STANAG 4481 defines the minimum technical standards for Naval shore-to-ship broadcast HF communication. The standard describes three data modem variants, namely the S4481-PSK Serial Tone Mode (STM) providing a BPSK modulated waveform offering data rate of 300 bps coded, the S4481-FSK Single-Channel Two-Tone FSK Mode providing data rates from 50 bps to 600 bps, and the S4481-FSK Multi-Channel (up to 16) Two-Tone FSK Mode providing date rates of 50 bps and 75 bps in a 3 kHz channel. The STANAG 4481-PSK waveform is a Non-Autobaud capable STM (Serial Tone Mode) waveform providing the BPSK modulated 300 bps coded data modem in a 3 kHz channel. The STANAG 4481-PSK modem waveform is identical to the 300 bps long interleaver (10.240 seconds) waveform of STANAG 4285. An example of the S4481-PSK waveform is the fleet brodcast transmitted on 6243.7 KHz/USB by the Royal Norwegian Navy (Figure 1).
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| Fig. 1 |
While the ACF bitmap of Figure 1 clearly shows the classic S-4285 framing, Figure 2 shows the BPSK modulation: notice that some decoders such as Sorcerer and K500 show the constellation related to the type of data modulation (BPSK, QPSK, PSK8) and NOT the "on-air" constellation (usually PSK8).
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| Fig. 2 - BPSK modulation at 300 bps Long interleaver |
User data, at least in this case, are encrypted using KW-46 (or equivalent device) given the presence of the M-sequence generated by the polinomyal x^31 + x^3 +1 (KW-46T uses that M-sequence to synch the KW-46R receive devices).
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| Fig. 3 - presence of x^31 + x^3 +1 M-sequence |
Direction Finding tries (TDoA algorithm) seem to indicate the area of Stavanger as the probable site of the transmitter.
