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.
Transmitting signals
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.
What's required
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.
After creating a coded response including identification and altitude information, the data is sent to be imposed on the radio wave that will carry it back to the ground station. Then the duplexer accomplishes the switching enabling the transmitter to couple with the antenna. This process takes about two microseconds from the time the initial interrogation is received. Once the signal reaches the SSR antenna on the ground the information is decoded, analyzed, and then displayed in front of the controller.
Most systems contain added protection that would prevent excessive interrogations. This means the transponder will reply to a maximum of 2,000 requests per second. The normal interrogation rate is about 400 per second.
The Mode S transponder is an enhancement to the air traffic control process. This feature provides a two-way digital data link for either air to ground or air to air communication. Airborne Separation Assurance (ASA) occurs when the system is interfaced with a Traffic Collision Avoidance System (TCAS). The purpose is to allow a TCAS equipped aircraft to fly within a continuously monitored airspace, allowing the flight crew to acquire and locate other aircraft that may pose a threat. This is accomplished by enabling a TCAS outfitted aircraft to transmit an encoded "All Call" request to any surrounding aircraft using a transponder.
When a nearby aircraft replies to the interrogation it is received and additional antennas can locate the source of the signal on a two-dimensional display. In addition if the target aircraft is using a Mode C or Mode S transponder, vertical data is added further aiding the pilots in recognizing a potentially dangerous condition. Traffic Alerts are displayed 40 seconds prior to a close encounter and a Resolution Advisory (RA) is issued by the TCAS about 25 seconds before the anticipated closest point in the paths of the two aircraft. Some aircraft contain dual transponders. One is operating and the other is used only if the primary unit should fail. Some of these installations will use a Mode S transponder as the primary and a Mode C as the backup. Here the TCAS system will only function with the primary unit in operation.
Regulations and maintenance
Federal Aviation Regulation (FAR) 91.413 addresses transponders and it reads as follows: "No person may
