Re: "lost" trains

[Eccles@ThisNameIsRubbish.co.nz]

On Fri, 7 Jan 2000 13:50:55 +1100, "nobody"
<dweebken@NOSPAM.yahoo.com> wrote:

> Cuttings and tunnels dont matter as long as they are not
>too deep, as beacons broadcast up to the satellite regardless, and broadcast
>devices could be installed in tunnels. 

The GPS satellites are transmit only [1] so they do not have a clue
where a particular receiver is. For GPS to work it must be possible to
receive some of the satellite signals.

I assume that by beacons you are referring to those used in
differential GPS. Fixed ground stations at accurately known locations
compare their "GPS" position with their actual position. From the
difference they can calculate the error in the GPS signals and this
information is broadcast for use by mobile receivers. The mobile
recivers use this error data *and* signals from the satellites to
determine their position.

[1] The satellite control stations transmit orbit data to the
satellites - so in this sense the saltelites can receive.