[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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