The author describes this mode as a 42FSK modulation, would it not be MFSK at 42 tone? So this is why I classify this mode under FSK type of modes.
ISCAT messages are free-form, up to 28 characters in length. Modulation is 42-tone frequency-shift keying at 11025 / 512 = 21.533 baud (ISCAT-A), or 11025 / 256 = 43.066 baud (ISCAT-B). Tone frequencies are spaced by an amount in Hz equal to the baud rate. The available character set is:
Transmissions consist of sequences of 24 symbols: a synchronizing pattern of four symbols at tone numbers 0, 1, 3, and 2, followed by two symbols with tone number corresponding to (message length) and (message length + 5), and finally 18 symbols conveying the user’s message, sent repeatedly character by character. The message always starts with @, the beginning-of-message symbol, which is not displayed to the user. The sync pattern and message-length indicator have a fixed repetition period, recurring every 24 symbols. Message information occurs periodically within the 18 symbol positions set aside for its use, repeating at its own natural length.
For example, consider the user message CQ WA9XYZ. Including the beginning-of-message symbol @, the message is 10 characters long. Using the character sequence displayed above to indicate tone numbers, the transmitted message will therefore start out as shown in the first line below:
0132AF@CQ WA9XYZ@CQ WA9X0132AFYZ@CQ WA9XYZ@CQ W0132AFA9X ...
sync## sync## sync##
Note that the first six symbols (four for sync, two for message length) repeat every 24 symbols. Within the 18 information-carrying symbols in each 24, the user message @CQ WA9XYZ repeats at its own natural length, 10 characters. The resulting sequence is extended as many times as will fit into a Tx sequence.
ISCAT-A, 21.5 baud ,Bandwidth 905hz ,TX duration is 1.716 secs
ISCAT-B, 43.1 baud, Bandwidth 1809hz, TX duration is 0.588 secs