[time-nuts] GPS question

[bg at lysator.liu.se]

On Thu, April 5, 2007 19:06, Joseph Gray said:
> Am I correct in assuming that the position given by a WAAS enabled
> handheld
> GPS unit is more accurate than the position reported by my Z3801?

Yes, IF you have decent reception for the handheld. You do not want a bad
multipathenvironment.

> Specifically, I tried a Garmin eTrex Legend Cx with WAAS enabled and used
> averaging mode for the waypoint.

Try logging the positions to a PDA or normal computer. Look at the values.
How much do they wander? Compare with the positions from the Oncore in the
GPSDO. This will give you a much better understanding or "feeling" of the
quality of the solutions. Can you split the GPSDO signal and reradiate to
the Garmin?

> On a related question, the eTrex reported that the position was +- 6ft. Is
> that really 6ft, or is that 6ft in addition to the 3M claimed accuracy of
> WAAS?

With a good antenna (&antenna siting) it is not hard to get an accuracy of
a few meters. You might approach that even with unaided GPS. But using a
highsensitivity receiver in marginal conditions (inside buildings etc) -
even if you get a fix, it will not be super accurate. Like in so many
other applications - add problems in one stage, and the downstream results
will suffer!