Primary Surveillance Radar sends a signal out and anything that reflects the signal back shows up on the radar display.
An aircraft must be in line of sight of the radar antenna to be displayed, so the radar range is usually increased by installing the antenna at higher elevations above all obstacles.
Secondary Surveillance Radar (SSR) incorporates a ground based antenna that transmits coded pulses which are received by aircraft transponders. The aircraft transponder sends a response back to the ground unit with various bits of information.
The information sent by the transponder depends on the transponder mode selected by the pilot:
The term ‘squawk’ is used to describe the 4 digit code selected on a transponder. For example, air traffic control may instruct you to “Squawk 4551”.
There are some transponder codes that are used in specific circumstances:
It is mandatory to have an SSR transponder in certain circumstances, such as in Transponder Mandatory Zones (TMZs), in some controlled airspace, and when flying at or above FL100.
If your aircraft is fitted with a serviceable Mode C transponder, it is mandatory for you to continuously operate it unless otherwise dictated by ATC.