just to update this thread:
garmin etrex 10 has dual gps/glonass
also, the iphone 4s uses gps/glonass
--- In PanoToolsNG@yahoogroups.com, "Keith Davison" <megakd@...> wrote:
> Yes, that is how DGPS works, the location of the base station, such as a single one on
> your farm, or one in a nationwide network, is known precisely. The base station GPS
> receiver continually compares the GPS fix for its location with the known location and
> transmits correction data via radio to the remote GPS receiver (in a tractor, for example),
> the remote unit then takes its own GPS received location and corrects this with the radio
> data from the base station.
> Obviously the nearer the remote GPS receiver is to the base station, the more accurate the
> correction will be.
> Free DGPS data used to be transmitted around the coastlines of some countries to aid
> marine navigation, Garmin used to sell a DGPS radio receiver that would connect to their
> handheld units and make the necessary corrections.
> Uncorrected GPS accuracy with handheld units varies with conditions, but you should be
> within 10m most of the time with spot fixes on a modern unit. Averaging over time with a
> static unit improves accuracy considerably, (under 3m by 15min) but clock errors limit
> uncorrected units to a maximum accuracy of about two metres.
> As mentioned earlier, if your unit succesfully receives WAAS or EGNOS, sub-2m accuracy
> is possible for spot fixes.
> Keith D.
> > I think that farmers use differential GPS where there is a fixed
> > receiver on a building and is radio linked to the vehicle mounted unit.
> > This gives a difference in readings this eliminating all satellite and
> > atmospheric variations. Use a bluetooth or USB GPS receiver I have managed
> > to get about 3 meters accuracy with them. Under bad conditions like under
> > wet foliage or between tall buildings the error can go up to 25 meters.
> > Quentin.