One more thought, if you go optical, then multiple sensors with overlapping=
 fields of view (venn diagram) will tell you when the object is in the over=
lap by hitting all the detectors, similar to the optical sighting method in=
 "The Dam Busters" where they had two beams of light that merge to one if t=
he aircraft is at the right altitude.  As you get lower, the overlap area s=
hrinks, allowing for easy localization once you find it at higher altitude.

--
David VanHorn
Lead Hardware Engineer

Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO =A080301 USA
phone:=A0303-417-1345 =A0x110
email:=A0david.vanhorn@backcountryaccess.com=A0



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