A transponder is a transmitter that sends out high-frequency radio signals that can be picked up by radar and by on-board collision avoidance systems. The signals tell air traffic controllers and other pilots that another aircraft is nearby.
Radar can pick up aircraft without transponders – they appear as a small dot on the screen – but transponders amplify the signal and provide a four-digit code that makes tracking easier.
A Mode C transponder responds to a hit by the radar beam by transmitting the aircraft’s identifying number, altitude, speed and course. This allows air traffic control to keep track the craft’s position and course and keep it safely separated from other planes in the vicinity.
However, because of their sleek design and the fact that primary radar systems “screen out” clutter caused by reflections off stationary objects such as mountains, gliders without transponders are often invisible to air traffic controllers and other pilots.
A major problem glider pilots face is that even very basic Mode A transponders – which respond with just a four-digit code – need power to operate. Although solar panels can be installed on gliders, batteries available to run transponders add unwanted weight and bulk.
Because low-priced transponders that require low power to operate and that are suitable for gliders are not currently available in the U.S., some sailplane pilots want to wait for a GPS-based system now in the works that will replace radar and transponders. But by that time, more lives could be needlessly lost.
