Submarine Communication

“Wireless” was introduced in the Royal Swedish Navy in 1899, the driving force being Charles Leon de Champs (later Admiral). He was soon accompanied by Ragnar Rendahl, a brilliant Swedish engineer working at AEG/Telefunken in Germany, who became the first “electro engineer” of our navy. After a decade, the merchant marine followed suit and Karlskrona Radio opened for public correspondence on July 15th 1910, its callsign SAA still kept for the communication centre of our navy.

HMS Svaerdfisken (the Swordfish) was the first submarine to get wireless. Using LF spark transmitters for morse telegraphy on wavelengths of 300, 450 and 600 m, submarine radio traffic did not differ much from ordinary means of communication. Thermionic valves (electronic tubes) were invented in the beginning of the century and with them, telephony became possible; the subs were the first Swedish units to get tube equipment. Insulated wires forward and aft of the conning tower were used as antennas. The loop for D/F (direction finding) was utilized also for radio communication, sometimes with better performance than the wires, sometimes not. For good reception, the antenna could not be lower than a few metres below the surface.

Admiral Erik Anderberg’s personal notes say that, as a young officer, he was in Varberg in 1925 commanding HMS Rigel. He contacted Grimeton to agree upon a test from SAQ to submarine HMS Illem (the Polecat), installed the receiving equipment and found that such underwater reception was possible. The Illern logbooks of July 11th-12th 1925 do not mention radio, but they confirm that the tests were made as the sub dived a few times outside Varberg. It is unlikely that VLF was used to submerged submarines before the Second World War (WWII); this will continue to be an important research area where help will be most welcome.

Submarine communication in the Second World War was vastly improved by the great sea powers beginning to use VLF, the Royal Navy from Rugby and the German Kriegsmarine from Nauen, later followed by “Goliath” in Kalbe an der Milde about 130 kms west of Berlin. Goliath, biggest in the world and built in great haste 1941- 43, had an output power of 1 MW and a large, highly efficient antenna, based on the principles of the Alexanderson multiple tuned system. It had global coverage with an antenna depth in the South Atlantic of 10 m below the surface. The HF return channel was ordinary morse, but at the very end of the war, the German Navy introduced a primitive burst transmission system called “Kurier.”

In 1941 and with considerable foresight, the signals department of the Swedish Navy drew up a plan for submarine communication settling for SAQ with its low frequency and high power as the main VLF transmitter to the Swedish submarines. HF communication from the submarines was considered “unsolvable”, here interpreted as possible only for subs on the surface.

In neutral Sweden, SAQ continued to be used for public correspondence that was increasingly important as one of the few possible communications links with belligerents as well as non-belligerents.

At that time, VLF had been recognized as a means of submarine communication. SAQ did double duty, e.g. for messages to the downed HMS Ulven (the Wolf), sent from “Per-Albins ledningsvagn” (the mobile communication center of the Swedish premier), the underwater reception confirmed by the radioman of HMS Delfinen (the Dolphin) which took part in the search.

In studies and notes of meetings in the years after the war, the Swedish Naval Staff showed continued excellent foresight in the area of submarine communication. Bengt Lundvall (later Admiral) reported from his visit to the Admiralty that, in addition to VLF Rugby, the Royal Navy had some 40 kW LF transmitters (frequency range  40-50 kHz) giving very limited depth of reception, but quite suitable in brackish waters like the Baltic. He also said that the British had removed the D/F loop and all wire antennas and installed two crossed coils, possibly with ferrite cores, with a reception depth of 20 m, most probably from Rugby. He added that a periscope-like retractable HF antenna mast should improve our existing HF transmitting system. No document mentioned burst transmission, an area possibly regarded too secret to be included in notes, not covered at all or discussed only in a smaller circle of people.

For the Baltic, an LF transmitter like the British one was built near Oskarshamn on the Swedish east coast; Ruda radio station online (SHR) went on air in 1959. The transmitter was designed for morse telegraphy only, an excellent means of communication, in particular under critical circumstances.

The Sjoeormen {the Sea Serpent) class, commissioned in the 1960’s, was a milestone in Swedish submarine development with radically improved VLF/LF reception, burst transmission on HF and new system principles. This system step created a powerful submarine communications system along the principle “standards above all” that has been unchanged. Our policy facilitated installation, maintenance and upcoming modifications as well as retrofitting the earlier series and using well tried equipment for newbuildings.

With few exceptions, Sjoeormen had radio equipment designed for the purpose or available as standard components off the shelf (COTS), some of them sligthly modified. The transistor had been invented already in the mid 1940’s, a technology step comparable to that of the thermionic valve, but it took some time for this technology to mature. Only the burst transmission system was fully solid state with discrete transistors and core memory.

The following survey will focus on the communication systems that are unique for submarines, i. e. longwave, shortwave and equipment for such use.

Longwave to the submerged submarine
In principle, the depth of VLF/LF reception depends on
•water salinity
•transmitter radio frequency
•transmitter power
•transmitter antenna efficiency
•distance to transmitter
•modulation method and signal processing efficiency
•data transfer rate
•sensitivity and selectivity of sub antenna and receiver system

Three parameters, viz. water salinity, transmitter radio frequency and transmitter antenna efficiency, interact. For the first two a lower frequency is better, for the latter a higher one. In the Kattegat, VLF is needed; in the Baltic LF is optimum.

The signal from the longwave transmitter goes along the ground and over the water surface and then almost vertically down to the submarine. The dominating attenuation is in the water and the locality of the transmitter is of less significance.

The Royal Swedish Navy needed to supplement SAQ with an LF transmitter of its own, optimised for the Baltic. It was to be installed near the small town of Trosa. However, the location was regarded as vulnerable and was changed to the village of Ruda west of Oskarshamn with a Swedish Philips 40 kW LF transmitter and a 200 meter high umbrella antenna, i.e. a mast with toploading wires. Ruda Radio opened for traffic on December 1st 1959; plans for a second installation did not materialize.

A decade later, the Swedish National Defence Research Institute (FOA) was asked to study how to cover the Baltic with another LF station. Neither geographical databases, nor sufficient calculating power were available. Thus, the study was based an manual methods, a simple but ingenuous way of overlaying two identically scaled overhead transparencies, one with circles of transmitter field strength, one a map with curves of needed field strengths for the desired depth of reception. The result was very astonishing: the optimum site was west of Oskarshamn, i.e. near Ruda. FOA then proposed two lower power transmitters. After trying different ways to implement these, they eventually came about, based on two disused Air Force jamming transmitters.

For greater depths, in particular in oceans, a high power VLF transmitter is needed. The station closest to SAQ is the German NATO facility of Ramsloh (Rhauderfehn, callsign DH038), 70 kms west of Bremen, in essence comparable to Goliath except for MSK (Minimum Shift Keying) modulation instead of morse telegraphy. It comprises eight 100 kW transmitter units, each with a 350 m mast. For depths of the order of 100 m, still lower frequencies are needed. Systems on about 80 Hz, used by USA (terminated) and Russia, need great transmitter powers and giant antenna systems, out of the question for a small nation.

For the two crossed ferrite loop antennas on the rear upper part of the conning tower Swedish Philips made a solid state tuner/amplifier. It was easy to specify: the best possible sensitivity and selectivity for weak signals and that at the expense of everything else. This unit was delivered before schedule, had excellent performance and a lower price than estimated — a very rare situation.

HF and burst transmission
In principle, a burst transmission radio system should
•expose the transmitter antenna over water as little as possible
•allow antenna tuning without transmitting power
•use high transmitter power
•have a well structured chain of receivers with diversity and redundancy
•forward messages to the combat centre in a quick and secure way
•have a flexible frequency choice based on wave propagation prognoses
•employ data rate and modulation suitable for the current wave propagation
•use efficient signal processing if possible with forward error correction
•avoid stereotype messages
•have a quick and secure ciphering system
•have a return channel to acknowledge, in this case VLF or LF

In the 1950’s, “Marinförvaltningens Telelab” (the electronics laboratory of the Navy Board), started work on a highly secret burst transmission system, “Snaggen”, including a better submarine HF antenna. Snaggen was tested on air, the antenna as a model scaled to a higher frequency. Signal processing had not come far those days and the first data code, primitive and binary, indicated symbol errors, no more, and we had to rely on the redundancy of the diversity receiver chain, the structure of which has been immune to all reorganisations. At that time, ciphering was onetime pad and messages were recorded on a standard teleprinter, its “bell” function used to alert the radio officer on watch. Since then, some refinements, not mentioned here, have been included; other refinements, not mentioned here, may await their turn.

It is difficult to get an electrically as well as mechanically good submarine HF antenna and our trials did not meet with success. However, contacts with the US Navy, which had one or two types of retractable antennas designed for nuclear subs, led to a contract with ITT. These antennas were excellent, although expensive, and very efficient — someone asked if a new coast radio station had gone on air.