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R the Comm C format. ELM transmittals to the ground use the Comm
R D format (Note: Comm C and Comm D are future applications;
R Comm D is not available with this transponder). All discrete
R Mode S interrogations and replies (except the all-call reply)
R contain the 24-bit discrete address of the Mode S transponder
R upon which 24 error detection parity check bits are overlaid.
R In the all-call reply, the 24 parity check bits are overlaid
R on the Mode S interrogation address and the transponders'
R discrete address is included in the text of the reply.
R The main function of Mode S is surveillance. To accomplish
R this function, the Mode S transponder uses the 56-bit
R transmissions (in each direction). In the 56-bit
R transmissions, the aircraft reports its altitude or ATCRBS
R 4096 code, and the flight status (airborne, on-ground, alert,
R Special Position Identification (SPI), etc.
R The discrete addressing and digital encoding of Mode S
R transmissions permit their use as a digital data link. The
R interrogation and reply formats of the Mode S system contain
R sufficient coding space to permit the transmission of data.
R These data transmissions may be used for air traffic control
R purposes, air-to-air data interchange for collision avoidance,
R or to provide flight advisory services such as weather
R reports, or Automated Terminal Information System (ATIS).
R Most Mode S data link transmissions will be handled as one
R 56-bit message included as part of a long 112-bit
R interrogation or reply.
R These transmissions include the message in addition to the
R surveillance data.
R Longer messages are transmitted using the Extended Length
R Message (ELM) capability. The ELM is capable of transmitting
R up to sixteen 80-bit message segments, either ground-to-air or
R air-to-ground. The ELM can be acknowledged with a single reply
R or interrogation. ELMs uplinked need not be replied to
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R individually, but can be acknowledged in a reply containing a
R summary of the received interrogations.
R ELMs do not contain surveillance data.
R The following table gives the definition of uplink format
R message fields:
R -------------------------------------------------------------------------------
R | DESIGNATOR | FIELD | INDICATION |
R -------------------------------------------------------------------------------
R | AP | address parity | 24-bit address and parity overlaid |
R | | | |
R | AQ | acquisition | differentiates acquisition from non-acquisi- |
R | | | tion interrogations |
R | | | |
R | DI | designator | identifies the coding contained in the SD |
R | | identification | field |
R | | | |
R | II | interrogator | identifies the interrogator |
R | | identification | |
R | | | |
R | MA | message | 56-bit MA field contains messages directed |
R | | Comm A | to the A/C |
R | | | |
R | MC | message | 80-bit MC field contains one segment of a |
R | | Comm C | sequence of segments transmitted in ELM mode. |
R | | | |
R | MU | message | 56-bit MU messages contain information used |
R | | Comm U | in air-to-air exchanges |
R | | | |
R | NC | number of C | number of segments transmitted in ELM mode |
R | | segments | |
R | | | |
R | PC | protocol | operating commands of the transponder |
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