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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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