Travis Marlatte wrote:
"Ron Lee" wrote in message
...

Actually the DOPs are related to satellite geometry and are unitless.
It is only when you multiply the DOP by the signal/user equipment
error that you get some sort of error in feet/meters.

The DOPs represent a statistical confidence. They do have a unit but you're
right, it's not in units of distance and it is not standardized.

No, Ron is correct that the various DOPs (PDOP, VDOP, HDOP, TDOP, etc.)
are unit-less measures based on the satellite geometry. They are
well-defined and standardized as specified in Section 3.1.3 of the
Navstar User Equipment guide:
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf

It's when the DOP, typically the HDOP, is translated into a
corresponding EPE (Estimated Position Error) or accuracy value that it
is converted into a measure with units of distance and these measures
are not standardized across manufacturers or even sometimes between
models of a given manufacturer.

I agree with some previous comments that the VDOP is generally somewhat
larger than HDOP due to the inherent geometry of the situation. You
usually have satellites on all sides of your position, but you can only
get signals from satellites above you, none below, and that reduces the
accuracy of the altitude calculation. An additional consideration in
the OP's case is that when the unit first gets enough satellite signals
to report a position it may only have enough for a '2D' calculation.
With three satellite signals, the unit can only determine the exact
time and two space coordinates, so it assumes the altitude based on the
last value previously seen and only calculates the lat. and long.
coordinates. Then when it gets one or more additional satellite
signals it can switch to '3D' mode and calculate the actual altitude as
well.