All-call reply

Chapter [chap:intro] shows that Mode S all-call replies can be generated by Mode S transponder to answer Mode S-only all-call interrogations and Mode A/C/S all-call interrogations.

Downlink format 11 is used for the all-call reply, and the length of the messages is 56 bits. The structure of an all-call reply message is quite simple. It only contains four fields, which are shown in Table 1.1.

The fields can be decoded as follows:

• CA: The definition of transponder capability is the same as in ADS-B messages (Table [tb:transponder_capability] in Chapter [chap:adsb-basic]).

• AA: The address refers to the 24-bit transponder address.

• PI: The decoding of PI is similar to the decoding of ADS-B parity (Chapter [chap:adsb_parity]).

For a message with downlink format 11, PI is overlaid with the interrogator identifier.¹ Hence, assuming there is no error in the data, the CRC will produce the interrogator identifier code for all-call replies. The interrogator identifier code can be zero. In this case the message is generated due to a spontaneous acquisition squitter (see Chapter [chap:acas]).

A decoding example is shown as follows:

MSG HEX:  5D484FDEA248F5
MSG BIN:  01011   101    010010000100111111011110  101000100100100011110101
          [DF=11] [CA=5] [AA=484FDE]               [CRC=22]

With the capability value (CA=5), we can see the aircraft has a Level 2+ transponder, with the ability to set CA to 7 and that it is airborne. The CRC remainder indicates the interrogator identifier code is 22.

import pyModeS as pms

msg = "5D484FDEA248F5"
pms.icao(msg)

484FDE