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

[Rob Kimberley]

Mike,

A quick answer to this - two reasons why...

1) Your Thunderbolt is using WGS84 coordinate system and not USGS, and..
2) The altitude fix is usually the most inaccurate parameter, as it is very
unlikely that you have a GPS bird directly overhead. Think of your position
fix as you sitting inside a sphere of probability. In the ideal world with
perfect GPS geometry it would be a sphere, but in reality it is an ellipsoid
due to less accuracy in the Z plane. 

Why, after setting (and assuming storing) a known accurate position it
changes again, I don't know, unless the unit is reverting to a survey mode.

I don't know this product intimately, so I'm sure others will proffer better
suggestions.

Rob Kimberley

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Michael Baker
Sent: 29 March 2008 12:50
To: time-nuts at febo.com
Subject: [time-nuts] Another Trimble Tbolt question....

Hello, All--

Thanks to the collective savvy of this list and the patient assistance of
Ken Winterling <wa2lbi> I was finally able to get my Tbolt to stop requiring
me to configure the COM-4 channel every time I ran the monitoring software.

So-- I had such good luck with that question, I'm going to pester the group
with one more Tbolt question:

In the "Position" area of the Tbolt monitoring display
my altitude is given as 1.4 meters.   The USGS topo map
and a recent survey both say that the ground level at my house is 84 feet
above MSL.  The antenna for the T-bolt is on the top of my house 19 feet
above the ground.

If I open SETUP / POSITION and "SET ACCURATE POSITION"
to, say....: 28 meters and enter that value, the indicated altitude changes
to 28 meters, but the next day or so, I always find that it has reverted to
1.4 meters.

Admittedly, having a (relatively) accurate altitude display is of minor
importance to me since my primary reason for having the Tbolt is for a
decent 10MHz reference.

However, as a matter of principle it would be nice to have the altitude
display be a little closer!

Any suggestions as to how to get the Tbolt to display a more correct
altitude?

Thanks!

Mike Baker
WA4HFR
Micanopy, FL
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