15 December 2018

STANAG-5030/MIL-188-140 VLF/LF multichannel broadcast to submarines (tentative)

The Navy ashore VLF/LF transmitter facilities transmit submarine command and control broadcast which is the backbone of the submarine broadcast system. The VLF/LF radio broadcast provides robustness, availability, global coverage, and has seawater penetrating properties. The 200Hz assigned bandwidth for VLF/LF broadcast and the low efficiency (and narrow bandwidth) of the aerials are limiting factors, but the use of Minimum Shift Keying (MSK), a form of Quadrature Phase shift Keying, can allow optimum use of this narrow bandwidth [1]. 
VLF/LF broadcasts to submarines are STANAG-5030 compliant but unfortunately it's a restricted document so no information is publicy available. Moreover, the new STANAG-4724 is currently being ratified by NATO member states as next evolution.  However, googling the web it's possible to retrieve (few) manufacturers brochures of VLF/LF modulators/demodulators, as the one shown in Fig. 1, and get some informations. These equipments can provide TDM multi-channel broadcast (up to four channels, all 50 baud) and mainly use modulation techiniques as MSK (MSK2 2x50 Baud channels and MSK4 4x50 Baud channels), OQPSK and OOK "on-off keying" (the latter usually associated with the Morse Code).

Fig. 1
Reference MSK modulation indicates zero-crossing transitions (eg +1/+1 to -1/-1 and viceversa, +1/-1 to -1/+1 and viceversa) cannot be allowed if phase discontinuity is to be preserved.

I analyzed some easily receivable VLF stations (DHO38, FTA, FUE, GQD, ICV, JXN, NSY, SXA, ...) and found that the phase-plane of some signals exhibits the expected transitions while others signals show odd transitions. The answer is to be found in the harmonics spectrum of the signals (Fig. 2): when the carrier is missing  the PLL algorithm locks onto one of the two spectral lines and causes the odd transitions shown in the phase-plane. The presence/absence of the carrier also makes me think of different solutions adopted by manufacturers since MSK should be coherently detected like OQPSK (that implies acquiring the carrier!) or non-coherently detected like FSK. 

Fig. 2 - carrier is missing in signals like FUE
My friend ANgazu pointed out the use of different filtering (Fig. 3). If a Gaussian filter with a Bt of 0.8 or less is in use, as in FUE, the side lobes are attenuated and the modulation is GMSK. NSY has many side lobes so, most probably, no Gaussian filter is in use and modulation is pure MSK. A special case is JXN that uses a cosine filter.

Fig. 3 - differing filterings
That being said, some equalization/correction is needed to emerge the carrier in the midlle of the two tones as shown in Figure 4:

Fig. 4 - FUE constellation after and before equalization
However (G)MSK doesn't seem to be the sole modulation used: using Diff=1 in the phase-plane it turns out that OQPSK-like modulations are used, as in case of FTA and DHO38 (Fig. 5)

Fig. 5
Indeed, MSK is a special case of Continuous-Phase Frequency Shift Keying (CPFSK) which is a special case of a general class of modulation schemes known as Continuous-Phase Modulation (CPM). It is worth noting that CPFSK is a non-linear modulation and hence by extension MSK is a non-linear modulation as well. Nevertheless, it can also be cast as a linear modulation scheme, namely Offset Quadrature Phase Shift Keying (OQPSK), which is a special case of Phase Shift Keying (PSK)... identifying the used modulation may become a nightmare!

data format
Traffic is encrypted and each channel may convey four different types of broadcasts, reference Figure 1:

VALLOR: a VLF/LF single-channel 50 Bd submarine broadcast operating as a backup to the VERDIN (1) system and using KW-46 encryption system (VALLOR is the codename for KW-46 system);
JASON: it's probably a proper feature of the shown product depicted (maybe a codename of an encryption system?);
CLEAR: most likely clear-text traffic (no encryption is used);
ECF: (Empty Channel Filler), in conditions where no messages are available for a transmission channel, Empty Channel Filler data is generated automatically at the transmitter equipment. 

Data are arrangend in a stream incorporating in a regular manner a symbol dedicated to synchronization and placed every r data symbols, i.e. in the same format defined by STANAG-5065 in which frames are delimited by the pseudo-random sequence generated by the polynomial x^31+x^3+1 (aka "Fibonacci bits"). These formats may also be related to the patent WO2009071589A2 [2]. Error Correction And Detection (EDAC) should be performed using Wagner coding.

transmit system
Figures 7a and 7b show simplified block diagram of the VERDIN (1) VLF/LF transmit system and a real-world equipment used by US-Ny. Shore-to-Sub broadcast is a continuous transmission sequence of prioritized messages which normally lasts two hours. It is generated by ISABPS (Integrated Submarine Automated Broadcast Processor System) and sent to the transmit terminal which is used to multiplex, encrypt, encode, and modulate up to four 50 bps submarine broadcast channels into VLF/LF radio frequency signals which is amplified/radiated by the VLF/LF transmitter antenna. [3]

Fig. 7a - VERDIN system
Fig. 7b - a VERDIN receiver

(to be continued here)

(1) VERDIN is a digital data, multichannel communications system operating in the VLF range from shore to deployed submarines. VERDIN permits transmission of up to four 50 Bd channels from an individual transmitter using time division multiplexing.The system is normally operated in a four-channel mode.

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