Primary Surveillance Radar (PSR): The 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.
Note that the range is the slant distance from the antenna and not the horizontal distance.
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.
The rotation speed of the antenna is usually between 5 and 12 rpm.
The antenna radiation patern is a narrow beam when seen from above and, whith some approximation, can be considered as a trapezium if seen from the side.

Secondary Surveillance Radar (SSR) 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,
reater 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.

In Mode A the aircraft's transponder provides positive aircraft identification by transmitting a four-digit code to the ground station.
The code system is octal; that is, each of the code digits may be any of the numbers 0-7.
There are thus 4096 possible four-digit codes.

In Mode C the aircraft's altitude, derived from on-board instruments, is transmitted to the ground station in addition to the identity.
Aircraft equipped with transponders supporting Mode S are assigned a permanent identification which can be selectively addressed by the ground radar.
This reduces problems of garbling between SSR returns from aircraft in close proximity.
Mode S also offers a wider range of data to be transmitted, including potentially an uplink of data from the ground station to the aircraft.