RegB wrote:
It has happened, a rider gets thrown and the horse takes off, gets
lost, can't be found days or even weeks later.

There was a saddled horse, presumed lying dead for 20 years, found near
San Francisco last month.

I assume some sort of "service" would be required, probably expensive,
but I would like to gather info anyway.

A smartphone with Google Latitude enabled will update it's location
periodically if there is a cellular signal. If the phone has GPS, the
position is very accurate.

Hmmm... leave the cellphone in a saddlebag so you can find the horse, or in
your pocket so you can call for help...

http://www.google.com/latitude/intro.html

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Clarence A Dold - Hidden Valley Lake, CA, USA GPS: 38.8,-122.5