Sorry for replying to an old message. On January 21, Franc Carter wrote: > Out of curiosity, how do you know it's off, whenever I think > about this sort of thing I have a really hard time working out > what might be a good 'source of truth'.
You build a very stable floor from concrete and mount a GPS receiver there. Receive signals for a month and compute the average of the reported location. Now you know the exact position of that concrete floor, with much higher accuracy than any single sample. Next day, when satellites report "you are at position X, Y", you know how much off they are, because you know your own position much better than the satellite. Actually, what you do know is how much delayed the time signal from each satellite is. If you can somehow (by GSM, WLAN, wire, 27 MHz CB radio, or carrier pigeons) get this delay correction information out to your mobile receiver, it can compute it's correct latitude and longitude. The WAAS/EGNOS systems do exactly this and the correction information is sent out via geo-stationary satellites. Since geo-stationary satellites orbit above the equator, if you are far north (in Scandinavia) you need a clear view of the southern horizon to receive this signal. And your GPS device needs to be prepared/enabled for WAAS/EGNOS. But if you are a commercial, public or military surveyor, you might have access to other systems than WAAS/EGNOS. -- Lars Aronsson ([EMAIL PROTECTED]) Aronsson Datateknik - http://aronsson.se _______________________________________________ newbies mailing list [email protected] http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/newbies
