Come to think of it, rather than following the usual inverse square law, power should drop off with the fourth power of distance (just like with radar) - there'

Come to think of it, rather than following the usual inverse square law, power should drop off with the fourth power of distance (just like with radar) - there'd be one inverse-square dropoff on the path from active transmitter to transponder, multiplied by a second inverse square loss on the return path. That's got to hurt performance.