Agreed that they are dispersion-field limited but their inherent
   broadband and "directionality" can, if located far enough from the
   signal sources and destinations, solve some interesting comms
   situations. For example, metropolitan "relays" able to leap tall
   buildings in a single bound:). They can complement broader NVIS HF
   which is less reliable and narrow band.

   On Mon, Dec 31, 2018, 5:08 PM grarpamp <[1]grarpamp@gmail.com wrote:

     On 12/31/18, Steven Schear <[2]schear.steve@gmail.com> wrote:
     > Retrorefltors are very useful. They are hard to beat for covert
     comms.
     > Unfortunately, optical ones are limited by weather. Much more
     versatile,
     > if possibly less covert, are radio types. Ultra lights the size of
     the Echo
     > are now easily deployed via inflatable and UV regidification.
     Retros point back, perhaps not so much useful for direct
     comms outside their relative dispersion return fields.
     Planar mirrors, lenses, over horizon, ionosphere, relays,
     lofted nuke comms, etc are different.
     [3]https://en.wikipedia.org/wiki/Retroreflector
     [4]https://en.wikipedia.org/wiki/List_of_passive_satellites

References

   1. mailto:grarpamp@gmail.com
   2. mailto:schear.steve@gmail.com
   3. https://en.wikipedia.org/wiki/Retroreflector
   4. https://en.wikipedia.org/wiki/List_of_passive_satellites