Primary Radar works on the echo principle that uses strong transmitted signals at a known time. These signals reflect from objects and the echoes are detected.

The Secondary Radar works on the principle of the "interrogation and response" technique. It uses a ground-installed transmitter and receiver that is the interrogator component of the system and the aircraft is also equipped with a transmitter and a receiver. There is therefore a two-way communication, unlike the echo principle.

• The Secondary Surveillance Radar (SSR) has several advantages over the primary radar. The major advantage being less power is required for the aerial. Since the minimum transmission power requirement is based on the distance to the aircraft, power echo creation is not necessary. The transmission completed by the transponder also results in stronger signals compared to the primary radar, meaning that the receiver sensitivity may be lower. These two factors combined lead to a smaller aerial requirement with lower installation costs.