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R  bits are overlaid. In the all-call reply, the 24 parity check 
R  bits are overlaid on the Mode S interrogation address and the 
R  transponders' discrete address is included in the text of the 
R  reply. The main function of Mode S is surveillance. To 
R  accomplish this function, the Mode S transponder uses the 
R  56-bit 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). Most 
R  Mode S data link transmissions will be handled as one 56-bit 
R  message included as part of a long 112-bit interrogation or 
R  reply. These transmission include the message in addition to 
R  the surveillance data. 
R  Longer messages are transmitted using the Extended Length 
R  Message (ELM) capability (if external ADLP is installed). The 
R  ELM is capable of transmitting up to sixteen 80-bit message 
R  segments at a rate of 8 per second, 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 
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  a_  Mode S interrogation message formats
R  All valid Mode S interrogation are listed here. 
R  The first 5 bits of the message indicate a UF message 
R  format. 
R  Items in brackets, e.g (AP:24) represent the "AP" field 
R  code, and the number of bits in the field "24". 
R  Underscored numbers represent unused fields and must not be 
R  processed. 
R 
R  - TCAS Surveillance: 
R  UF=0 (0 0000) 3_ (RL:1) 4_ (AQ:1) 18_ (AP:24) 
R 
R  - Surveillance, Altitude Request: 
R  UF=4 (0 0100) (PC:3) (RR:5) (DI:3) (SD:16) (AP:24) 
R 
R  - Surveillance, Identity Request: 

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R  UF=5 (0 0101) (PC:3) (RR:5) (DI:3) (SD:16) (AP:24) 
R 
R  - Mode S All-Call Interrogation: 
R  UF=11 (0 1011) (PR:4) (II:4) 19_ (AP:24) 
R 
R  - Long TCAS Interrogation (COMM-U): 
R  UF=16 (1 0000) 3_ (RL:1) 4_ (AQ:1) 18_ (MU:56) (AP:24) 
R 
R  - COMM-A Interrogation, Altitude: 
R  UF=20 (1 0100) (PC:3) (RR:5) (DI:3) (SD:16) (MA:56) 
R  (AP:24) 
R 
R  - COMM-A Interrogation, Identity: 
R  UF=21 (1 0101) (PC:3) (RR:5) (DI:3) (SD:16) (MA:56) 
R  (AP:24) 
R 
R  - Uplink Extended Length Message (UELM)/COMM-C 
R  Interrogation: 
R  UF=24 (1 1) (RC:2) (NC:4) (MC:80) (AP:24) 
R 
R  NOTE : PC, RR, DI and SD subfields are undefined for____ 
R  UF=20/21 broadcast interrogations. 
R 
R  b_  Mode S reply message formats
R  All valid Mode S replies are listed here. 
R  The first 5 bits of the message indicate a UF message 
R  format. 
R  Items in brackets, e.g (AP:24) represent the "AP" field 
R  code, and the number of bits in the field "24". 
R  Underscored numbers representing unused fields must be set 
R  to zero. 
R 
R  - TCAS Surveillance: 
R  DF=0 (0 0000) (VS:1) 2_ (SL:3) 2_ (RI:4) 2_ (AC:13) 
R  (AP:24) 
R 
　
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