Why is radar NOT a useful technique for searching for near-Earth asteroids (NEAs)?
The answer is that the parameter space (time delay, Doppler frequency, and direction as functions of time) is simply too huge. Arecibo and Goldstone achieve the sensitivity needed for NEA observations via their large size and hence large antenna gain, which produces extremely narrow beams. These telescopes need a pointing ephemeris good to half an arc minute and a reasonably accurate Doppler ephemeris to be able to detect a known NEA. The phrase “functions of time” is critical, because radar waveforms mandate that radar detection involves time exposures, and hence demand that the target be tracked not just along its sky path but also along its time-delay (range) and Doppler-frequency (radial velocity) paths. This is why a decent ephemeris for an object is needed if there is any hope of detecting the object.
