In order to carry out the observations of 2012 DA14, we must carefully check the radar pointing calibration. Remember that this asteroid will fly past quite quickly, so it is essential to check that the alignment is correct.
To do this, we plot the anticipated asteroid trajectory. We then search for radio calibration sources (point-like radio "stars") that are in the sky and work out when they would cross the asteroid trajectory.
We then can observe these sources as they are in that position. Because these are the anticipated positions of the asteroid, we can check to make sure everything is ready for the critical observations.