• Replies to ATC radar on Modes A, C and S with altitude and ident
• Antenna diversity as required for TCAS II (TDR-94D)
• Data link capability for TCAS II crosslink and ATC data communications
• ICAO Level 3A for receiving extended length messages
• Capable of GPS/FMS position reports ADS-B
• Provisions for ICAO FANS
• High reliability with crystal controlled solid-state transmitter
• Flexible air data interfaces for compatibility with all aircraft
• Diagnostics for easy fault isolation and shop maintenance
• Provides new Mode S Elementary and Enhanced Surveillance functionality
• Optional ADS-B capability provides non-radar area surveillance

The TDR-94D transponder is the airborne portion of the Air Traffic Control Radar Beacon System. The active transponder replies to ground based Secondary Surveillance Radar (SSR) interrogations to provide air traffic controllers with aircraft position identification and altitude.

The transponder also replies to TCAS II interrogations from other aircraft. If a conflict must be resolved between two TCAS II-equipped aircraft, the complementary evasive actions are coordinated via the Mode S digital data link. The TDR-94D is compatible with TCAS II receiver/transmitters that comply with RTCA document DO-185A, also identified as DO-185 Change 7.0.

Finally, the TDR-94D serves as a modem for data link, performance, navigation information and ATC transactions between the aircraft and Mode S secondary surveillance radar.

The TDR-94 and TDR-94D are similar except for the diversity function in the TDR-94D. Two antenna ports and receivers in the TDR-94D service antennas on top and bottom of the aircraft to help ensure line of sight air-to-air data links in a TCAS encounter.

Two transponders are usually installed, but only one is used at a time. The unused transponder is used as a hot standby for system redundancy.

The TDR-94D is an all solid-state, crystal-controlled receiver/transmitter. It receives pulsed and phase-shift modulated interrogations from ground and airborne SSRs and replies with pulse amplitude modulated transmissions at high-peak power.

The transponder is a complex digital terminal with connections to the flight deck controllers, air data systems, TCAS receiver/transmitter and optionally to a flight management system.

The TDR-94 and 94D reply with Mode A (squawk ident), Mode C (barometric altitude) or Mode S in response to the corresponding type of interrogation from the ground station or TCAS II.

Mode S transponders also broadcast, or squitter, their address. The address is a 24-bit identification unique to each individual aircraft. The address is assigned by the country of registry and is usually derived from the aircraft registration number.

The TDR-94D is classified as a Level 3A Mode S transponder according to the International Civil Aviation Organization standards and recommended practices. As a data link terminal, it receives short (Comm A) and long (Comm C) uplink messages and transmits short downlink messages (Comm B). Separate messages may be linked in chains to transmit large amounts of information.

The TDR-94D incorporates features that allow future system growth such as the Aeronautical Telecommunications Network (ATN), TCAS IV and Automatic Dependent Surveillance-Broadcast (ADS-B). Full compliance as an ATN data link requires addition of a Transponder Interface Function (TIF).

The TDR-94D incorporates some TIFs via high-speed ARINC 429 ports for Global Positioning System (GPS) and Flight Management System (FMS) use. With these interfaces, the TDR-94D facilitates long-range surveillance without a SSR. Air data and FMS-derived data are available to ATC via a Mode S interrogation/reply data link, reducing voice communications.

The TDR-94D incorporates standard interfaces for uplink of differential GPS corrections with formats compatible with ARINC 743 (GPS). The signal-in-space is in accordance with RTCA requirements.

Software upgrades to meet the requirements for European Elementary and Enhanced Mode S Surveillance and ADS-B are available.

The TDR-94D is operated from a variety of flight deck devices; the CTL-92 Transponder Control, radio tuning unit or FMS. Transponder-control information is echoed such that information on multiple tuning sources is always coordinated.

The TDR-94D is specifically designed for TCAS II-equipped business aircraft and regional airliners with 28 V DC electrical systems and limited space. The TDR-94D is housed in a package specially designed for efficient heat transfer and resistance to high intensity radiated fields.

The TDR-94D accepts either ARINC 429 or Commercial Standard Data Bus control and altitude information that facilitates flexible system design. ARINC 575 and Gillham (encoder) altitude are also accepted. Data link interfaces are high-speed ARINC 429 digital buses.