priyom.org

Protocol

F01 is based on the ITA2 alphabet. At the normal settings, it uses the speed of 200 bd, the shift of 500 Hz, 1.5 stop bits, the mark frequency of -250 Hz, and the space frequency of +250 Hz.

Block format

The payload, 5-digit groups, is packed into 25-character blocks, which are sent directly one after another without line breaks, of the following fixed structure:

=8251  65380484843868036 
=8462  30549583192393891 
=7853  29392548929506269 
=8554  549410102838013150
=8635  516060140587158102
...
)69729 0608400000+++++237
ABBBCCCDDDDDEEEEEFFFFFGGG
  • A: Indicates the block type, and takes the form of one of the following characters:
    • '=' indicates that the message will continue in the next block.
    • ')' indicates that this is the last block of the message.
  • B: Unknown. Related to error detection. Always 3 digits.
  • C: Block number. Counted from 1. Padded with spaces from the right if shorter than 3 digits.
  • D: First 5-digit group of the 3-group fragment contained by the block.
  • E: Second 5-digit group of the 3-group fragment contained by the block.
  • F: Third 5-digit group of the 3-group fragment contained by the block.
  • G: Unknown. Related to error detection. Padded with spaces from the right if shorter than 3 digits.

If this is the last block of the message, the D, E, and/or F slots can be replaced by +++++ if there is no group to place in them. Decoding a message comes down to putting together the 3-group fragments from the blocks in the right order.

There is also an optional block of different format that precedes all the other blocks, and contains a 5-group header in the common Russian intelligence format:

(2111110009777423182507200839 
ABBBCCCCCDDDDDEEEEEFFFFFGGGGG
  • A: Always '(' - matches ')' from the last block.
  • B: Unknown. Related to error detection. Always 3 digits.
  • C: First group of the header: message type. F01 usually uses 11177, sometimes 11100.
  • D: Second group of the header: link ID, fixed per schedule.
  • E: Third group of the header, unknown.
  • F: Fourth group of the header. Contains the message date as day of the month in the first two digits, always zero as the third digit, and the serial number between 01 and 99 inclusive in the last two digits.
  • G: Fifth group of the header: group count in the first 4 digits.

All blocks are repeated through the broadcast for 6-8 minutes for redundancy.

Null messages

Null messages are represented by this single block repeated for about 6 minutes:

)5761  00000++++++++++162

Multiple messages

It is possible to transmit more than one message in the same broadcast with F01. In that case, the total group count is filled up with the group 00000 until it is a multiply of 10, then followed by the next message.

If a full 5-group header was included, only the header of the first message in the TX is distinguished using a separate block, while in the following messages it is mixed with normal message groups.

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