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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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