[ErikJ]

I thought about that source of interference as well, but at these wavelengths very slight differences in distance will result in going from constructive to destructive interference (and back). I figured that then on average the chances of a missed detection are probably pretty slim. That brings my main concern back to the stray signal problem.

We have essentially a Direction of Arrival (DoA) problem - for more info just google for more info, or search IEEExplore if you have access. A similar problem has been solved many times over, especially in test ranges for antennas and radars. They have essentially the same problem we do - you can't distinguish a reflection off a wall from a reflection from the object. Now those guys use some pretty fancy techniques to eliminate those issues, but we can take a page from their notes. Since we have a priori knowledge of the source location (something I didn't have in the example I cited previously - in that case my beacon was moving), we can ignore signals coming from regions we don't expect to see on in. Now, this brings up some potential navigational pit falls, but if you used IR search/track in combination line following you'd be in good shape. If you're just randomly searching for a beacon then stray signals *could* become an issue. It's hard to say for sure (especially for me to say since I'm an EM guy not an E/O guy) until somone tries it out. However, since again the potential source of the stray signal is moving (quickly with respect to wavelength), this might go away just like the true multipath problem.