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R 1EFF : 001-049, 101-105, 151-199, 1 34-52-00Page 38 1 1 May 01/05 1 1 1CES 1 The discrete addressing and digital encoding of Mode S transmissions permit their use as a digital data link. The interrogation and reply formats of the Mode S system contain sufficient coding space to permit the transmission of data. These data transmissions may be used for air traffic control purposes, air-to-air data interchange for collision avoidance, or to provide flight advisory services such as weather reports, or Automated Terminal Information System (ATIS). Most Mode S data link transmissions will be handled as one 56-bit message included as part of a long 112-bit interrogation or reply. These transmission include the message in addition to the surveillance data. Longer messages are transmitted using the Extended Length Message (ELM) capability. The ELM is capable of transmitting up to sixteen 80-bit message segments, either ground-to-air or air-to-ground. The ELM can be acknowledged with a single reply or interrogation. ELMs uplinked need not be replied to individually, but can be acknowledged in a reply containing a summary of the received interrogations. ELMs do not contain surveillance data. The following table gives the definition of uplink format message fields:

R 1EFF : 001-049, 101-105, 151-199, 1 34-52-00Page 39 1 1 May 01/05 1 1 1CES 1 -------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | -------------------------------------------------------------------------------| AP | address parity | 24-bit address and parity overlaid | | | | | | AQ | acquisition | differentiates acquisition from non-acquisi-| | | | tion interrogations | | | | | | DI | designator | identifies the coding contained in the SD | | | identification | field | | | | | | II | interrogator | identifies the interrogator | | | identification | | | | | | | MA | message | 56-bit MA field contains messages directed | | | Comm A | to the A/C | | | | | | MC | message | 80-bit MC field contains one segment of a | | | Comm C | sequence of segments transmitted in ELM mode. | | | | | | MU | message | 56-bit MU messages contains information used | | | Comm U | in air-to-air exchanges | | | | | | NC | number of C | number of segments transmitted in ELM mode | | | segments | | | | | | | PC | protocol | operating commands of the transponder | | | | | | PR | probability | contains commands to the transponder to | | | of reply | specify the reply probability to the mode S | | | | only all-call interrogations (Allows | | | | operators to acquire closely spaced A/C). | | | | | | RC | reply | designates the transmitted segment as initial,| | | control | intermediate or final | | | | | | RL | reply | indicates if message is short or long | | | length | | | | | | | RR | reply | length and content of the reply information | | | request | requested by the interrogator | | | | | | SD | special | contains control codes affecting the link | | | designator | protocol | -------------------------------------------------------------------------------

R 1EFF : 001-049, 101-105, 151-199, 1 34-52-00Page 40 1 1 May 01/05 1 1 1CES 1 The table below gives the definition of downlink format message fields:

 -------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | -------------------------------------------------------------------------------| AA | address | contains the A/C mode S address | | | announced | | | | | | | AC | altitude | information containing the A/C altitude | | | code | | | | | | | AP | address | 24-bit address and parity overlaid | | | parity | | | | | | | CA | transponder | reports transponder capability | | | capability | | | | | | | DR | downlink | requests extraction of downlink messages from | | | request | the transponder by the interrogator | | | | | | FS | flight | flight status | | | status | | | | | | | ID | identification | contains the Mode A identification code | | | | | | MB | message | 56-bit MB field contains the message | | | Comm B | transmitted to the interrogator | | | | | | MD | message | 80-bit MD field contains one segment of a | | | Comm D | sequence of segments in the ELM Mode | -------------------------------------------------------------------------------
(4) Operation (Ref. Fig. 006) The transponder operation is under the control of a processor which drives all the transponder functions and performs the self monitoring. Each function or module has dedicated tasks, exchanges data with the processor and receives commands. The transponder consists of nine main sections.
 (a) Antenna switching circuit This circuit ensures the connection with the antennas. It transmits and filters the Radio Frequency (RF) signals from the antennas to the receiver circuit. This circuit switches to the top or bottom antenna according to the signal received from the
R 1EFF : 001-049, 101-105, 151-199, 1 34-52-00Page 41 1 1 May 01/05 1 1 1CES 1 antenna selection circuit. The interrogation or reply signals are routed to the antennas via coaxial cables.
　
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