Shine a 4 milliwatt 645 nm wavelength dollar store pet laser onto a red reflector lens at night and it explodes into color.
Slender reflective fibers would easily reflect a relatively low power satellite based laser a couple hundred miles back to the satellite and show up on satellite imaging. A very weak magnet on each fiber could orient the fibers in specified directions with respect to the earth's magnitic field it the time it would take to fall hundreds or thousands of feet from a plane.
It would be very difficult to tramp over the fibers without disturbing the orientation.
When Al Quada tries a night time ambush, everyone on the planet knows about it.
> The mililary needs some material that can be crop dusted onto road
> > beds that can only be disturbed by digging and not by vehicle traffic.
> Ordinary traffic performs a certain amount of "gardening" on a dirt
> road, meaning some of thetaggant_will_ be mixed into the roadbed
> material. How deeply depends on the exact nature of the material (sand/
> clay/organic dirt, salts, concrete, asphalt, etc.) the kind and degree
> of traffic, the weather _and_ climate, and so on.
>
> Worst case you get washboarding which requires periodic repair,
> meaning deeper penetration by thetaggant. It will however be more
> evenly distributed than the sort of localized dugskullery you're > talking about.
>
> Hence the sort of disturbance due to traffic will be easily
> differentiable from that due to digging.
>
> > The material must be deposited with a characteristic "thumbprint" that
> > can be identified with the appropriate sensors but cannot be
> > duplicated, i.e., it cannot be swept up and spread over a freshly > > planted bomb.
>
> > This method wouldn't reveal the old bombs but it would make it easy to
> > spot where a roadbed was recently dug up.
>
> Just off the top of my head, how about microscopic polymer chips
> like those used in dynamite. Rather than the complex layering used to
> indicate batch numbers etc. it would be infused infused with an
> additive which, when exposed to UV, fluoresces in the IR, not the
> visible. If laid down by ground vehicle or say Predator or other drone
> during low traffic periods it would have a fairly even characteristic
> distribution, and any disturbance will be immediately visible by
> inspection by personnel wearing IR goggles during the day, and at
> night with IR goggles and the assistance of a UV lamp. Inspection
> could also be done by suitably-equipped drones of course.
>
> Attempts to "sweep up" and use the dust to cover new bombs will not
> replicate the dust distribution as laid down originally.
>
> Mark L. Fergerson