Maritime - DEFENCE MANAGEMENT JOURNAL, Issue 47

Sound thinking

Robert Kerr, of RRK Technologies Limited, outlines the problems, and potential solutions, in the ongoing battle to achieve better communications on submarines

The importance of communications, like many things about submarines, is not new. Communications intelligence, for example, in the form of Enigma and HF direction finding, was a key factor in the defeat of the U-boat campaign during the World War II. Likewise in the more recent Falklands conflict, communications again played a significant part in the deployment and use of the submarines involved. In the latter case, it was the limitations of the Radio Frequency (RF) communications available that had an impact on how the submarines were operated.

Unlike the U-boats, which spent a significant portion of their patrol surfaced or at Periscope Depth (PD), modern submarines now spend most of their patrol fully submerged. When below PD as far as RF communications are concerned, the submarine is incommunicado, with the notable exception of Extremely Low Frequency (ELF) and Very Low Frequency (VLF) broadcasts.

However, ELF is extremely expensive to operate, has a very, very low data rate and hence can only be used as a simple alert and was never a UK national asset. VLF, on the other hand, provides moderate data rate (teleprinter speeds) to a depth only slightly deeper than PD, but does not provide worldwide coverage. Communication turnaround from placing a signal on the submarine broadcast to receiving a reply or acknowledgement from the submarine when it returns to PD can be measured in hours, sometimes many.

Clearly, this was not conducive to using the submarine as a responsive and flexible member of a joint task group. One of the factors that influenced the decision to sink the Belgrano, or at the very least the timing of that decision, was the latency of communication with the submarine. Only being able to communicate with the submarine every few hours certainly concentrates the mind on what can happen in intervening period.

Prior to the Falklands conflict, concepts of operation had started to be developed to improve submarine integration with the task group, with control often being exercised by a Submarine Element Coordinator (SEC) embarked at sea with the task group battle staff. However, despite the movement of more operational control to sea, the communication latency involved in the submarine reading the broadcast meant it continued to be treated as a supporting, but often distant, player to the task group. This way of operating submarines remains essentially unchanged today, despite the vast increase in connectivity that is available to both the fleet in general and the submarine when at PD. The communication latency problem described still exists and the submarine is still an unresponsive partner as far as the surface fleet is concerned. The surfeit, if there ever is such a thing, of data connectivity now available at PD only further emphasises the paucity of connectivity available to the submarine when operating deep in its natural environment.

That is not to say that there is no ability to talk to the submarine under water. Through water acoustic communications have been available to submarines since the 1920s, when the precursor to the current NATO Standard Underwater Telephone (UWT) was first fitted to UK submarines. In reality, despite using more reliable electronics, the current incarnation of UWT is little changed in capability over that fitted in the 1920s. Do not let the term 'telephone' deceive you; using this equipment bears little resemblance to using what normal people refer to as a telephone. The user has to speak slowly and clearly, often using simple codewords with special meanings. This limited facility can, however, provide communications out to a couple of miles and on very rare occasions greater ranges can be achieved. The best that can be said about it is something is better than nothing and at least it is a standard that everyone adheres to.

Considerable effort has therefore been expended over recent years to improve on the capability to communicate to dived submarines.

An initial area of research was deployable and recoverable RF antennas. These are buoys that contain satcom antennas, which are deployed to the surface on the end of a cable by the dived submarine, enabling the submarine to receive and transmit signals while deep. Examples of recoverable systems are the UK's Recoverable Tethered Optical Fibre (RTOF) buoy and the German Callisto system. A single-use expendable system is in development by the US as part of its Comms at Speed and Depth programme, called the Tethered Expendable Communications Buoy (TECB). Of these systems, Callisto is the furthest along the development path with a number of systems currently at sea on German submarines. These systems, however, either have limited duration on the surface, or produce a detectable wake if the submarine is moving, and are still dependent on the submarine deploying them. So while they, in part, address the latency issue, they do not completely remove it.

In acoustics, digital signalling technology has been applied to the problem of communicating with the submarine, with most systems using keyboard input rather than voice, since this is inherently more efficient. They range from high power systems using coherent modulation schemes designed to maximise data rate, such as the US MF ACOMMs programme, to lower power spread spectrum systems designed to maximise reliability (and, to an extent, provide some degree of covertness) such as the UK developed Proteus system. The performance offered by these systems varies, for MF ACOMMS ranges out to circa 35nmi with a data rate of 1 to 2Kbits/s can sometimes be achieved and for Proteus operating in spread spectrum mode 100 to 200bits/s out to a range of 12 nautical miles can be achieved with good reliability. The ranges of both these systems, however, limit their usefulness in providing task group connectivity to the submarine due to the sheer geographical scale of naval operations.

A further class of acoustic system has therefore been developed to address the task group connectivity issue, the tactical pager. These systems employ signals optimised for reliable detection at ranges beyond 100nmi along with data compression techniques to maximise the information throughput available at these ranges. The capability can be further enhanced by mounting the paging transmitter on an A-sized sonar buoy equipped with a satcoms link that can be launched from the submarine whilst dived, as well as from surface ships or aircraft. During a recent Royal Navy trial, a paging system demonstrated the ability to pass simple information such as contact positions to an unalerted submarine with high reliability to ranges well beyond 100nmi.

One final area of research that should be mentioned and which the USN currently has great interest in, is the use of blue/green lasers to communicate to dived submarines. While potentially offering very high data rates, there are a number of inherent difficulties with this technology that are still to be overcome, such as cloud cover, variable water clarity, and the visibility of the laser spot at the sea surface, and it is therefore many years from practical fruition. There has been some discussion of mounting lasers on low earth orbiting satellites to provide global coverage, but this will require a multibillion dollar investment. It is likely, therefore, should the technical problems be solved that laser comms will be first deployed to enhance local communications between maritime patrol aircraft and submarines.

Submarine communications are currently undergoing a number of exciting and innovative developments. While no single one of these developments is a panacea to the age old problem of submarine communications, in combination they offer a very significant advance on currently deployed capability. One could speculate on the capabilities of a submarine fitted with a broad range of these systems. Such communication capabilities would transform the submarine's ability to rapidly respond to the changes in the tactical and strategic picture and would therefore greatly enhance the Royal Navy's ability to project power across the entire gamut of naval operations. While submarine communications have never been easy (as many a submarine captain would attest to when trying to clear the broadcast at PD in the middle of an Atlantic gale), we are now finally moving from the desire to have better connectivity to the dived submarine to having the ability, procurement permitting, to deliver it.