[time-nuts] Another Trimble Tbolt question....
GandalfG8 at aol.com
GandalfG8 at aol.com
Sun Mar 30 05:46:10 EDT 2008
In a message dated 30/03/2008 07:47:57 GMT Daylight Time, darrell at shaw.ca
writes:
My guess is that at least some of the difference that you see in your
altitude is from the propagation delay in the cable between your antenna and
the GPS module.
Guessing still further, I would think that the position as reported would be
some distance immediately below the antenna. How long is your cable?
If the antenna is stationary, you could move the GPS module to any position
within the sphere allowed by your length of cable, and the reported position
would stay the same.
-----------------------------
I think that's all rather optimistic, as mentioned previously height
estimation is the most innacurate parameter to be reported by GPS units.
It's also important to remember that the "mean" sea level reference is not
necessarily the same as "local" sea level.
I am running a Thunderbolt from an antenna at most 10 feet away from the
unit with a cable not much longer, so not too much propagation delay.
The antenna is quite low but has a good view of the sky and at the moment is
tracking seven satellites.
Altitude will always vary by at least a few metres whenever it makes a
survey, it's currently showing 58.8 metres but previous survey was around 64
metres.
That's not bad when you consider that I live in the middle of the Clyde
estuary in Scotland, with the water's edge somewhere around 50 feet away from the
antenna, and that even with allowance for tidal variations the antenna is,
at the most, 3 or 4 metres above the physical water level:-)
regards
Nigel
GM8PZR
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