Precise Orbital Tracking of an Asteroid with a Phased Array of Radio   Transponders
  Deflecting an asteroid from an Earth impact trajectory requires only small velocity changes, typically of the order of microns per second, if done many years ahead of time. For this, a highly precise method of determining the need, magnitude, and direction of a deflection is required. Although the required precision can be achieved by much less accurate extended observations, an intrinsic resolution of $\mu $m/s permits the live monitoring of nongravitational orbital perturbations (Yarkovsky effect), and of a deflection effort itself. Here, it is proposed to deploy on the asteroid's surface multiple radio units to form a phased array capable of measuring radial velocities relative to Earth to about 1 $\mu$m/s and ranges to 5 m. The same technology can also be used for scientific applications such as very-long baseline radio astronomy, milli-Hertz gravitational wave detection, or mapping of the solar wind. 
Bernhard W. Adams