The TDR-94D Transponder replies to ground-based Secondary Surveillance Radar interrogations. The TDR-94D provides air traffic controllers with aircraft position identification, altitude and enhanced surveillance data. The TDR-94D is compatible with TCAS II receiver/transmitters that comply with RTCA document solid-state, crystal-controlled receiver/transmitter specifically designed for TCAS-II. The TCAS II system installations requiring antenna diversity.The TDR-94D Transponder is a Diversity version; i.e., it is capable of transmitting and receiving on either of two antennas. The TDR-94 is a non-diversity model. The TDR-94/94D can receive and send data link messages that are required for ATC automation. The added data link capability allows the transponder to perform additional air traffic control and airplane separation assurance (ASA) functions. The TDR-94/94D is an integral part of the Air Traffic Control Radar Beacon System (ATCRBS).

The TDR-94 or TDR-94D Transponder is normally mounted in a MMT-150 Modular Mounting Tray and controlled by an ATC control like the CTL-92/92A Control.  The TDR-94/94D operates in mode A by responding to normal beacon radar interrogations. This mode provides identification of transponder reequipped airplanes on the ground controller’s plan position indication (PPI). If the installation is equipped with a compatible altitude encoder, it can operate in mode C and provide altitude data long with the normal reply. In the mode S scheme, each airplane is assigned a unique identification code which allows automatic and unique interrogation and display of a specific airplane.