We are talking about an angle of typically 0.1 millirad with respect to a direct reflection - it is not "side-scattering" in the conventional sense.Bistatic radar does work, but trying to do any real computation with side scatter radar reflections is challenging at best. Trying to do doppler on the return to see if the reflector was heading our way or even earth orbit crossing i think is beyond the limits of current technology. Something light minutes away outside the main bean would have a terrible S/N ratio.
No. For the same price you get a much smaller antenna and orders of magnitude lower power, which means the range gets much shorter. The probability that something would fly through the volume where the space-based radar happens to be is tiny.Would it be better in anyway to have a radar in outer space at a convenient location, be it some kind of earth stationary orbit, Lagrange point or other?
Don't misrepresent the article please. It is not talking about asteroid detection there, it is talking about measuring known asteroids. The amount of radar power they reflect tells us something about their composition. This requires them to be in range and at a known place, however.