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.

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.

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.. :)

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.

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.