will benefit the most because they can average a number of readings.
Finally, the clocks in some cheap GPS receivers are jittery (kind of like cheap Ethernet cards used to be), and may limit their performance even without SA. After all, they didn't have to be that good with SA turned on.
That said, it's great that SA is coming off.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
The 100M error attributed to SA is the maximum error that can be affected. There is also a large amount of error in the C/A signal itself without the P signal, so consumer stuff will still be ~100 times less acurate than military. The whole point is that with SA you could be right on, you just wouldn't know it since the error moves around a lot.
As far as terrorists go, the Soviet GLONASS system is apparantly almost as accurate as GPS with no encryption.
Si vis pacem, para bellum
The only thing more annoying than a Libertarian is an (un|mis)informed Libertarian
Keep in mind that the US government doesn't care how much something costs. "A billion here and a billion there, and pretty soon you're talking about real money." -- (US Senator Everett Dirksen, Illinois)
Anyway, the point I'm making is that the US government is much more interested in political power, and the abilty to effectively shut down tranportation facilities is a real potent tool. And it looks like it is in the interests of the US government (!= interests of the average citizens of the US) to keep this technlogy in the hands of the US military.
That and the technology to build a real competitor to the GPS satellite system is now available. Consider the costs over doing something like the Iridium, and putting up a competitor to the GPS satellites would be relatively trivial. Other than the fact that the GPS satellite uses a hyper accurate clock, it isn't all that much more complicated than the original Sputnik satellite put up by the USSR. It just transmits the current time and its position (with some accuracy references.) Ground stations are necessary to control the satellites mainly to make sure the clocks are accurate.
With the US government eliminating the intentional errors, they have dried up the market for all potential early users of a GPS alternative. I always thought it was a stupid thing to do anyway... deliberately engineer something to be less accurate than it could be. And introduce bugs and other problems in trying to cope with those intentional errors.
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 Soviet military and space agency built a GPS system at exactly the same time as the Americans. The system is still functional, and many of the GPS receivers from European firms actually accepts both of the signals, and can use either on for positioning.
The Russian system gives you many more satelites in the arctic regions due to the large area of Russia that's located in the arctic.
Just cuz you ain't paranoid, doesn't mean they're not after you.
Over at one of the military gps status sites (over here) they mention the mission being global nav, time, and nuclear detection.
Nuclear detection?
Anyone have any other info on this? I'm mostly just curious, I didn't think those satellites also had that feature. Or are they simply the standard relays for another detection facility?
Just wondering,
Jason
I have a little Garmin GPS III+ that has a little moving map display, "here you are" arrow, routes, waypoints, altitude, and it's about to get a whole lot more accurate.
/.-ers.
Having a web-pad or some other PDA (handspring!) with GPS capability would also be very cool, and it would have more memory than my Garmin (which has enough memory for about 10 MapSource counties with street-level detail, and the entire US with highway detail).
Of course, the Garmin isn't open source, and thus would be utterly useless to some
But I don't know how I ever got along without it. I'm still amazed at what this little box can do.
-CausticPuppy "Of all the people I know, you're certainly one of them." -Somebody I don't know
GPS satellites orbit in the area between low earth orbit and geosynchronous orbit. This means they move across the sky relatively slowly (geosynchronous satellites have no apparent motion, while low earth orbiting satellites are only usable in the sky for 10-15 minutes). This altitude means receivers can pick up the signals easier (geosynchronous satellites require heftier antennas or a parabolic reflector) and their slower relative velocities make them easier to lock on to and position over longer periods of time.
As far as the actual question, the other comments pretty much said it best. A satellite can be temporarily disabled or introduce error into the signal only as it's within range of the region in question. Even if you're 1/8th of the way around the world and end up locking on to this satellite as it starts introducing error into the signal, there are still several other satellites in range of your position that your receiver could lock on to instead, compensating for the error. This means the actual affected area can be isolated as much as they need to.
I 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.
Now I'm going to go off topic for a second to speculate that if the DOJ breaks MS up, Billy Borg will buy Cuba (I bet Fidel would sell if you offered him, say, $5 billion,) evict everyone, and erect a Giant Flying Windows Logo visible from Space.
I'm trying to teach myself to set people on fire with my mind... Is it hot in here?
There's some more info on the announcment at the International GPS Executive Board. There are some announcments from the secretary of commerce, so I think that's where some of the motivation came from.
There's also a great representation of the difference in accuracy with and without sa on. take a look at:
http://www.igeb.gov/sa/diagram.shtml
To quote:
Anyways, have fun out there.
Jason
This doesn't mean that *you*, the cell-phone user can locate where you are - it just means that the phone company can, so that 911 can locate you (if you're not paranoid, and for some reason believe the official explanations), or so anybody with a badge can locate you (if you *are* paranoid), or so any 2600-script-kiddie or at least any good social engineer can locate you.
If you do want to check out the paranoia options, spend a while thinking about the requirements that the 911 center be able to locate you any time your phone is on without you acknowledging it, and the lack of requirements for a standard locatee user interface.....
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
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.
- (c) 2018 Hank Zimmerman
Well, yes, you do have to broadcast the result. But if John Q. Terrorist needs an accurate signal, he can get it. Rebroadcasting the correction is relatively trivial, technically. If you build/control the transmitter, you could be quite certain that you were getting the right information.
Even easier, you can buy commercial DGPS transmitters fairly cheaply, or if your target is near the coast (as 90% of the US population are) then you can simply use the USCG DGPS broadcasts! IMHO, SA has never made any sense at all in terms of blocking terrorists. If it were that useful, then we'd have seen a GPS bomb by now.
SA is probably more about military applications, which I don't know much about. Probably the DoD had reasons to be concerned, but either decided that they could be addressed or that the role of SA was going to be obsoleted by widely available technologies.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
from Trimble Navigation, "Differential GPS Explained":
Summary of GPS Error Sources:
Per Satellite Accuracy Standard GPS
Satellite Clocks 1.5 m
Orbit Errors 2.5 m
Ionosphere 5.0 m
Troposphere 0.5 m
Receiver Noise 0.3 m
Multipath 0.6 m
SA 30 m
Typical Accuracy 50 m
The former USSR already has an equivilant to American GPS. The GLONASS constellation has been operational since about 1986.
The GIS company I work for has receivers that use both GPS and GLONASS for most of our survey work. When used in differential mode, these things are capable of sub-meter accuracy. (usually less than 0.5 meters error)
Still, turning off SA is great news.. :)
Yes, one day I may actually learn to spell...
The reason that pinpoint accuracy is important when looking at long range ballistic trajectories is for mid course corrections. When an ICBM leaves the Ukraine headed for Florida, it has to travel through a whole lot of poorly modelled atmosphere. Accordingly, at various points along the track, the missile has to make small adjustments to compensate for un-modelled forces in order to hit its target area. These mid-course corrections are very sensitive to small errors, so position inaccuracy at these critical points can lead to large errors in the missles final destination. Given this, a loss of precision of a few hundred meters means you hit Havana instead of Disney-Land. It does not mean that you hit Epcot instead of Space Mountain.
(ps. I like Disneyland and am not in any way condoning launching anything at in. )
I see a trend here. Last year pictures of a North Korean missle launch site were commissioned and taken by a private satellite, revealing a rather unimpressive facility not nearly matching the threat advertised by the Pentagon. Then it was pictures of Area 51 by a Russian satellite released to the public. Now we have the GPS degradation turned off.
Let's be honest here. It was only a matter of time. The longer restrictions were in place, the more likely it is that private or non-U.S. satellites would walk right on by, rendering the degradation irrelevant. Just like the restrictions on what satellite pictures U.S. satellites are allowed to take, they become pointless as more private and foreign instruments proliferate.
This is a good thing, IMO. It's becoming harder for governments to hide information from their citizens. When you read some of the now released transcripts on how the CIA manipulated the media in Iran and Vietnam and countless other hotspots how can you not have positive feelings on these developments.
I would like to nominate David Brin for a Slashdot interview, given all of the recent transparency and privacy stories that have come up lately.
See
http://www.wombat.ie/gps/saoff.gif
for a GNUPlot graph of SA being
switch off.
I'd guess that the US government will retain the ability to dither the signal.
It's Linux, damnit! Pay no attention to renaming attempts by self-aggrandizing blowhards.
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.
Current GPS hardware will work as usual, but instead of the ~50-foot precision, it'll be much more precise. The press release suggests that the improved system will be ~10x more accurate, which would seem to indicate ~5-foot (1-2 meter) precision. Anyone have any more specific numbers?
-David
Now maybe my GPS-controlled car will stop driving off the side of the road!
Kevin Fox
Kevin Fox
Now secret missle in basement is accurate enough to destroyt moose and squirrel once and for all!
I suspect this was done to try to head off anyone building a competitive GPS network. I believe the EU (which feels it needs to have a home grown alternative to every single thing developed in the US) has been planning a military precision GPS competitor system for a while now. Maybe the US gov't doesn't want that or other systems to actually happen.
It's not quite reliable enough for automatic landing of aircraft (differential GPS with ground based transmitters is necessary for that), but is good enough for MANY other applications.
I know that in a coal mine that my brother in law worked at, they had Loran navigation for the big trucks. On foggy days they could operate when they couldn't see the ground. Imagine driving up and down a mountain in a 300 tonne truck without seeing the road.
I think it's almost time for me to buy a GPS for my bike. Or maybe one that plugs in to my Palm Vx.
This is too damn cool.
I only wish it wasn't another example of a military spin-off technology. It could have been developed in the civilian arena with a little governmental support. I can understand how bitter it is for the USAF to swallow the entire cost of the constellation of satellites, only to see Radio Shack selling $100 units at good profits.
Greg
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.