4 ms·

There's something I've always wondered and maybe here is the right place to ask it, but what is the practical and theoretical accuracy of the GPS? Can it track

There's something I've always wondered and maybe here is the right place to ask it, but what is the practical and theoretical accuracy of the GPS? Can it track the position of a device in, say, a radius of 2 feet? It always looked to me like GPS measures are pretty coarse by design or are limited by the government for security purposes (e.g. launching missiles).

brazzy 12y ago
Nominal accuracy of "pure" GPS is 15 meters. Using Differential GPS (which uses additional signals from ground stations)gets you down to 10cm in the best case. There used to be an additional limitation called "selective availability" where the military deliberately introduced errors in the clock siginal to reduce accuracy to about 100m, but this was switched off in 2000.
- userbinator 12y ago
  Note that most modern GPS receivers usually achieve much better than 15m accuracy in normal use. http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/ I can personally say that ~1.5-2m accuracy is definitely achievable even with a cheap entry-level receiver like the ones integrated into smartphones, when there is a clear view of the sky and good weather. Edit: It seems my phone does have DGPS, so that accuracy is not just "raw GPS".
  - cliveowen 12y ago
    That's really outstanding, I didn't expect that.
cyanoacry 12y ago
GPS comes in two flavors, SPS (standard positioning service, your standard civilian positioning) and PPS (precise positioning service, for the military). The SPS specification[1] says you should expect 95% of reported locations to be within a 9m diameter circle (ref 3.8.3). Realistically, though, you can get much better, and a lot of receivers get within 2-3m. Up until about 2000, there was thil thing called Selective Availability[2] (aka SA). This limited civilian performance to much worse (95% ~20-30m) by some clock dithering techniques. During-SA and post SA accuracies are very different. In order to support airplanes using GPS for precision landing, the FAA rolled out WAAS[3], which is a wide-area augmentation system that broadcasting atmospheric correction data (specialized just for the US; there are other national systems). GPS+WAAS gets you to within a meter or so. [3] If absolute position isn't important, but relative position is (say, if you're measuring the movement of two tectonic plates relative to each other at a fault), there's this neat thing called RTK. This gets you relative positions (in the field) on the order of millimeters[4]. [1] http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf http://www.gps.gov/technical/ps/2008-SPS-performance-standar... [2] http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Selective_availability http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P... [3] http://www.navipedia.net/index.php/WAAS_Performances http://www.navipedia.net/index.php/WAAS_Performances [4] http://hemispheregnss.com/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=522&articleid=28&documentid=197 http://hemispheregnss.com/DesktopModules/EasyDNNNews/Documen...

brazzy 12y ago

Nominal accuracy of "pure" GPS is 15 meters. Using Differential GPS (which uses additional signals from ground stations)gets you down to 10cm in the best case. There used to be an additional limitation called "selective availability" where the military deliberately introduced errors in the clock siginal to reduce accuracy to about 100m, but this was switched off in 2000.

userbinator 12y ago
Note that most modern GPS receivers usually achieve much better than 15m accuracy in normal use. http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/ I can personally say that ~1.5-2m accuracy is definitely achievable even with a cheap entry-level receiver like the ones integrated into smartphones, when there is a clear view of the sky and good weather. Edit: It seems my phone does have DGPS, so that accuracy is not just "raw GPS".
- cliveowen 12y ago
  That's really outstanding, I didn't expect that.

cyanoacry 12y ago

GPS comes in two flavors, SPS (standard positioning service, your standard civilian positioning) and PPS (precise positioning service, for the military). The SPS specification[1] says you should expect 95% of reported locations to be within a 9m diameter circle (ref 3.8.3). Realistically, though, you can get much better, and a lot of receivers get within 2-3m. Up until about 2000, there was thil thing called Selective Availability[2] (aka SA). This limited civilian performance to much worse (95% ~20-30m) by some clock dithering techniques. During-SA and post SA accuracies are very different. In order to support airplanes using GPS for precision landing, the FAA rolled out WAAS[3], which is a wide-area augmentation system that broadcasting atmospheric correction data (specialized just for the US; there are other national systems). GPS+WAAS gets you to within a meter or so. [3] If absolute position isn't important, but relative position is (say, if you're measuring the movement of two tectonic plates relative to each other at a fault), there's this neat thing called RTK. This gets you relative positions (in the field) on the order of millimeters[4]. [1] http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf http://www.gps.gov/technical/ps/2008-SPS-performance-standar... [2] http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Selective_availability http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P... [3] http://www.navipedia.net/index.php/WAAS_Performances http://www.navipedia.net/index.php/WAAS_Performances [4] http://hemispheregnss.com/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=522&articleid=28&documentid=197 http://hemispheregnss.com/DesktopModules/EasyDNNNews/Documen...