Primary Radar antenna rotates (usually at 5-12 rpm) emits a pulse of radio wave.
Upon reaching an aircraft (or other object) the wave is reflected and some of the energy is returned to the antenna.
The PSR output data uses polar coordinate system: it provides range and bearing of the targets found in respect of the antenna position. The range is determined by the time difference of the emitted and received pulse (the speed of propagation is the speed of light) and the bearing is obtained from the antenna azimuth. Some examples of radars that use the primary radar principle:

• ATC area or Aerodrome Surveillance Radar

• Airborne Weather Radar (AWR)

• Airport Surface Movement Radar

• Precision Approach Radar (PAR)

Secondary Radar requires an airborne transponder which responds to the receipt of a pulse from a ground-based antenna by transmitting a return signal.
Because the transponder transmits a much stronger signal than that which is reflected off an aircraft in primary radar systems, greater range and reliability can be achieved with secondary radar and cheaper and more efficient ground equipment can be used. Additionally, information such as altitude and a code can be added to the returned signal from the transponder which is then displayed on the operator’s screen.

Some examples of radars that use the secondary radar principle:

• Secondary Surveillance Radar (SSR)

• DME