29 March 2018

FSK 500Bd/1000, CIS Navy Akula ("shark")

CIS Akula ("Shark") is a FSK 500Bd/1000 burst waveform used by Russian/CIS Navy in  ship-shore links, most likely by submarines. Akula is one of the most interesting signals you may meet in air: fast, unpredictable and unfrequent; see below for a little story of this signal.
Back to the signal, the waveform consists of FSK bursts modulated at speed of 500Bd and 1000Hz shift (Figure 1). A distinctive sign are the last bits of the demodulated bitstream: a sort of EOM mark "1771/" (Figure 2).

Fig. 1
Fig. 2
I worked several good quality recordings and found that they can be successfully descrambled using the polinomyal x^5+x^3+x+1, after the removal of the scrambler the resulting bitstream exhibits an interesting 6-bit period (Figure 3).

Fig. 3
The same 6-bit period (Figure 4) can be obtained by descrambling the bitstream after differential decoding: in this case the scramble polynomial is x^4+x^3+1 (thanks to KarapuZ).

Fig. 4

Legacy Akula (Shark), or the so-called 49th channel, was originally a ship-shore superfast telegraph system used to transmit reports from submarines, the received transmissions were immediately relayed to HQ Navy on all available communication channels.
Transmissions did consist of ten groups of 5 digits and 0.72 secs in air. The main equipment of Akula is the sensor P-758 and the receiver P-759 (Figs 5,6), with their ancillaries, and appeared in the fleet in the late 50's. In total, more than 4,500 sets were produced. [1]

Fig. 5 - P-758
Fig. 6 - P-759
"In parallel with the development of land-based communication systems of the Navy the technical means with high-speed, security and automation were designed for surface ships and submarines. The experts of the Naval Research Institute of Communication designed special HF very-high-speed (VHS) secured communication link later named Akula (Shark). Then existing systems could not detect and not even saying of taking a bearing of VHS transmissions. In addition thanks to the usage of increased capacity (up to 15 kW) radio transmitting equipment at submarines and a set of geographically distributed land-based receiving radio centers the high-fidelity reception was possible even at range of 8-10 thousand kilometers. Navy commissioning of VHS communication means marked the new qualitative stage in the development of naval communication systems."

There is an interesting story about the so-called "Project Boresight" and “lost” Soviet submarines that confirms the Akula's undetectable feature (thanks to Dave for the link):

Akula, with minor variations ("Dolphin", to be precise), is still used for long-range operational and near operational-tactical communications of the Russian Navy, perhaps the P-758IS equipment is used (Figure 7).

Fig. 7 - P-758IS

Frequencies (all CFs):

3399 4414 4882 5338 5555 5784 6772 6836 6852 6864 6908 6920 7316 7620 
7690 7734 7674 7748 8300 8500 9155 9202 9264 9372 9955 9628 10116 10192
10208 10314 10478 10659 10664 10816 10860 10888 10928 11024 11155 12312
12368 12693 13146 14266 13404 13406 14206 14208 14266 14840 14860 16104
16248 16264
Thanks to Dave who collected the logs.


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