Radar observations
 

As well as receiving radiation generated by astronomical sources, radio telescopes are used frequently in radar studies of the various bodies of the Solar System. Pulses of radio energy are directed towards the body and then their faint echo is received after a delay depending on the distance of the object. In planetary radar studies, observation of both the time delay between transmitting and receiving the radio pulse and any Doppler shifts in the wavelengths making up the pulse may be combined in building up a radar map of the planetary surface. In the simple case depicted in figure 1, it is assumed that the axis of rotation of the planet is perpendicular to the direction to the observer, this direction CRE cutting the planet’s surface at R.
Then the ring AFBD defines a zone on the planetary surface giving the particular time delay since, within that zone, the planetary surface is at the same distance from the observer. If the planet is rotating on its axis, the reflected pulse will exhibitDoppler shifts. Again there are zones on the planetary surface providing particular values of Doppler shift. For example, the zone NARBS obviously defines a region which is neither approaching nor receding from the observer and so will produce zero Doppler shift.
Areas J and K, however, lie on a zone all points of which are producing the same positive Doppler shift.

Figure 1. The production of a radar map by measuring the echo time and the wavelength spread of a radio pulse.
Although the real case is more complicated than the simple case depicted in the diagram, it can be seen that by examining the reflected pulse very carefully, and selecting particular time delays and Doppler shifts, identifiable points on the surface can be picked out, thus providing the basis of a radar map.
Finally brief comment may be made on the role of radio astronomy in the field of space probes. Without the application of the large radio receivers to pick up the weak signals of data or pictures transmitted by interplanetary probes, their voyaging out into the Solar System would have been futile.