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Communicating when Underwater By Dr David Maddison Today, we take communication in most places for granted, and for the most part, it is possible. But underwater (and underground), things get a lot more difficult. Still, there are ways to get a message across. This article will concentrate on the challenges underwater; we will cover underground communications in a follow-up article next month. A round cities and even in rural areas, we can connect to phone towers with our mobile phones, or we can communicate via radio directly to other radios or via repeaters (eg, CB radio). We can use satellite phones or shortwave radios in remote areas, including at sea. All these methods rely on transmitting radio waves through the atmosphere, either line-of-sight to a tower, bouncing off the ground or atmospheric layers, line-of-sight to a satellite overhead, or directly from transmitter to receiver. Transmitting through water or underground is much more difficult for the reasons explained below. Communicating through liquid or solid matter Why would you want to communicate underwater or underground? Think of vehicles like submarines or underwater drones, or when people are in a cave or mine, or buried in snow. 14 Silicon Chip Common radio frequencies used for general above-ground communications are in the medium frequency (MF), high frequency (HF), very high frequency (VHF), ultra high frequency (UHF) and super high frequency (SHF) bands, from about 300kHz to 30GHz – see Table 1. These frequencies generally don’t penetrate very far into the ground or saltwater. Useful radio penetration into the ground or saltwater is generally only possible with wavelengths in the extremely low frequency (ELF) to very low frequencies (VLF) bands, from 3Hz to 30kHz. An unfortunate characteristic of these frequencies is that they have enormously long wavelengths, and consequently, vast antennas are required. However, some tricks can be used to lengthen antennas electrically. Also, receiving antennas don’t have to be as long as transmitting antennas; loop antennas can also be used for Australia's electronics magazine reception. Apart from the large antennas needed, the bandwidth and hence data transmission rate at those low frequencies is so low that voice cannot be transmitted, only simple codes. See Figs.1-3 to get an idea of the vast inductors and coils used for VLF transmissions. Why do longer radio wavelengths have greater penetrating power? Conductive materials usually block electromagnetic waves; hence, the use of metals to shield electronics from interference or shielding braids in coaxial cables. Conductors mostly block radio waves because they contain free electrons, which are caused to oscillate by the radio wave and reflect or absorb energy in doing so. The lower the frequency, the less energy is absorbed because there is less coupling of the wave with the electrons. Note also that extremely thin layers of metal do allow the transmission of some electromagnetic waves. siliconchip.com.au Figs.1-3: examples of 1960s RF variometers (variable inductors) and RF coils in a “helix house” as part of the final drive for a US Navy VLF antenna for submarine communications. These are at the US Naval Communications Station in Balboa, Panama and were made by Continental Electronics. Source: www.navy-radio.com/xmtr-vlf.htm Also, alternating currents mostly travel in the outside surface of conductors, to the ‘skin depth’, which becomes lesser as the frequency increases. The skin depth is greater in more poorly conducting materials. Seawater is also electrically conducting, although not nearly as conductive as metals. Seawater is an electrolyte that conducts mainly because of dissolved free mobile ions from common salt, primarily sodium (Na+) and chlorine (Cl−), but also others like magnesium (Mg2+), calcium (Ca2+) etc. These mobile ions absorb and reflect most radio waves at frequencies except the lowest. Freshwater is much less conductive than seawater, making radio penetration into freshwater much greater than seawater. Still, submarines rarely travel in freshwater. The electrical conductivity of seawater is typically in the range of 3-6S/m (Siemens/m), compared to the conductivity of copper at 5.8×107S/m and aluminium at 3.8×107S/m. So these metals are about 10 million times more conductive than seawater. Nevertheless, the electrical conductivity of seawater is still a problem for radio communications. However, for above-ground communications, this can be a benefit; it is possible to use seawater as the ground plane or counterpoise of an antenna. Some rocks have a high metal content, making them also somewhat conductive; this is an important consideration for antenna siting. Submarines Submerged submarines cannot communicate at regular radio frequencies, and can only receive radio signals at ELF, SLF, UHF and VLF frequencies (3Hz-30kHz; see Table 1). Because of these low frequencies, information transfer is extremely slow, far too low for voice frequencies, and only simple codes or Morse code can be transmitted. Only nine countries are known to operate VLF transmitters to communicate with submarines: Australia, Germany, India, Norway, Pakistan, Russia, Turkey, the UK and the USA. Table 1 – radio frequency bands per the ITU (International Telecommunication Union) Frequency name Abbr. Freq. range Wavelength Some common uses <3Hz >100,000km None known 3Hz-30Hz 100,000km10,000km Submarine communications Super low frequency SLF 30Hz300Hz 10,000km1,000km Submarine communications Ultra low frequency ULF 300Hz3kHz 1,000km100km Submarine communications, mine and cave communications Very low frequency VLF 3kHz-30kHz 100km-10km Submarine communications, radio navigation systems, time signals, geophysics Low frequency LF 30kHz300kHz Radio navigation, time signals, longwave AM commercial broadcasting in Europe and Asia, RFID, amateur radio (certain countries) ...continued overleaf No ITU designation Extremely low frequency siliconchip.com.au ELF 10km-1km Australia's electronics magazine March 2023  15 Table 1 (continued) – radio frequency bands per the ITU (International Telecommunication Union) Medium frequency MF 300kHz3MHz 1,000m-100m AM commercial broadcasting, amateur radio, avalanche beacons High frequency HF 3MHz30MHz 100m-10m Shortwave & amateur radio, 27MHz CB, long-range aviation & marine communications, radio fax, over-the-horizon radio Very high frequency VHF 30MHz300MHz 10m-1m Aircraft communications, amateur radio, emergency services, commercial FM broadcasts Ultra high frequency UHF 300MHz3GHz 1m-10cm TV broadcasts, microwave ovens, radars, mobile phones, GPS, wireless LAN, Bluetooth, ZigBee, satellites, Australian UHF CB Super high frequency SHF 3GHz30GHz 10cm-1cm Wireless LAN, radar, satellites, amateur radio Extremely high frequency EHF 30GHz300GHz 1cm-1mm Satellites, microwave links, remote sensing 300GHz3THz 1mm-0.1mm Remote sensing, experimental uses No ITU designation Table 2 – radio wave penetration in water for 50dB attenuation Frequency 10Hz (ELF) Source: https://jcis.sbrt.org.br/jcis/article/view/362 100Hz (SLF) 1kHz (ULF) 10kHz (VLF) 1MHz (MF) 10MHz (HF) 1GHz (UHF) Seawater 440m 140m 44m 14m 1.4m 0.44m 0.044m Freshwater 29000m 9200m 2900m 920m 92m 29m 2.9m Submerged submarines cannot transmit messages because the antenna required would be infeasibly long and the power requirements too high. Nevertheless, very long antennas are trailed behind submarines when they have to receive these signals; certain types of loop antennas can also be used. Submarines can transmit and receive at all typical frequencies if they surface, partially surface, float an antenna buoy to the surface or connect to a seabed “docking station”. However, a submarine that has surfaced or partly surfaced runs the risk of being found, either via its radio transmissions, or radar or optical reflections from its antenna masts or buoy. Its wake could also be detected by an aircraft or satellite. For a table of submarine radio communications options and the associated risks, see Fig.4. To minimise radar reflections from submarine periscopes and antenna masts, radar-absorbing materials (RAM) are applied – see our article on Stealth Technology in the May 2020 issue (siliconchip.au/Article/14422). Besides radio, submarines can communicate via acoustic and optical means, which we will also cover. descend to 600m. Escape from submarines is possible to a depth of about 200m and rescue with another submersible to about 600m. Submarines don’t always operate at their maximum depth, though; they choose the depth corresponding to the thermal layer that is most likely to prevent sonar detection for the particular sea conditions they find themselves in. The ABC news article at www.abc. net.au/news/11570886 states that the typical operational depth of an Australian Collins-class submarine is 180m. Radio signal penetration Table 2 shows the depth at which radio signals can be received through water for an attenuation of 50dB, which is a power reduction of 10000:1. That doesn’t necessarily mean that signals can’t be received deeper than that; it depends on the original signal strength and the sensitivity of the receiving equipment. Sources differ on the exact penetration of these frequencies into seawater, but they broadly agree with what’s shown in the table. Attenuation changes with salinity and temperature. Depending on the radio frequency, it is likely that a submarine will have to alter its depth to be able to receive radio signals. Fig.5 shows radio wave attenuation for Submarine operating depths The operating depth of submarines is said to be from the surface to 300m-450m below for modern Western nuclear submarines. Some sources claim that Russian Yasen-M boats can 16 Silicon Chip Fig.4: submarine RF communications options and associated risks. LDR = low data rate, MDR = medium data rate, P/D = periscope depth, ESM electronic support measures (intelligence gathering through passive listening). Based on: https://man.fas.org/dod-101/navy/docs/scmp/part06.htm Australia's electronics magazine siliconchip.com.au To receive VLF signals, submarines are typically equipped with both. The Ambrose Channel pilot cable (ULF) Fig.5: radio attenuation for a range of water conductivities and frequencies. Seawater (the most conductive) corresponds to the top two curves. Original: from a 2012 paper by Emma O’Shaughnessy quoted at www.quora.com/Whycant-radio-waves-transmit-through-water The Ambrose Channel is the only entrance to the Port of New York and New Jersey. Delays due to bad weather were once a huge and expensive problem, so in 1919-1920, they laid a cable on the bottom of the channel, which carried a 500Hz, 400V AC signal that could be detected about 1km away. Ships carried two induction coils and an amplifier to receive the signal. By switching between coils, they could determine which side was closer. The signal was mechanically keyed with Morse code that spelled NAVY. Arguably, this was the first use of what could be interpreted as a ULF signal for underwater communications. different frequencies and water conductivities. has been tested, as we will investigate shortly. The Grimeton Radio station (VLF) Optimal frequency in the ELF to VLF range Receiving electric versus magnetic fields As per Table 2, VLF is the highest useful frequency range for communication with submerged submarines. The lower the frequency, the better the penetration into seawater. Still, as the frequency reduces, so does the rate at which data can be transmitted. The complexity and cost of the transmitter also increase dramatically as the frequency drops. For this reason, VLF has been chosen as a happy medium for submarine radio communications, although ELF Radio signals have an electric field component and a magnetic field component. An example in everyday use is a long-wire antenna on an AM radio vs a ferrite rod or loop antenna. The long wire is sensitive to the electric field, and the ferrite rod or loop to the magnetic field. It is much easier to build an antenna to receive the electric field component, but it is also much larger. Long-wire antennas are possibly more sensitive but also more prone to electrical noise. Fig.6: an Alexanderson Alternator at the Grimeton Radio Station. Source: https://w.wiki/6DPN The Grimeton Radio station is a World Heritage listed Swedish radio station that operates at 17.2kHz and 200kW. It uses no electronics but generates a carrier wave for Morse Code with a high-frequency alternator called an Alexanderson alternator (see Fig.6). It is an obsolete technology that was even obsolete when the transmitter was built. It was used for transatlantic wireless telegraphy from the 1920s to 1940s. Later, it was used by the Swedish Navy for submarine communication. It was in service until 1995 but now operates twice yearly – see siliconchip.au/link/ abik for the transmission schedule. There is an Australian reception report at siliconchip.au/link/abil, meaning the signal travelled 14,000km – almost to the other side of the planet. For further information, see http://dl1dbc.net/SAQ/ and https://w. wiki/67Wd Goliath (VLF) The first use of VLF radio waves to communicate with submerged submarines was by Nazi Germany in WW2. Their Goliath transmitter could communicate with submarines anywhere in the world to a depth of between 8m and 26m, depending on water salinity, temperature and the distance from the transmitter. It used a 1MW vacuum tube transmitter tuneable between 15kHz and siliconchip.com.au Australia's electronics magazine March 2023  17 Fig.7: the Belconnen transmitter towers in 1951. Source: https://bpadula.tripod. com/australiashortwave/id45.html Fig.8: the Naval Communication Station Harold E. Holt, call sign NWC. Source: https://w.wiki/6DPP 60kHz (20km to 5km wavelength) at 12 specific crystal-controlled frequencies, plus other frequencies with reduced power below 19kHz. The operation modes of Goliath were: a) Morse code, mainly at 16.55kHz, using on-off keying b) Hellschreiber at 30-50kHz with AM tone pulses (see our articles on Digital Radio Modes in April & May 2021; siliconchip.au/Series/360) c) Low-quality voice at 45-60kHz with very low bandwidth (see Table 3) Modes a) and b) could use Enigma encryption. After the war, the transmitter system including the antennas was disassembled and taken to the then Soviet Union in 3000 rail cars, and reassembled about 150km from Moscow. It is still used today, operated by the Russian Navy, to transmit messages to Russian submarines along with time signals! Its call sign is RJH90 and it operates between 20.5kHz and 25.5kHz according to a specific schedule; see https://w.wiki/6DP5 Belconnen Naval Transmitter Station, Australia (VLF) The Royal Australian Navy transmitter facility at Belconnen, ACT, consisted of three 183m-tall VLF transmitting masts 400m apart. They were orientated east-west for maximum transmission directivity into the Pacific and Indian Oceans – see Fig.7. The complex was completed in 1939 and operated until 2005. At the time of its completion, it was the most powerful naval transmitting station in what was then the British Empire. It operated at 44kHz and was used to communicate with surface ships and submarines. For submarine communications, we can estimate that a 44kHz signal would penetrate seawater to a depth of 10m for about 50dB attenuation. The original power was 200kW but was upgraded to 250kW after an overhaul in 1959-1961. In conjunction with a similar facility in Rugby in England, communications could be made anywhere in the world. One report from an ex-technician states that the antenna system was “an ‘inverted L’ type with a huge capacitive top hat” supported by three towers. He also said that “the final ‘tank circuit’ was housed in its own building, and fluorescent lights did not need to be connected to power”. The facility also contained HF transmitters that served both military and civilian purposes. At the peak of its operations, it had 38 HF transmitters ranging from 10kW to 40kW and 50 antenna systems. In 1956, it broadcasted radio to the world about the Olympic Games in Melbourne. Naval VLF transmitter operations were transferred to Harold E. Holt Communications Station at North West Cape, Western Australia, in 1995. We don’t know how far away submarines could receive transmissions from Belconnen when submerged. Still, for the alternative site in Rugby in England, the page at siliconchip.au/ link/abim indicates that submarines could receive 16kHz signals with an antenna depth of about 7m and a range of about 3200km with loop antennas. The reception range increased dramatically when not using loop antennas; presumably, long wires were used instead. Also see the video titled “Track 6 Belconnen Transmitting Station” at https://youtu.be/lX39drhaI7g Naval Communication Station Harold E. Holt (VLF) The Naval Communication Station Harold E. Holt (Fig.8) is based in northwest Western Australia, was built in 1968 and is a joint Australia/ Fig.10: a side elevation view of the VLF antenna system at Cutler, Maine shown in Fig.11. 18 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.9: the Naval Communication Station Harold E. Holt antenna system. US facility for communicating with submarines. It operates at 19.8kHz and 1MW, so we can surmise a penetration depth into seawater of approximately 10m. However, due to its high power, the actual depth may be greater. The antenna consists of a central 387m-tall tower surround by six 364m-tall towers and a further six 304m-tall towers; see Fig.9. It is described as a ‘trideco’ antenna. The wires from the central mast to the 12 surrounding towers create a capacitor ‘plate’, with six ‘panels’ parallel to the ground and driven at the centre (see Figs.10 & 11 of a similar antenna). Rather than the central mast being the radiating element, there are six vertical wire “downleads” that radiate the VLF waves. There is a “counterpoise” system at ground level or Table 3 – Goliath system voice Frequency -3dB bandwidth 15kHz 30Hz 20kHz 63Hz 30kHz 250Hz 60kHz 1230Hz Source: siliconchip.au/link/abjd siliconchip.com.au Fig.11: the US Navy VLF antenna system at Cutler, Maine, which is very similar to the one at Harold E. Holt. Note the vast dimensions. Australia's electronics magazine March 2023  19 buried within the ground (it is not clear which). The antenna design is extremely efficient at 70-80% compared to other VLF antennas with efficiencies of 15-30%. There is not a lot of information available on this antenna and transmitter system but a very similar US Navy system is installed at Cutler in Maine, USA. See siliconchip.au/link/abj7 Fig.12 shows a submarine VLF receiver from 1972, the same era as this transmitter. US Navy ELF program Fig.12: the configuration of submarine VLF receiving equipment with the AN/BRR-3 set circa 1972. It operates at 14-30kHz with a loop antenna, longwire buoy antenna or whip. Source: www.navy-radio.com/manuals/01011xx/0101_113-03.pdf Fig.13: part of a 23km arm of the ELF antenna in the forest at Clam Lake, Wisconsin. Source: www.navy-radio.com/commsta-elf.htm 20 Silicon Chip Australia's electronics magazine As described earlier, transmitting at VLF frequencies allows a submarine to receive signals up to a submerged depth of around 14m. The submarine can be deeper than this, but it must trail a buoyant antenna at the reception depth. ELF frequencies from 3Hz to 30Hz and SLF from 30Hz to 300Hz offer much deeper radio penetration, allowing submarines and their antennas to remain at normal operating depths. Losses with SLF are very low – see Table 4. Experiments with ELF and SLF started in 1962 using a leased 70km length of HV power line in Wyoming that was disconnected at night. In 1963, a 176km antenna was built from Lookout Shoals, North Carolina to Algoma, Virginia. This was driven with 60A at frequencies between 4Hz and 500Hz with a radiated power of 1W. Signals were detected by the submarine USS Seawolf 3200km away, at an unspecified depth. In 1968, there was a proposal to build a transmitter that operated at 40-80Hz. The SLF system was called Project Sanguine and would have had 9700km of cable covering 58,000km2 or ~40% of the US state of Wisconsin. One hundred underground power stations were to produce 800MW of electrical power for transmitters. A small-scale test was performed at Clam Lake, Wisconsin, with two 23km crossed antennas (see Fig.13). The antenna was made of 15mm diameter aluminium cable mounted on 12m timber utility poles. That project was abandoned in 1973 for various reasons, but small-scale research continued. The system was designed with extensive redundancy to withstand a nuclear attack. In 1981, the then President Ronald Reagan revived the project at a much siliconchip.com.au smaller scale, and construction started in 1982. The existing 46km Clam Lake antenna was kept, while a new 91km antenna was built in Republic, Michigan, in the shape of the letter F with two 23km segments and one 45km segment, 238km away from Clam Lake (see Fig.14). There is no significance to the F-shape; it was due to land availability. An important siting consideration for the antennas was the very low conductivity bedrock in those areas. This enabled more rock to form part of a much larger antenna, as the current must flow much deeper to complete the electrical circuit. The signal generated travels in the natural waveguide between the Earth and the bottom of the ionosphere – see Fig.15. The antennas were ground dipoles, as shown in Fig.16. The antenna is fed from the halfway point by a power plant transmitter (P) at 300A and 76Hz or 45Hz. The ends of the antenna are grounded in 91m-deep boreholes. An alternating current passes between the grounded ends of the antenna (I) through the bedrock and along the above-ground wires. The arrows point in just one direction for clarity, but the direction of the current flow alternates. This current creates an alternating magnetic field (H) that radiates ELF waves, shown in yellow. The radiation pattern is directional, with the strongest signal coming from the ends of the wires. Hence, antennas must be built in at least two or more orthogonal directions for omnidirectional use. When combined, the effective radiated power of the two systems was 8W, from an input power of 2.6MW – an efficiency of just 0.0003%! Due to the low bandwidth of the system, it took about 15 minutes to transmit a three-letter coded message. Usually, the message contained instructions on where and when to surface, come close to the surface or release an antenna buoy to receive a more comprehensive message. The system would constantly transmit an ‘idle’ message, indicating to a submarine that they were still within the receiving range. The system became operational in 1989 and covered about half the world’s surface. It was decommissioned in 2004, with the US Navy stating that VLF systems had evolved to siliconchip.com.au Table 4 – losses & antenna efficiency for the SLF band Frequency 45Hz Propagation loss per 1000km 0.75dB Loss per 1m seawater penetration 0.23dB Relative transmitting antenna efficiency -4.4dB 76Hz 140Hz 1.2dB 2.0dB 0.27dB 0.36dB 0.0dB 2.5dB Source: www.navy-radio.com/commsta/elf/elf-1402-81A.pdf Fig.14: a map showing the location of the Clam Lake and Republic transmitter antennas in red. P G G H I 14 mi (23 km) Australia's electronics magazine Fig.15 (above): electric field lines radiated from an ELF/SLF transmitter travel in the natural waveguide between the Earth and ionosphere. A similar radiation pattern applies to VLF. The deepest sub receives ELF, another receives VLF with a buoyant antenna, while another floats a buoy. Fig.16 (left): a ground dipole of the type used in Project ELF (one of the 23km segments). Source: https://w. wiki/6DPK March 2023  21 the point that this system was unnecessary. In the video at https://youtu.be/ eC1cqwGkOwY, a technician who worked on submarines comments that the ELF/SLF receivers were synchronised with the transmitter using caesium beam clocks. If a noisy signal were received from one direction, the receiver delay would be adjusted so the same signal could be picked up, coming from the other side of the world. TACAMO Fig.17: schematic view of two trailing VLF antennas behind a Boeing E-6A, part of the TACAMO communications system. Source: https://nuke.fas.org/ guide/usa/c3i/e-6.htm TACAMO (“Take Charge and Move Out”) is a US system of communications links designed to survive a nuclear attack, keeping in contact with its submarine fleet if land-based transmitters are destroyed. To establish VLF communications, long antennas are trailed behind a Boeing E-6B Mercury aircraft (based on the Boeing 707; see Fig.17). The E-6B has two trailing antennas, one 8km long and the other 1.5km long. Once deployed, the aircraft goes into a tight banking turn. The longer wire hangs as vertically as possible, while the other wire trails behind the plane, forming an L-shape. The transmitter used is the 200kW AN/ART-54 High-Power Transmitting Set (HPTS) consisting of a Solid State Power Amplifier/Coupler (SSPA/C) OG-187/ART-54 and Dual Trailing Wire Antenna System (DTWA) OE-456/ART-54. For more details, see the TACAMO comms flight manual for the E-6A at siliconchip.au/link/abj8 (the earlier version of this aircraft). April 22nd, 2015, even though they could have repurposed it for several other uses, including by SBS, who wanted to use it for a radio tower. See my video of the tower titled “Woodside Omega Navigation System Tower VLF Transmitter, Victoria, Australia” at https://youtu.be/S_T7hd0oXUE From Table 2, we can see that a 10kHz signal would penetrate seawater to a depth of around 14m with 50dB of attenuation. Australian Omega transmitter (VLF) Oberon submarine VLF communications equipment We covered the Omega navigation system in detail in the September 2014 issue (siliconchip.au/Article/8002). The Omega system was shut down on September 30th, 1997. After that, the Omega transmitter at Woodside, Victoria, was modified for reuse by the Royal Australian Navy for submarine communication until December 31st, 2008 (see Fig.18). It was converted for use at 10-14kHz to support a 100-baud, two-channel MSK (minimum-shift keying) transmission with a 100kW antenna input power and a radiated power of 36.5kW. Its designation was VL3DEF. Sadly, the tower was demolished on Oberon-class submarines are now obsolete; they were designed in Britain, built between 1957 and 1978 and served five countries, including Australia. The last Oberons in use were decommissioned in 2000. While it’s hard to find information about VLF and other communications for submarines presently in use, there are details on the obsolete Oberon communication schemes. Fig.19 shows their various antenna options: ALK a VLF aerial in a recoverable buoy ALM an omnidirectional VLF aerial comprising a series of loops in the fin 22 Silicon Chip Russian Zeus ELF/SLF transmitter The Russian Navy has an ELF/SLF transmitter called ZEVS (Zeus) on the Kola Peninsula, east of Finland. It was first noticed in the West in the 1990s and usually operates at 82Hz with MSK modulation, although it is thought to be capable of transmitting from 20Hz to 250Hz. It is believed to have two ground dipole antennas of 60km, driven at 200A to 300A. Apart from military purposes, it is also used for geophysical research. Australia's electronics magazine Fig.18: the former 432m-tall Omega Tower Woodside, a frame grab from the video at https://youtu.be/S_ T7hd0oXUE Note the concrete helix building to the right. ALN a telescopic HF/UHF mast ALW a buoyant, disposable VLF wire aerial AMK a UHF/IFF (IFF = identification, friend or foe) combined antenna associated with the ECM (electronic countermeasures) mast AWJ an emergency whip aerial for use on the surface only Fig.20 shows the VLF receiver used on these boats. They operated at 14-22.5kHz with 150Hz bandwidth and were only suitable for telegraphy reception, not voice or transmission. VLF data rate There is not much published information on data rates for VLF comms. Still, Continental Electronics Corporation (https://contelec.com/case-­ history-lfvlf), a major manufacturer of naval VLF equipment, states on its website that: Very Low Frequency (VLF) communications transmitters use digital signals to communicate with submerged submarines on at frequencies of 3-30 kHz. The Navy shore VLF/LF siliconchip.com.au Fig.19: antenna options for the Oberon class submarine, once used by Australia. The original is from a manual published by San Francisco Maritime National Park Association (https://maritime.org/doc/oberon/operations/index.php). transmitter facilities transmit a 50 baud submarine command and control broadcast which is the backbone of the submarine broadcast system. We assume this is with optimal frequency and conditions. One baud is about one bit per second, so this is 6.25 bytes per second; the actual rate will be less due to parity bits etc. That works out to about 300 characters per minute. The average word length is about five characters, so about 60 words per minute can be transmitted under optimal conditions (this paragraph would take ~30s). That rate could be doubled or even tripled with data compression. Continental Electronics also made equipment for the Harold E. Holt VLF transmitter mentioned above. receive VLF comms while the submarine stays more deeply submerged. A submarine can still remain fully submerged for higher frequencies but deploy a buoy with the appropriate antennas. Alternatively, the boat can surface and risk being detected, as shown in Fig.4. Figs.21 & 22 show a buoy from GABLER Maschinenbau GmbH that can be deployed from a submarine via a reel mechanism, using 8mm-thick buoyant wire that is up to 6km long. The buoy has various sensors, antennas and a camera. Its buoyancy can be controlled so the antenna can remain Fig.20: a CFA receiver, type 5820AP 164474, as used on Oberon-class submarines. Source: http://jproc.ca/ rrp/rrp2/oberon_cfa.pdf just submerged for VLF reception. A 30m antenna rod for HF reception is at the end of the cable, just before the buoy. The system allows for the reception of VLF signals (7-30kHz), the reception and transmission of satellite communications when the buoy is on the surface, and the reception of HF signals at the surface. Regarding satellite communications, it can receive and transmit to Iridium, NEXT and other systems, and it can receive GPS, Galileo, GLONASS and BeiDou navigation signals. Unmanned aerial vehicles (UAVs) can also be controlled from the buoy. Buoyant antenna systems Ideally, a submarine should not have to surface to receive or send signals. As already discussed, a submarine can deploy a wire antenna to receive VLF. This antenna floats to a shallow enough depth that it can Fig.21: the GABLER reel mechanism and buoy for trailing submarine antenna system. Source: www. gabler-naval.com/wp-content/ uploads/2021/05/GABLER-Naval_ BWA_2021-05_EN.pdf Fig.22: components of the GABLER digital buoyant wire antenna system: 1) Submersible winch. 2) Antenna tow cable with VLF antenna 3) Towed Digital Antenna and Satcom Controller (TDASC), incorporating HF antenna. 4) Inboard control and interface unit. Source: same as Fig.21. siliconchip.com.au Australia's electronics magazine March 2023  23 Underwater acoustic communications Underwater communications can also be acoustic. The earliest example of this was with bells, but today, ultrasonic transducers are used. There are many difficulties with underwater acoustic comms, such as multipath propagation, strong signal attenuation, environmental noise and variation in acoustic properties of water due to temperature and salinity layers. Many modulation modes have been developed for underwater acoustic comms, such as frequency-shift keying (FSK), phase-shift keying (PSK), frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), frequency and pulse-­ position modulation (FPPM and PPM), multiple frequency-shift keying (MFSK) and orthogonal frequency-­ division multiplexing (OFDM). Acoustic signals are only transmitted from a submarine when stealth is not a concern, as submarine or shipbased sonar systems can determine the origin of such signals. “Gertrude” underwater acoustic telephone During WW2, the USA developed an underwater telephone called the AN/BQC-1 (see Fig.23) and variants, nicknamed Gertrude. It used SSB (single side-band) acoustic communications at 8.3-11.1kHz or a CW signal at 24.26kHz. Voice communications were possible to about 450m, but calls could be heard at about 1.8-4.5km distance. It was used to communicate with other Fig.23: the “Gertrude” underwater telephone from WW2. 24 Silicon Chip submarines and surface vessels. Some versions of this device are still used today, but for stealth reasons, modern submarines try to avoid using them. JANUS (acoustic) JANUS is an open-access NATO standard for underwater acoustic communications for military and civilian use (see www.januswiki.com/tiki-­ index.php). It is a standard that serves a similar purpose as IEEE 802.11 for WiFi but for underwater acoustic use, allowing devices from different manufacturers to interoperate. Devices announce themselves at a shared frequency of 11.5kHz and then can negotiate a different frequency or transmission protocol. The system has been tested at distances up to 28km. The present JANUS standard frequency is defined by STANAG 4748 and uses 9.44-13.6kHz. The present frequency band for military underwater telephony (UWT) is 8087-11087Hz (STANAG 1074/1475), which overlaps somewhat with JANUS. There is a proposal to reserve 4375-7625Hz for military use and 24.75-31.25kHz for civilian purposes. UT3000 (acoustic) The ELAC UT3000 2G (see Fig.24) combines analog and digital underwater communications into one device and is compatible with STANAG, JANUS and other standards. It can deliver up to 1400W of acoustic transmission power. It performs functions such as telephony, telegraphy, digital data transmission and reception, noise measurement and distance measurement. It also has an emergency beacon mode and operates from 1kHz to 60kHz. CUUUWi (radio/acoustic) CUUUWi (‘cooee’) is a communications gateway between underwater and above-water mobile phone and satellite phone users for voice and text – sees Fig.25-27. It was developed under an Australian government grant by L3Harris Technologies. The system is designed to find (from distress signals) and then communicate with stricken submarines, or provide encrypted communications with submarines (or other underwater platforms) at speed and depth. A gateway surface vehicle (or fleet), such as an unmanned surface vessel (USV), is required to receive radio communications from surface vessels or satellites and convert them to acoustic communications for underwater reception. A range of up to 10km (20km in good conditions) is possible. The system can also be used for subsea platforms, including autonomous underwater vehicles (AUVs), seabed sensors, submarines, ships and divers. The system is compatible with various NATO standards, including JANUS. It can detect standard 8.8kHz underwater beacons and 37kHz emergency locator pulses, as commonly fitted to submarines, and will soon be on aircraft ‘black boxes’ and maritime voyage recorders. Surface modes include satellite communications, 4G/3G/GSM and VHF. Underwater modes include underwater telephone (UT3000), HAIL (Hydro Acoustic Information Link) Fig.24: the ELAC Sonar UT3000 2G acoustic underwater communications device. Source: www.researchgate.net/figure/UT3000digital-underwater-communication-system_ fig2_281904054 Australia's electronics magazine siliconchip.com.au IridiumSATCOM Voice/SMS + CUUUWi Command & Control IridiumSATCOM Surface Vessel Voice/SMS + CUUUWi Command & Control Shore Operations Wi-Fi (<50M) CUUUWi Gateway 500Kb/s (<100M) Rich Data Fig.26: the GPM300 MASQ acoustic modem, part of the CUUUWi system. CUUUWi Gateway Voice/SMS (<10km) AUV APFA ultrasonic modem supporting rapid data channel CCSM ● HAIL ● UT3000 & MASQ Fig.25: the CUUUWi system with communications between satellites, surface vessels, a submarine and an AUV (autonomous underwater vehicle). Source: www.l3harris.com/sites/default/files/2020-09/ims-maritime-datasheetCUUUWi_0.pdf and MASQ (Multichannel Acoustic Signalling Quality of service). Deep Siren (radio/acoustic) Raytheon, Ultra Electronics Maritime Systems and RRK Technologies Ltd developed Deep Siren Tactical Paging (See siliconchip.au/link/abjc) for the US Navy. It uses disposable buoys deployed from a submarine to transfer messages from Iridium satellites to the submarine via an acoustic data link. The range of the system is 50 nautical miles (92.5km) or more from the buoy to the submarine, and the submarine can operate at normal speed. In contrast, a sub has to run at reduced speed when towing antennas, such as those on a floating buoy or VLF cable. The buoy can be deployed from a surface ship, aircraft or from a submarine’s garbage chute(!). System testing started in 2008 and it was demonstrated in 2011. Its current operational status is unknown. TARF (acoustic/radar) Translational Acoustic-RF Communication is an experimental system developed by the Massachusetts Institute of Technology (MIT). Sound waves from an underwater source cause vibrations on the surface that can be picked up via a sensitive radar operating in the 300GHz range. See the video titled “Getting submarines talking to airplanes, finally” at https:// youtu.be/csYtAzDBk00 siliconchip.com.au Range limits of underwater acoustic communications Nature may have the answer to this. It is said that humpback whales communicate acoustically and can be heard by another up to 6400km away. Underwater Optical Communications (UWOC) There were hopes in the 1980s that airborne or spaceborne lasers could be used to communicate with submarines. With the SLCSAT (Submarine Laser Communication Satellite) and similar proposals, the idea was that a laser beam would be directed toward the ocean in the approximate submarine area and a communications channel would be established. Blue lasers for such a system were developed by Northrop Corp, and a highly sensitive laser detector by Fig.27: an 8.8kHz emergency location pinger with a battery lasting 300 days. These can be picked up by the CUUUWi system and would help locate aircraft black boxes, submarines in peril etc. Lockheed Corp. As far as we know, this system was never put into service. From UWOC in use today and reported below, it appears that underwater optical links in seawater can only work over a few tens of metres. The attenuation and scattering of light in seawater are just too great. However, an optical link could presumably be established between a buoy on the ocean surface and an aircraft. Blue-green lasers have been developed for naval use that can transmit data at 90Mb/s over water for up to 10km, but when used underwater, the data rate drops to 7-10Mb/s over 10-20m (as described at siliconchip. au/link/abin). Aqua-Fi (optical) Basem Shihada et al. from the King Abdullah University of Science and Relevant videos and links ● VLF signals that individuals have received: www.sigidwiki.com/wiki/ Category:VLF ● 1972 US Navy manuals for VLF communications: www.navy-radio.com/ manuals/shore-vlf.htm ● An experimental, compact piezoelectric VLF antenna: siliconchip.au/link/ abit and www.nature.com/articles/s41598-020-73973-6 ● The companion site for the Australian VLF transmitter at Belconnen, “16 kHz VLF, Rugby, England”: https://youtu.be/Unlg2gY2Zrs ● On the Goliath transmitter, “The Radio Network that Communicated with Nazi Subs”: https://youtu.be/OSNCvJN5Xoo ● “Project E.L.F. – The history of communicating with submarines underwater - #HamRadioQA”: https://youtu.be/eC1cqwGkOwY ● “Reception of signals from submarines on VLF”: https://youtu.be/ UYaK3tWXbn0 Australia's electronics magazine March 2023  25 Technology in Saudi Arabia developed an underwater Internet access architecture that used a Raspberry Pi computer and off-the-shelf green LEDs or 520nm lasers to transmit data. They obtained a maximum data transfer rate of 2.11MB/s. They did not specify the communications distance, but diagrams in the PDF at siliconchip.au/link/abio suggest up to 10m for LEDs or 20m for lasers. However, the picture of the lab demonstration shows a distance closer to two metres. Using online SDR radios to listen to VLF signals You can use a computer sound card or audio input to receive VLF signals with a PC, antenna and software only. There are many articles and videos on how to do this. For example, see: www.prinz.nl/SAQ.html | siliconchip.au/link/abj9 | www.vlf.it siliconchip.au/link/abja | siliconchip.au/link/abjb There is an experimental online VLF-HF SDR receiver (EA3HRU) at http:// sdrbcn.duckdns.org:8073/ in Pallejà, Barcelona, Spain. Select VLF mode in the menu. Blue laser diodes Reported in Nature Portfolio (www. nature.com/articles/srep40480), a 450nm blue GaN laser diode modulated by quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) can transmit data through seawater at a rate of 7.2GB/s over 6.8m or 4.0GB/s over 10.2m. Underwater data nodes (optical or acoustic) Underwater data nodes could be established for submarines or AUVs so that they can establish a high-­ bandwidth connection with their command centre without surfacing (see Fig.28). This would allow them to receive information much faster than VLF or ELF radio, and transmit it too, without having to release a floating antenna or buoy. A faster data channel could be established than with satellites, so there would be less exposure time for the antenna buoy or periscope. This would also provide an alternative means of communication if satellites and landbased transmitters are destroyed. Fig.28: an underwater communication range of 1020m is within the capability of a blue-green laser. Source: www. mobilityengineeringtech.com/ component/content/24599 26 Silicon Chip A screen grab from the online SDR radio EA3HRU in VLF mode. The idea is that an underwater vehicle would manoeuvre close to the communication node on the seabed and establish a comms channel by optical or acoustic means. China’s laser sub-hunting system (optical) It is not hard to imagine that the following laser system built to hunt for submarines could also be used to communicate with them if the laser system was modulated with data. According to ABC News (www.abc. net.au/news/11570886), China has developed a blue-green laser system for shining light from aircraft into the ocean and looking for a reflection indicating the presence of a submarine. The laser is beamed from an aircraft at an altitude of 1.6-3.2km and will find a submarine as deep as 160m. The article notes that a Collins-class submarine has a typical operational depth of 180m. The objective is to build a satellite that can find subs as deep as 500m. This system is similar in principle to the Australian-developed LADS (Laser Airborne Depth Sounder) for seafloor mapping, which could be adapted for submarine communication. However, as noted above, optical communications underwater are of limited range. See our previous article on sonar in Australia's electronics magazine the June 2019 issue (siliconchip.au/ Article/11664). LUMA LUMA X is an underwater optical modem (www.hydromea.com) that can transfer data at up to 10Mbit/s over 50m, enough for HD video – see Fig.29. It is suitable for use with autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs). Next month Underground communications pose some similar challenges to underwater communications. There are quite a few different aspects to communication underground, so we’ll cover them in a separate article in next SC month’s issue. Fig.29: the Luma underwater optical modem. Source: https://files. hydromea.com/luma/Hydromea_ LUMA_X_datasheet.pdf siliconchip.com.au