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Radar transponders

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Primary and secondary radar

Primary radar is a system where the ground-based antenna transmits a radar pulse, then listens for the small amount of return energy that is reflected from an aircraft. The time delay between the transmission of the pulse and the receipt of the reflected return is a measure of the range.

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.

Traffic alert and collision avoidance system (TCAS) is an airborne system which is capable of interpreting the transponder returns of nearby aircraft and displaying the positions of these aircraft on a cockpit display. TCAS can warn the crew of impending collisions and advise avoidance manoeuvres provided it receives the altitude information from nearby aircraft. For this reason, mode C (the ALT selection on a typical transponder) should always be selected by all aircraft outside controlled airspace.

TCAS is fitted to most commuter aircraft that operate in Class D, E and G airspace. It is therefore in everybody’s interest for all VFR transponder equipped aircraft in Class E or G to squawk code 1200 with ALT selected.

ADS-B approval and operations

Automatic dependent surveillance–broadcast (ADS-B) is a means by which aircraft, aerodrome vehicles and other objects can automatically transmit or receive data such as identification, position and additional data, as appropriate, in a broadcast mode via a data link. An aircraft, which is fitted with serviceable ADS-B transmitting equipment that complies with an approved equipment configuration, must operate the equipment continuously during the flight in all airspace at all altitudes unless the pilot is directed or approved otherwise by ATC.

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