They make the whole system work
By Jim Sparks
Air Traffic Control (ATC) has jokingly been called "an organization funded by the railroads to discourage travel by air." However, considering the total number of aircraft operations daily and the rare incidents of collisions, it is evident that the system works! One device makes all this possible - the transponder.
A transponder will send an identifying coded signal in response to a transmitted interrogation from a ground-based radar station. An air traffic controller can then view the identified blip on a screen and know who it is and provide direction to the flight crews maintaining adequate separation with other blips. More recent versions enable aircraft to recognize other aircraft in the area and subsequently provide alerts to the flight crew so they can avoid potential hazards.
Transponder components on aircraft include a receiver-transmitter, control head, digitizer, and antenna.
On the ground, primary surveillance radar transmits a narrow radio frequency (RF) beam using a rotating antenna. Any target in the beam path will reflect some energy. The elapsed time between the transmission and the energy return can then be calculated and by taking into account antenna position a precise bearing and distance can be displayed on a two-dimensional radar screen. A secondary surveillance radar (SSR) provides the controller with aircraft identification and altitude information. This is accomplished by transmitting groups of pulses. The first provides aircraft identification data and the second gives altitude information.
The basis of TCAS
Transponder Mode "A" is the reference for identification and Mode "C" is the term applied to altitude information. And Mode "S" is now used to enable aircraft to have in-flight communication with other aircraft permitting position and course comparisons. This becomes the basis for a Traffic Collision Avoidance System (TCAS).
Codes distinguish each aircraft. They are typically assigned prior to flight and entered in the transponder control head by a flight crewmember. Certain codes are used when an aircraft is in distress such as 7700 or 7777, and 1200 represents that the aircraft is flying using Visual Flight Rules (VFR). This is recognized by the ground-based equipment and is then highlighted on the air traffic controller's screen making them aware of the situation.
Altitude encoding is a feature associated with transponder Mode C and the response uses a gray code also known as a Gilham code. This is a special binary format using 11 pulses and is located between the main pulses of the transponder output signal. Each pulse represents a specific altitude increment.
Received interrogation signals are passed through a detection circuit called a duplexer within the transponder which will control the switching of the antenna from receive to transmit. The signal is further monitored and conditioned to make it electrically manageable and to validate the pulse pair groups and ensure the aircraft does not respond to an inaccurate signal.
Once the pulses are determined to be valid they are passed on to a decoder. It is here that the determination is made regarding the mode of operation. It is the decision at this point that will decide the format of the transponder response to the ground-based equipment. While the reply is being prepared a suppression signal is also generated to other L band equipment installed on the aircraft. This will include devices such as Distance Measuring Equipment (DME) and is used to block or deactivate the receivers within these systems during the short response transmission.
Transponder systems include an "IDENT" switch. This flight deck located device can be selected by the flight crew at the request of an air traffic controller. Activation will cause the identification of the aircraft on the controller's screen to enlarge allowing the aircraft to stand out on the display. This is accomplished by adding an additional pulse to the transmission and is often referred to as a SPIP.
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