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

[Magnus Danielson]

From: Darrell Robinson <darrell at shaw.ca>
Subject: Re: [time-nuts] Another Trimble Tbolt question....
Date: Sat, 29 Mar 2008 23:41:36 -0700
Message-ID: <002e01c89231$1f524fc0$9400a8c0 at shaw.ca>

> I am not an expert.
> 
> 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.

No, that cancels out. The geographical position comes from the time differances
between the signals and that is how they are perceived by the antenna. The
cable will maintain these time-delays differances fairly accurate (only the
doppler frequency offsets skews the delay) but the receiver will receive them
at a later time due to cable delay. Therewill be a systematic offset of the
time solution thought, due to the cable delay. The reason for that is that the
receiver actually finds a position in [X Y Z T]t for the antenna, not the
receiver, so the cable delay on the antenna can be seen as the additional
output delay on the PPS.

So no, the height is not that greatly affected by the cabel delay. I would look
at what types of ellipsoid-geoid compensations is performed, and use of a local
datum.

> 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.

Don't bother.

The receivers position is irrelevant, as it is the antenna position that is
important.

Cheers,
Magnus