I agree completely but you forgot to include:
grass and *angled* concrete
grass and *angled* asphalt
Ultrasound bounces *very* well from hard surfaces and if the hard
surface is angled away from the receiver the reflected sound just
disappears...from the standpoint of the receiever that is. Gee is that
grass I'm looking at or a concrete driveway with a slight angle.
re: grass and gravel / pebbles
I never tried this one but I would expect a lot of scatter, hence
attenuated response, hence confusion with grass.
I favor the beacon approach. I'd personally like to not have the
beacons, but realistically, it is probably the most practical,
particularly if they could (somehow) be made passive. Besides, on my
*piece of the rock* there is no physical divider between my property and
my neighbor's, so I'd have to do *something* there, unless of course I
can convince him to let me wonder over there now and again.
Tom