[Disclaimer, although I work for a company that manufactures GPS equipment I don't claim to be an expert].
WAAS is broadcast from 2 satellites over the US, and contains a simplistic model of the atmosphere conditions over the US on a grid based system (100km I think). This allows a GPS receiver to model the current conditions and produce a better position fix. DGPS (as broadcast by US coastguard) is a RTCM correction stream, which contains similar information about atmospherics but is related to a single point (the reference station). DGPS is usually transmited by radio around 110KHz and can be received by a DGPS beacon (although can be streamed via internet). It is usually fed into a GPS receiver via serial port. You can set up your own reference station, but it is not simply a GPS receiver broadcasting the inaccuracies in it's computed position. It is more involved than that, you will need a receiver capable of doing direct carrier/phase measurement (ie. a post processing capable receiver), at which point the computation might already be in the firmware. It is possible to kick some consumer grade receivers (garmin etrex for example) into a debug mode where you get this information, but I am unaware of any project to compute a RTCM stream from this information. <subliminal ad ;-> If you want to purchase a pre-built solution see: http://www.point-inc.com/products/gsr2700rsx.html </ad> The further you (the rover) are from the reference station the less accurate your position fix will be, a base line of less than 20km is normally required for surveying level accuracy. Corrections are normally transmitted via UHF/VHF radio modem or GPRS cell modem. A third option is a virtual reference station which will compute a fake reference station close to the point at which you are surveying. Cheers, Mungewell. _______________________________________________ talk mailing list [email protected] http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/talk
