What to Do?
It should come u no surprise to anyone in the submarine community that our old way of communicating is simply no longer acceptable. Although it is unlikely that an SSN will ever be able to match the communications capabilities of platforms which will always have many large (and dry) antennas a hundred feet or more above the air-water interface, improvements are required in both communications accessibility and transmit/receive throughput. At the recent Submarine Technology Symposium, however, this subject did not attract the attention one would have expected-other than the two frequently voiced and broadly-hued observations that ” … we must communicate better” and ” … we mustn’t compromise our ‘core competency’ of Stealth”. Indeed, a pessimist could draw the conclusion that although all admit its existence, operators view better connectivity u a technical problem and technicians view it as an operational problem.
Rear Admiral Tom Elliot, the recently reported Deputy Director, Submarine Warfare Division (N87B) was a notable exception to the above general statements. In his Keynote Address for the second day of the proceedings, he drew heavily on his work, under CINCPACFLT Admiral Archie Clemens, on Information Technology for the 21st Century or IT21. It would appear, fortunately, that both legacy and emergent technologies and techniques are coming together that offer dramatic improvements without violating any Jaws of physics. An example of their work was related wherein the data rate on an existing onboard communications system was increased by orders of magnitude through nothing more complex than replacing the installed modem. Prerequisites to properly employing and exploiting these technologies and techniques, however, include the defining of some terms and the controlling of some expectations. What is it the submarine needs to do and when and from where does it need to do it?
Connectivity Data Rates
First, we cannot forget that data rate is to data as power is to work. One of Rear Admiral Jerry Holland’s memorable one-liners is that ” … time is a dimension of any process”. and it is not just bandwidth but the time-bandwidth product that determines how much communications capacity exists-an observation that is probably trivial to everyone in the Navy except submariners, since others tend to assume that the t in the time-bandwidth relationship is 24 hours/day of constant, active, bi-directional traffic. Even under present battle group operational scenarios, SSNs don’t have to be told all that much, and have even less to say. What is important is that the SSN be quickly available for on demand communications, and that its equipment suite be capable of sending and receiving the information required. The real reason for better data rates to support transferring relatively limited time-bandwidth products is to constrain the total time spent doing it.
Another issue of importance to submariners (and really, everyone) is that there are too many subjective and vastly different terminologies as to just what is a high or low data rate. As has been done with both the RF (radio frequency) and acoustic spectrums, quantitative values are needed in lieu of the now largely qualitative descriptors. Certainly, that rate at which ELF (extremely low frequency) is received-in the order of 5 to 10 baud-represents a stake in the sand for ELDR (extremely low data rate) communications. Similarly, an order of magnitude around typical VLF (very low frequency) data rates might define VLDR, and so on. If the nomenclature continued on in sync with classic frequency bands, then the end of the descriptive road would happen with EHDR (extremely high data rate) equating to anything more than 3000 megabaud-maybe not high enough to describe transfer by some exotic radar or photonic means, but certainly more than adequate to encompass most other probabilities for several decades, particularly for submarines. Therefore, at least for the purpose of this article, the following are defined:
|Extremely Low Data Rate (ELDR)
|Very Low Data Rate (VLDR)
|Low Data Rate (LDR)
|Medium Data Rate (MDR)
|High Data Rate (HOR)
|Very High Data Rate (VHDR)
|Ultra High Data Rate (UHDR)
|Super High Data Rate (SHOR)
|Extremely High Data Rate (EHDR)
This logic would show most current on-line home computers operating on the Internet at about the MDR level, not a bad point of reference (in fact, all that telephone companies are required to assure of a standard line is 9600 baud-right in the middle of the MDR band).
The Connectivity Envelope
Connectivity is the principal submarine issue-for the sake of discussion, let it be defined as the ability, on demand and while submerged, to establish a bilateral link with· some other entity or thing (i.e., some component within a network or system of systems). In fact, submarine associated terms that might ultimately attain some degree of accepted meaning are connectivity depth (as opposed to periscope depth or even communications depth), or even better, connectivity envelope, analogous to current operating envelopes. Similar to operating envelopes, connectivity envelopes would be speed and depth dependent, but would also vary as a function of frequency, data rate and perhaps even sea state. In the vertical dimension, the envelope would be defined as a function of the range of non-broached keel depths from which communications can be conducted, and similarly, a range of allowable speeds in the horizontal dimension. In the simplest example such an envelope would be rectangular, but as for an operating envelope, there is no reason it would have to be.
Relative comparisons benefit from some measure of effectiveness (MOE) or system of units. A mental construct for first-cut descriptive units to describe connectivity envelopes could be the envelope’s area (in foot-knots; perhaps the most absolutely meaningless units since barn was defined as the probability measure for neutron absorption by a nucleus). Of note, it would do the Submarine Force and its customers a disservice to even intimate that within the next several decades any HDR connectivity envelope would have a foot-knot MOE so large that it would not impact that platform’s mobility to some degree. It is difficult to imagine a case where the foot knots of an HDR connectivity envelope would ever equal the foot-knots of a platform’s operating envelope.
Submarine Operational Needs
Much has been said and written about the data rate at which a submarine must be able to communicate. The value most typically heard is the T1 rate (about 1.5 megabaud-VHDR by the standards above). It should first be realized that Tl is just an AT&T designator for a quality level of land-line service, and has little justification in practice to set a submarine operational requirement. If instead, real mission requirements are addressed in conjunction with an appreciation of onboard assets available to reduce the communications requirements (e.g., the need to quickly transmit high resolution processed and compressed still imagery), required rates drop to more reasonable and technically achievable values. As the quantity of raw data expands, it becomes more and more important to reduce it to information at the point of origin (fortuitously the means to do just that continue to become smaller, faster and cheaper) before transmission. Ship the wine, not the grapes.
Maintaining the covert nature of submarine operations also remains a high priority, if not so much for platform survivability as it would have been in war with the Soviet Union, certainly to maintain ubiquitous uncertainty in the minds of potential adversaries. Because of that, a real need exists for transmission from the submarine to have the greatest LPI (Low probability of intercept) characteristics possible. Several techniques exist to enhance LPI transmission, not the least of which is to clear the transmission as quickly as possible and/or to have as narrow a beamwidth as possible so that the transmission can be pointed at the intended receiver with little energy propagating in other directions. It is proposed that, given a level of circuit discipline which would result in transmission of information and not just data, two-way HDR connectivity as defined above will meet the needs of submarines and their chains of command well into the 21st century. To slightly modify another favorite statement of Jerry Holland, “Real information in time is better than information (data) in real time.”
Supporting Equipment Concepts
If HDR connectivity will likely meet all of the submarines downlink needs and uplink responsibilities, just what type of hardware is needed to provide that rate and in what connectivity envelope? Certainly, the most traditional submarine thing and posture for bilateral communications is at periscope depth with a hull-mounted retractable antenna mast. This option remains a viable, if not critical one, and programs are well along to provide mast-mounted HDR connectivity at EHF frequencies to orbiting MILSTAR satellites. Since EHF permits very directional but compact antennas, transmissions by this means would be LPI to a large degree, since it would be unlikely that any but the intended receiver would be in the narrow and positionally stabilized beam. The disadvantage of this means is that it provides a very small connectivity envelope (perhaps 15 feet by 8-10 knots).
A parallel option, being pursued through a joint effort between Naval Underwater Warfare Center and the Spears Communications Group, Ocean Systems Division of Sippican, Incorporated, and with which the writer has been involved, involves development of enhanced versions of the legacy Buoyant Cable Antenna. These concepts exploit a higher loading density of in-line electronics, and imbedded arrays of antenna transmit/receive elements. In one such conceptual system, a buoyant antenna module some six feet long and six inches in diameter, and at the air-water interface, would be towed by the submarine to provide HDR connectivity through an envelope of perhaps 200 feet by 6 knots, an increase of about an order of magnitude by a feet-knot MOE. Although such a HDR buoyant cable antenna certainly could be made retrievable and even replaceable while submerged by use of a new lockout/streaming mechanism, the mechanical engineering and physical installation considerations of such a capability could likely become the longest path of development/deployment. An alternative approach would be to develop a clip-on capability to rapidly provide CINCs with much improved SSN connectivity while better but longer term options were developed, not unlike the way 1970s STASS towed acoustic arrays provided much needed acoustic advantage years before retrievable acoustic towed arrays were fielded in number.
To continue the ST ASS/retrievable towed array analogy, two-way communications through 15 knots and 400 feet (a connectivity envelope of some 350 feet by 12 knots)-much more in line with what is considered operational speed and depth for most submarine missions-could likely be obtained from yet another legacy technology which, like the BCA, was upgraded to what increased electronic component density and advanced materials can bring. In one such concept, being independently investigated by Sippican/- Spears, the remotely actuated sensor platform (RASP) would be a retrievable hydrodynamic body tethered to the ship via a BCA-like fiber optic cored high-strength cable. While externally reminiscent of the communications buoys so many of us have towed at one time or another, this device .. would not be your Father’s Oldsmobile”.
- With autonomous control surfaces, it would maintain its own depth rather than constantly being winched in and out from the ship.
- It would provide the VLF/LF link but also have an electable mast with an HDR phased array antenna, electro-optical and in-air acoustic sensors and ESM capability.
- It would provide a significant degree of above layer acoustic sensing.
- Information, not data, would flow up and down the fiber optic link since most processing and modulation would be done in the buoy rather than aboard the ship.
- The spatially stabilized, narrow beam width HOR phased array would provide a similar degree of LPI to uplinks as that obtainable from the HDR mast.
- A standard bus architecture would allow extraordinary mission/sensor flexibility while also providing an easy communications upgrade path to accommodate the rapid expected changes in commercial and military satellite constellations.
Intuitively, there is a greater degree of technical risk associated with the development of a RASP when compared to an HDR BCA, but a RASP would probably benefit significantly from BCA- oriented developments in off-board electronics, antenna elements and lightweight/high strength tow cable construction.
Submarine connectivity at HDR rates is essential if Joint Forces are to fully exploit the special attributes that SSNs offer. These rates should properly first be achieved through the current development of mast-mounted directional antennas. However, for many years during the Cold War, when U .S. submarines enjoyed an extraordinary level of acoustic advantage, a continuing concern by the Force was to remain aware enough of emergent technologies not to be surprised by the arrival of viable non-acoustic detection methodologies. In fact, the Submarine Force was among the leaders in investigating candidate phenomenological, and took early engineering steps to defeat many. Prudence dictates that this same awareness and concern still be exercised, particularly as post-Cold War mission sets have submarines communicating more, closer to shore, and in shallower waters. Whatever above-surface, non-acoustic detection methodologies might be enabled by the same extraordinary and relatively inexpensive improvements in signal processing which are affecting so many other endeavors, they would probably be significantly mitigated if the large hull of the submarine had the choice of remaining further from the air-water interface while meeting its communications requirements. The continued contribution to U.S. forces of an SSN’s stealth, mobility, firepower and endurance will be enhanced by an accelerated near term development of a clip-on HDR BCA and the midterm development of an HDR RASP.