R 1EFF : 051-099, 1 34-52-00Page 7 1 1 May 01/05 1 1 1CES 1 The transponder contains data link capability, which allows it to receive uplink messages and uplink extended length messages. The transponder can transmit Downlink Messages and Downlink Extended Length Messages (DELM). The transmitter has the capability of transmitting sets of 16 segment DELMs at a rate of 8 per second.
R **ON A/C 106-149, 221-299, 301-399, 401-499,
A. Principle (Ref. Fig. 002) An airborne transponder provides coded reply signals in response to interrogation signals from the ground secondary radar and from aircraft which is equipped with the TCAS. The ATC/Mode S system has the capability for Modes A, C and S. Mode A: identification Mode C: altitude range Mode S: In a Mode S environment, the transponder responds to ATCRBS-formatted interrogations with ATCRBS-formatted replies, but, in addition, it can be discretely addressed directly by Mode S interrogators. Discrete addressing means that only the specifically addressed Mode S transponder replies. Top and bottom antennas and associated antenna switching and signal processing features provide the Diversity Antenna features that allow compatibility with TCAS II. The diversity antenna allows selection of signal receptions from either the top or the bottom antenna based on the characteristics of the received interrogation signals. The transponder selects the antennas (lower or upper) which receive the best transmission signal from the ground ATC secondary radar . This improves air-to-air surveillance and communication. The ATC/Mode S transponder, is also used in collision avoidance. In this role, the transponder performs as an ATCRBS and Mode S transponder for ATC requirements, while simultaneously serving as an integral part of TCAS. The Mode S transponder has two basic functions related to TCAS:
(1)
Approximately every second, a Mode S transponder makes a transmission known as squitter. These transmissions alternate between the top and bottom Mode S antennas. Squitter is coded with the aircraft Mode S address and equipment capabilities. A ground station and airborne TCAS listen for squitter as the initial indication that a Mode S-equipped aircraft has entered the surveillance area.
(2)
The communication between two TCAS-equipped aircraft occurs from the TCAS antenna of one aircraft to the Mode S transponder antenna of the other aircraft. Aboard the TCAS-equipped aircraft, data received by the Mode S transponder is passed to a TCAS computer, and the TCAS computer determines the content of reply messages. The Mode S reply includes the flight number transmitted by the Flight Management and Guidance Computer (FMGC).
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