This post goes to complete the one of Christoph where he noticed the presence of LFSR delimited 7-bit frames in STANAG-4285 payloads. Now, given that:
1) the HF waveform STANAG-4285 is largely used for NATO naval broadcasts;
2) the markers consist of the bits of pseudo-random sequence generated by the polynomial x^31+x^3+1, as specified by STANAG-5630 and already seen in FSK 50Bd/850;
3) those bits (termed "Fibonacci bits") are used by KW-46 cryptographic equipment to provide synchronization;
we can assume that NATO naval broadcasts are secured with KW-46 encryption.We analyzed several STANAG-4285 transmissions in both 600L and 1200L sub-modes picked up in well-known frequencies (belonging to British, Danish, Dutch, French, German, Norwegian and Spanish Navy) as:
1200L_10186.2
1200L_10264.1
1200L_13410.1
1200L_7554.6
1200L_9095.0
600L_10733.3
600L_12958.3
600L_13057.7
600L_5361.8
600L_8170.2
600L_8408.2
600L_8612.2
660L_11538.3
and in all we verified the presence of the Fibonacci bits.
By the way, NATO naval broadcast consists of a continuous coded stream of data which is typically be transmitted over multiple HF frequencies at the same time, providing an uninterrupted data flow from shore to ships. In the absence of messages to be transmitted, "jam" messages (pseduo-random chars) is injected into the data stream.