  We show that dark-matter candidates with large masses and large nuclear
interaction cross sections are detectable with terrestrial radar systems. We
develop our results in close comparison to successful radar searches for tiny
meteoroids, aggregates of ordinary matter. The path of a meteoroid (or suitable
dark-matter particle) through the atmosphere produces ionization deposits that
reflect incident radio waves. We calculate the equivalent radar echoing area or
`radar cross section' for dark matter. By comparing the expected number of
dark-matter-induced echoes with observations, we set new limits in the plane of
dark-matter mass and cross section, complementary to pre-existing cosmological
limits. Our results are valuable because (A) they open a new detection
technique for which the reach can be greatly improved and (B) in case of a
detection, the radar technique provides differential sensitivity to the mass
and cross section, unlike cosmological probes.
