[Talk-GB] Very accurate GPS devices
Further to Ed's post, are there really GPS devices with one inch accuracy? I could imagine letting a device sit for some time, and then averaging the position, which might lead to increased accuracy at the cost of long exposure time... is that method used for surveying and so on? Matthew signature.asc Description: This is a digitally signed message part. ___ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
Re: [Talk-GB] Very accurate GPS devices
Matthew Gates wrote: Further to Ed's post, are there really GPS devices with one inch accuracy? I could imagine letting a device sit for some time, and then averaging the position, which might lead to increased accuracy at the cost of long exposure time... is that method used for surveying and so on? He was referring to a combine using a DGPS system which has a base station at a fixed point on the farm whose location is well known. It then compares that known location to one calculated from the satellites in the normal and broadcasts the difference to the mobile received on the tractor/combine which uses the difference to correct it's own calculated position. What that allows you to do is to compensate for inaccuracy caused by local atmospheric conditions as you are generating a correction based on a local base station. Tom -- Tom Hughes ([EMAIL PROTECTED]) http://www.compton.nu/ ___ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
Re: [Talk-GB] Very accurate GPS devices
On Fri, 29 Aug 2008, Tom Hughes wrote: He was referring to a combine using a DGPS system which has a base station at a fixed point on the farm whose location is well known. It then compares that known location to one calculated from the satellites in the normal and broadcasts the difference to the mobile received on the tractor/combine which uses the difference to correct it's own calculated position. A DGPS station actually works out how big an error each received satellite signal has and transmits that data to the (mobile) GPS receiver, which then applies the correction _before_ calculating the location. (i.e. the DGPS signal contains the timing errors for each satellite rather than the errors in the coordinates, since the errors in the calculated coordinates would depend on which satellites the GPS is using, which is something the DGPS transmitter doesn't know). I suspect that a system that accurate is probably not just using DGPS though - it probably has a set of ground-based transmitters at known locations that it uses for ranging as well. - Steve xmpp:[EMAIL PROTECTED] sip:[EMAIL PROTECTED] http://www.nexusuk.org/ Servatis a periculum, servatis a maleficum - Whisper, Evanescence ___ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
Re: [Talk-GB] Very accurate GPS devices
On Fri, Aug 29, 2008 at 2:52 PM, Tom Hughes [EMAIL PROTECTED] wrote: Matthew Gates wrote: Further to Ed's post, are there really GPS devices with one inch accuracy? I could imagine letting a device sit for some time, and then averaging the position, which might lead to increased accuracy at the cost of long exposure time... is that method used for surveying and so on? He was referring to a combine using a DGPS system which has a base station at a fixed point on the farm whose location is well known. It then compares that known location to one calculated from the satellites in the normal and broadcasts the difference to the mobile received on the tractor/combine which uses the difference to correct it's own calculated position. What that allows you to do is to compensate for inaccuracy caused by local atmospheric conditions as you are generating a correction based on a local base station. ... and then the final stage is carrier-phase enhancement, which gets you down to the one-inch accuracy. It tells you what fraction of a wavelength of the GPS carrier signal you are out by compared to the base station, but not how far away you are. So if you can use DGPS to get down to a 20cm location, the CPGPS will tell you whereabouts in that particular box you must be. The guy from the OS at SOTM did a fairly good job of explaining it the difference between DGPS and CPGPS - I don't think wikipedia helps much. http://en.wikipedia.org/wiki/Global_Positioning_System#Accuracy_enhancement Cheers, Andy ___ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
