Common ultrasonic transducers have a half angle of about 15deg. so they
project a cone of 30deg. Signal can be receieved from outside 30deg, but
at greatly reduced repeatability.
>
> To reiterate: I would prefer a system which did not depend on
> pre-programmed knowledge. A much more satisfying (to me) and reliable
> system based on ultrasound would simply map landmarks and boundaries
> early in the proceedings and use these as points of reference to help
> compute future relative position fixes.
>
You'll get no argument from me there ;-)
-- Dave Everett Email: deverett nospam at idx.com.au (c) 1996 - Copyright remains with the author unless explicitly stated