[Editor’s Note: Dr. Becken graduated/rom the Naval Academy in the class of1947. Following PG School at Monterey and UCLA, he became an Engineering Duty Officer (Elex). Thereafter, his naval assignments were all related to ASW, sonar and undersea warfare. Upon retirement, he joined Raytheon Company, Subma-rine Signal Division, where he has been Head ofEngineering and Director of Technology. Dr. Becken is a Past Chairman of the ADPA Undersea Warfare Systems Division, and is currently a member of the Advisory Council to the NSIA Undersea Warfare Executive Comminee.]
Fifty years have elapsed since the invention of the UQC underwater telephone. In comparison with the operational enhancements made in other fields, submarine tactical acoustic communication capabilities, at least as measured by installed operational equipments, have not improved significantly in the interim. The word tactical is stressed, meaning two-way communications between ships and submarines, not bell-ringer one-way systems, which require the called submarine to come to periscope depth to communicate using electromagnetic communication links, or special purpose strategic communications systems which have received some limited use in the past.
The lack of progress might be explained by a number of factors. Certainly in the early days the intractability and complexity of the acoustic medium presented an obstacle. However, enough has been learned about the medium so that excuse can no longer be used legitimately. There also have been periods where the desire for absolute stealth has been paramount and to radiate any energy was anathema to the submarine community. On the other hand, there are some who might argue that Murphy’s Law (if something can go wrong, it will) was responsible for the lack of progress in the past. No matter in which direction the reader’s personal preferences might lean, a review of the history of developments in this field contains some valuable lessons, which might be applicable to other development programs in the future . In addition, now that the submarine community has come out of the closet and there is greater emphasis being placed on electromagnetic communications and on new satellite antennas, it might be timely to reevaluate the potential contribution of acoustic communications to today’s tactical solutions. First, I will provide some history and a view of lessons learned and then conclude with arguments in support of a reexamination of the potential contribu-tions of acoustic communications to improved submarine/battle group tactical operations. My list of lessons learned include the following:
- Never base a system design upon some hypothetical scenario as to how the system will be employed when in fleet use.
- Make certain that a system design is not tuned to one particular operating environment. The corollary is that there really are differences between the Atlantic and the Pacific!
- Without prototypes to evaluate, no matter how imperfect, the operating forces have difficulty in defining their operational requirements.
- When introducing a new capability to the fleet, keep it simple. It is better to solve a problem in small, sequential steps rather than in a single giant leap.
- A successful program requires a clearly identified program sponsor with a broadly recognized need and reasonable continuity in project management.
- If you want to communicate using acoustics, you have to make a noise!
Prior to the invention of the UQC-1 underwater telephone, acoustic communications between ships and submarines either did not take place at all or was limited to Morse Code by on-off keying with installed searchlight active sonars. In fact, the Submarine Signal Company, where submarine referred to underwater not a submersible ship, was founded in 1901 before the discovery of rad io because of the invention by Elisha Gray of an underwater bell which could be controlled by an electromagnet. The bell was used both as a navigational aid and as an early means of communications between ships and submarines. In fact Dr. Reginald Fessenden, the famous physicist, was hired by the Submarine Signal Company to invent a device which could overcome the very slow communication rates of the underwater bell. The Fessenden Oscillator was the result, useful for commu-nications, but more importantly, the basis for the invention of the fathometer'”.
As the significance of transmission frequency upon range became better understood during the 1950s, a lower frequency version of the UQC, the WQC-2, was developed. To my knowledge, all ASW ships and all submarines are equipped with the WQC-2, which includes the higher frequency UQC band for communication to other ships, such as NATO forces, for example, all of which have a UQC capability.
The search for better acoustic communications capability received impetus during the middle 1950s because of the planned introduction of SUBROC, the submarine launched, long range nuclear depth bomb. The problem then, which reoccurred during the later attempts to develop a submarine launched ASW standoff weapon, was that the ability to fire a weapon at long ranges had outstripped the fleet’s ability to generate accurate fire control solutions at those same ranges. While SUBROC’s large warhead lessened to some degree the fire control accuracy requirements, passive sonar, bearings only, fire control solutions of that day were insufficient to support SUBROC. During this same period, propagation research under the long range active detection (LORAD) program at the Navy Electronics Laboratory (NEL) in San Diego was producing very long ranges using low frequency, 1.5 kHz, FM and pseudo random noise (PRN) transmissions. At that time, just about all active sonars employed CW waveforms and the signal processing benefits of large time/bandwidth products were just beginning to be appreciated. From this work stemmed the concept of secure submarine communications (SESCO).
The idea behind SESCO was very straightforward. The only problems were that it required submarine tactics at variance with the way their commanders had been trained to operate and, in addition, it had a few technical flaws. The operational concept was that two submarines would operate in consort. If the range and bearing between them could be established with reasonable accuracy, then their individual bearings to the same target would permit a triangulation fire control solution to support SUBROC. The SESCO concept was based upon a very long PRN code of 232 bits, good for the duration of a patrol, in that it would not repeat itself over that period. Each submarine left port with PRN clocks synchronized to a radio standard. In fact it was sufficiently difficult in those days with the technology available, that the PRN clocks were removed from the systems and carried to a central location on a tender, for example, in order to complete the lock.
With each system having identical generators, it was possible to determine range between submarines by establishing the time interval by cross correlation between the receipt of signal and the occurrence of a correlation spike. Communication information was encoded by superimposing four frequencies on a 2 kHz bandwidth.
These were the days before fly-before-buy as it is known today and acoustic modems were specified, prior to operational testing, as part of the BQQ-1 sonar systems slated for what became the Permit Class SSN. In addition, 20 independent SESCOs, BQC-2s were procured for older submarines, nuclear and non-nuclear. The newer submarines would use the BQS-6 array and transmitters while the independent SESCOs were to have their own deck-mounted array.
The initial systems, which were of the BQC-2 variety, were tested in about 1960 with very unhappy results. In order to minimize the time to synchronize and the length of the transmis-sion, the tactical concept assumed that each submarine would follow a prescribed track. This was the first fatal flaw. Subma-rine commanding officers do not follow prescribed tracks. They did not then and they probably would not want to now. Once a target was detected by one submarine, his inclination was to investigate first and communicate second. By the time he did decide to pass on information about his contact, he had moved from the predicted position he would have occupied had he followed the prescribed track. Accordingly, his consort, if he did detect that a transmission had occurred, was not able to correlate quickly because the transmitting submarine was not at the range expected. The transmitting submarine, on the other hand, since he received no reply, assumed that his message had not been received because the transmitting source level was too low and, accordingly, raised his transmitter power. The receiving submarine, having by now obtained synchronization and wanting to reply, would lower his transmit source level because the received signal would appear so strong. Naturally, the original transmitting unit would fail to hear the reply, etc., etc. While all of these attempts to communicate were transpiring, the target would counterdetect the transmissions with the result that the system failed miserably.
Post exercise analysis identified two of the three causes that were responsible for the failure. The first is the number one lesson learned which was previously listed. It is impractical for a system designer to base his concept upon an approach which constrains a tactical commander unrealistically. Some might argue that the constraint was not unrealistic in that the potential benefits outweighed the limitations . Let’s face it. Our submarine Navy owes its success in large measure to innovative, independently thinking commanding officers. To expect them to operate counter to their natural instincts without extensive reindoctrination was not reasonable. The broader lesson learned is that we should design robust systems which do not rely upon some special tactic or operational approach, since it is never possible to predict the operational situation which the system will face eventually when in the hands of the operating forces.
The second system flaw was not as obvious as the first. SESCO was designed as a cryptographically secure, not a covert system. It was believed, however, that covertness would not be compromised if source levels used were the minimum necessary to establish contact and if transmissions were short. In addition, the PRN code was expected to provide some degree of covertness because of bandwidth, and the code was only known to the message addressee. The factor which was overlooked was transmit directivity. The array used with the BQC-2 was a sparsely filled, truncated cone of transducer elements with no baffling. As such, its directivity was poor and its sidelobes were high. Accordingly, any other listener, even if off the acoustic transmission axis, could detect the presence of the transmission, if not the intelligence. This constituted an unacceptable liability. It was some time before the third system flaw was appreciated and I will delay that explanation to the appropriate time in the narrative.
It was at about this time that I became involved in the program in a small way. I was assigned at NEL as the LORAD project officer, when it became apparent that array directivity was a problem. Since the SESCO concept had derived from the work at NEL, there was a natural desire to make the system work. We believed that at least a few BQC-2s should be installed with better arrays in operating submarines. An array concept was defined based upon the use of a ceramic transmit cylinder positioned at the focal point of a parabolic compliant tube reflector. The idea was to install the arrays in the bow buoyancy compartment of Guppy diesel electric submarines using a monkey legs mechanical train system to provide plus or minus 120 degree azimuthal coverage and the largest practical aperture for maximum directivity. The laboratory went to the Bureau of Ships {BUSHIPS), after obtaining the agreement of the local submarine division commander to install the systems in the four submarines of his division with help from the local submarine tender, and offered to build the arrays and install the systems at no cost to the Bureau . It seemed like an offer too good to refuse and an excellent opportunity for the fleet to obtain some hands-on operating experience with some fairly advanced equipment. Unfortunately, for reasons which I will explain, the BUSH IPS project manager could not see his way clear to turn over four of the 20 unused SESCO equipments for installation, and thus, a valuable opportunity to obtain practical operating experience was lost.
What influenced his decision was the appearance of a new communication modem called SPUME, which had been developed at the Marine Physical Laboratory of Scripps under Office of Naval Research sponsorship. Dr. F. Noel Spiess, the Director of MPL was not only a first class scientist and engineer, he was also a World War II submariner and maintained a very active role in the Naval Reserve. As a trained submariner, he was worried about covertness. His concept was based upon the transmission of a short burst of multiple tones. The presence or absence of specific frequencies would represent the intelligence. Since the transmissions would be short, about 100 milliseconds, they would not be likely to alert either a passive scanning sonar such as the BQR-2 or a mechanically trained passive system credited, at that time, to the Soviets. When NEL appeared on the scene requesting four BQC-2s, the BUSHIPS project manager was preparing a plan for a comparative evaluation of SESCO and SPUME. The evaluation would take place in six months and would settle, once and for all, which of the two communication concepts should be implemented in the fleet . Installing four SESCOs in the Pacific would just confuse the issue, in his opinion. The evaluation did take place, not in six months but in two years. It was unsuccess-ful for the same three reasons noted earlier, two of which I have discussed and the one I have yet to describe. The opportunity to give the fleet some capability with which to experiment, even if limited, was lost and the BQC-2s were consigned to Mechanics-burg and presumably became scrap.
The third reason for failure of the earlier SESCO tests as well as the more recent SESCO/SPUME evaluation was environment related and due to the, at the time, poorly understood phenomenon of multi-paths. As noted earlier, underwater sound propagation is complex due to refraction and reflection effects. Sound originat-ing at a source does not necessarily reach the receiver at the same time when the sound paths are of varying length. This multipath effect can be divided into two classes, depending upon path length differences. Refraction effects under certain conditions can lead to small path length differences, on the order of a wavelength. Thus a communication system which retied on fixed tones can experience dropouts due to path differences introducing a 180 degree phase reversal, and destructive interference. Major path length differences up to several seconds can occur between major propagation paths-direct, convergence zone, and bottom bounce. Accordingly the signal from longer transmission length pulses, such as used in SESCO, can be significantly distorted by the random addition of multiple signals over an extended time period. It was multi-paths, more than any single factor, that contributed to the SESCO/SPUME failure.
It is logical to ask why multi-path effects were not appreciated as a result of development testing which must have occurred prior to production. The answer is that there ace regions in the ocean where multi-path effects are minimum, as for example, in the Pacific Hawaiian area where most of the LORAD and SESCO testing had occurred. However, the environmental conditions in the Atlantic, where the Submarine Development Group conducted operational testing, differed markedly from Hawaii and represent-ed some of the worst multi-path conditions that might be found. The moral of the story is obvious. It is dangerous to assume that a system which performs well in some test environment will perform that same way across a broad spectrum of environments.
The whole sequence of events gave underwater acoustic communications a terrific black eye. Human nature being what it is, the pendulum swung violently from the get equipment into the fleet mode to let’s go back to basic research. A period of relatively low level 6.2 exploratory development effort ensued for about ten years, during which a better understanding of the mechanisms involved in acoustic propagation and their effect on communications was developed. In my opinion, however, the pendulum swung far too far and the fleet went too long without the opportunity to experiment with acoustic communications even if the equipment available did not meet all of the operational requirements. The overreaction became so severe that for a time, the governing operational requirement specified absolute covert-ness, even with a hostile interceptor on the acoustic axis at a range closer than the ship with whom it was desired to communicate. Patently, such a requirement defied the laws of physics and led to my somewhat tongue-in-cheek statement that in order to communicate with acoustics, you must make a noise.
By the 1970s enough progress had been made that the time appeared ripe to try again to produce some operational equipment. Experimental modems had been laboratory sea tested, sub-to-sub, sub-to-ship, which generated renewed confidence that the multi-path problem could be solved and the Navy embarked upon the advanced development of a system called SAMAC, submarine acoustic modem and controller.
SAMAC reached the test and evaluation stage but was not accepted for fleet use. In my opinion there were two reasons for its lack of acceptance: cost and program sponsorship. Because the Navy technical community had been frustrated for so long in their attempts to provide the fleet with an effective acoustic communications systems, now given a new opportunity, they over-specified the requirements, calling for levels of automation which priced the system beyond that which the Navy could afford. This resulted in the fourth lesson learned-when introducing a new capability into the fleet, keep it simple.
The second problem and the fifth in the list of lessons learned is the one about the need for consistent program sponsorship . The old saying it takes two to communicate certainly applies in this case. An effective tactical acoustic communications capability requires the cooperative sponsorship of the air, surface and submarine ASW communities. An attempt was made during the 1970s to focus the attention of all involved parties by the creation of an integrated acoustic communication program (lACS) within the then Naval Electronics and Communications Command (NAVELEX). While NAVELEX could generate a plan, it never appeared possible to obtain a support consensus from the OPNAV program sponsors, at least support sufficient to generate needed program funding . While a recurring theme at industry briefings by OPNAV and fleet personnel for many years from analyses of fleet exercises had been the need for better acoustic communica-tions, that perceived need has never resulted in consistent support from all the parties concerned. At the time that SAMAC was undergoing evaluation, I suspect that the submarine community’s dedication to strategic ASW and concerns over Soviet submarine radiated noise quieting were responsible for the loss in interest in underwater acoustic communications.
Much has happened since the days of SAMAC and the lACS program. The emphasis upon strategic ASW by SSNs in Arctic waters had decreased dramatically. Littoral warfare is the major concern, including integration of the SSN into the battle force with an accompanying emphasis upon direct satellite communications between a submarine commanding officer and the battle force commander. There is an ever increasing demand for very wide communication bandwidths to enable the transmission of massive amounts of data and imagery. This demand has led to the search for a periscope mounted satellite dish antenna compatible with that need. It also would appear from the willingness of the submarine to use RF that earlier reservations about coming to periscope depth in order to communicate, with the potential loss of sonar contact being tracked, is no longer of particular concern. Superficially at least, it might appear that any acoustic communications capability has been left behind that which can be provided by RF.
Before anyone gives up on the need for acoustic communications however, several other factors should be considered. The emerging application of unmanned underwater vehicles (UUVs) is begging for a vehicle control and data transfer solution independent of an umbilical cord between the UUV and the submarine. Research in progress at the Woods Hole Oceanographic Institute and at Northeastern University suggests that underwater acoustic communication rates up to 20,000 bits per second may be possible using such techniques as adaptive equalization. Data compression algorithms, developed to support the needs of satellite imagery. have lessened bandwidth demands. Also, there still may be tactical situations where a submarine commander would like to avoid coming to periscope depth in order to communicate. With the ready availability of relatively inexpensive commercial off-the-shelf computers and signal processors, it may now be appropriate to revisit the potential of underwater acoustic communications links between ships and submarines and to treat th is field more than just a source of lessons learned and examples of how not to run development programs in the future, but as a source of solutions to future submarine communication problems.
