[time-nuts] Another Trimble Tbolt question....

[Magnus Danielson]

From: GandalfG8 at aol.com
Subject: Re: [time-nuts] Another Trimble Tbolt question....
Date: Sun, 30 Mar 2008 05:46:10 EDT
Message-ID: <d1d.1f0e59bf.3520bb62 at aol.com>

Nigel,

> 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

Considering your position, it seems to agree with modelled differences, just
look briefly at the second coluourfull map of the earth on this link:

http://cddis.gsfc.nasa.gov/926/egm96/egm96.html

and you see that what you have observed does indeed match the deviation in
readings that you see.

Infact you have provided a nice example of the disagreement between the
WGS-84 ellipsoid reading (default readout) and that of the actual geoid.
There exists models with which the geoid position can be calculated from the
ellipsoid position.

In the mean while, the 30,072 m I am reading out of my receivers survey-state
right now could be close to a geoid reading. But then I suspect that Novatel
have made some form of compensation. I should expect some 25 m or so above the
ellipsoid.

But then, I expect my GPS antenna to have a poor position in terms of multipath
and a move is upcomming.

Cheers,
Magnus