GPSmap76CSx, tech. question

I am pretty sure the "D" symbol means that there is a "Differential" signal for that Sat.

The Differential signal is a Time and Distance correction which is transmitted to correct the terrestrial drag on a signal.

It used to be something that the Department of Defense turned off and on as they saw fit in order to make GPS units much less accurate so the "bad guys" could not use a GPS to pinpoint a location for whatever reason.

The differential signal harkens back to the days of Loran.

Tim

Thanks a lot, that makes sense!

It sounds like you may be confusing two very different things. A differential signal is indeed, as you stated, extra error correction information that can be used by a properly equipped GPS to get a more accurate solution to the GPS equations. In short, the atmosphere introduces some variables to the signal even for simple things like temperature, and these can affect GPS accuracy. But the error is pretty consistent over a large area. So if this error is measured at some known point and information about the error is broadcast to GPS receivers (either by a local radio signal or over satellite), the the GPS can correct for the measured error. WAAS does this by sending the correction signal from a geosynchronous satellite, but unfortunately it doesn't seem to be used by the current Garmin Nuvi line. And no, a high sensitivity receiver does not in any way eliminate the benefit of a WAAS signal. Thinking that it does would be like claiming that you don't have to speak clearly as long a you yell very loud.

The "something that the DoD tuned off" that you talk about is the selective availability signal. This was a deliberate extra error that was being added to the civilian gps signal to deliberately make it less accurate. It was turned off in 2000. Although a differential signal could be calculated locally to correct for this error also, it really has nothing to do with a differential signal used to improve GPS accuracy.

Thanks for that extra clarification.

GPS is much more technical than most people think, or appreciate.

The GPS companies want to make it as user friendly as possible, just like the computer.

No need for the average user to know what happens behind the curtain.

But the people here at POIFACTORY are way beyond the "average" user.

Joy to listen to dedicated people!