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.

Comments

Don't expect too much.

[hey!]

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.

Possible New Uses

[jamesl]

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.

There already is competition...

[Croatian Sensation]

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.

GPS Mission includes nuclear detection?

[Jafa]

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

Paranoia: Cell-based location vs. GPS

[billstewart]

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.....

About GPS and signal degredation

[Kartoffel]

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.

Re:About GPS and signal degredation

[Eil]

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.

Re:It was pretty silly anyway...

[singularity]

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.

USSR Equivilant = GLONASS

[DJGreg]

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.. :)

Why accuracy is important (500km not 500m)

[goodviking]

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

[Aravaipa]

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.

Re:Selective denial of GPS on a regional basis - H

[mr_death]

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.

Thank god!

[KFury]

Now maybe my GPS-controlled car will stop driving off the side of the road!

Kevin Fox

At Lastski!

[Anonymous Coward]

Now secret missle in basement is accurate enough to destroyt moose and squirrel once and for all!

Competition is the Reason

[Aaron M. Renn]

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.