||A maximum of 128
bits is transmitted to allow for the transmitter to key. If the
transmitter keys up and settles on frequency quickly, it is possible
to see most of the 128 bits, if the transmitter is slow then much
less than 128 bits is seen. The received signals therefore does
not always contain all 128 bits during this pre-key sequence. The
sequence is also designed to compensate for variable response times
of receiver squelches and AGC settling times. At 2400bps, 128 bits
equates to about a 20th of a second (53.333mS to be precise).
||16 bits of alternate
bits to allow the decoder to lock into the bit speed.
||2 bytes with a
unique bit pattern allows the decoder to precisely character syncronise
to the following 8 bit bytes.
||Start of header
||Type of transmission.
(AGCS-1, AGCS-D etc).
number. Filled with a special sequence if a squitter burst.
acknowledged. Advises other station if a re-send is necessary.
type of message being sent. Usually a letter and a number. See
||Single byte which
increments on each successful transmission.
||If STX present,
then text message follows.
number. Usually made from the current time (minutes/seconds). Only
transmitted in downlink data blocks.
international flight number. Only transmitted in downlink data blocks.
||The text message
itself. Just as with AX25 packet, the actual length is variable
but limited to a maximum of 220 characters.
||End of text marker.
If ETB is transmitted instead then more text follows in a subsequent
||End of packet
block marker byte.