In air navigation, a flight transponder (transmitter-responder) is a device that emits an identifying signal in response to an interrogating received signal. Transponders are used as Identification of Friend or Foe (IFF) systems in military aviation, and also in Air Traffic Control (ATC) Secondary Surveillance Radar (beacon radar) systems for general aviation and commercial aviation.

When the transponder pulse is high (1), the ELT remains off, and when the transponder pulse is low (0), the ELT gets switched on.

Primary radar is limited by terrain and weather, and also detects unwanted objects such as automobiles, hills, and trees. It also cannot estimate the attitude of an aircraft. Secondary Surveillance Radar (SSR) over-comes these limitations, but it depends on a transponder in the aircraft to respond to interrogations from the ground station in order to make the plane more visible. Depending on the type of interrogation, the transponder sends back a transponder code (or “squawk code”) or altitude information to maintain separation between planes. Mode S (Mode Select) is designed to help avoid over-interrogation of the transponder (having many radars in busy areas), and to allow automatic collision avoidance.

Transponders automatically transmit a unique four-digit code when they receive a radio signal sent by radar. The code gives the plane's identity, and radar stations establish speed and direction by monitoring successive transmissions. This data is then relayed to air traffic controllers. But when the transponder shuts down due to a technical glitch or other reason, unpredictable loss of human lives, nature, and economy may occur. These conditions have necessitated active research to design a better and more effective system.

Presently, other parallel systems to track an aircraft in case of transponder failures are:

  1. Automatic Dependent Surveillance - Broadcast (ADS-B): A cooperative surveillance technology in which an aircraft determines its position via satellite navigation and periodically broadcasts it, enabling it to be tracked. ADS-B is automatic in that it requires no pilot or external input.
  2. Global Navigation Satellite System (GNSS) aircraft tracking solutions: Such systems can transmit the GNSS position of the aircraft to a server on the ground via a communication network. The server collects the data and makes it available to aircraft operators.
  3. Transponder killing (shutdown): This will not necessarily make a flight invisible. Coverage will lapse and vary, but ATC radar can often portray a transponder-less plane as a primary target — a blip that provides the aircraft's position and speed, but not its altitude.

Redundant Transmitting System in Aircraft (RTSA) can effectively transmit an emergency signal during mishaps occurring due to lack of proper safety systems. The proposed RTSA system uses NOT Gate and Emergency Locator Transmitter (ELT) at a frequency that is readily available (121.5, 243, or 406 MHz). The figure shows that the digital pulse of the transponder when high (1) does not actuate the ELT, but when the digital pulse is low (0), the ELT is actuated. The system is completely automated by actuation, and is independent of any type of human interference and control.

A traditional radar system and ADS-B cannot be independent from an aircraft or from the flight crew. The primary radar blip is not completely reliable since it also detects unwanted objects. GNSS can cause interference problems, which can only be solved using robust receivers, making the system more complex. A technical malfunction/power loss or a voluntary disconnection will render an air traffic controller completely blind. In such a situation, RTSA will prove reliable.

Parameters such as minimal power consumption for actuation, and resistance to vibration, shock, and radiation were considered. With the incorporation of the RTSA system, a significant drop is expected in loss of life, search operation costs, and aircraft damage. Recent incidents of aircraft disappearing without a trace necessitate an auto-control redundant system that can avoid such mishaps.

The RTSA system is designed to maintain independence from external influences. It provides simplicity in design, installs easily, is free from signal interference, and is comparatively cost-effective.

This article was adapted from SAE Technical Paper 2015-01-2443. To obtain the full technical paper and access over 200,000 resources for the aerospace, automotive and commercial vehicle industries, visit the SAE MOBILUS site at: here .

Aerospace & Defense Technology Magazine

This article first appeared in the June, 2017 issue of Aerospace & Defense Technology Magazine.

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