At the Pacific 2004 show in Sydney, the Australian company Nautronix displayed its HydroAcoustic Information Link (HAIL), which it sees as a text-based replacement for the usual underwater telephone, which garbles its messages so often that it is called a "say-again machine." Nautronix argues that by substituting text for voice, HAIL drastically improves the reliability and clarity of underwater communication. It considers HAIL attractive because it can use the unmodified transducers of existing underwater telephones: the new elements are the device producing the waveforms going into the transducer and the signal processor at the other end. Because such devices are relatively inexpensive, there is no great barrier to installing HAIL. In the current context of U.S. naval tactics, HAIL or an equivalent offers far more; it may become a key enabler of network-centric undersea warfare. Nautronix has made no such claims, but the possibility seems to be inherent in the characteristics of its system.
HAIL follows a long tradition. Voice links are attractive because they seem simple, and because so much of communication is in the way words are said, not just in the words themselves. Unfortunately, voice messages cannot easily be replayed and displayed. Text offers the important advantage that it can be displayed and reread. Graphics are even better, because they are easier to interpret, particularly when those involved are under stress. These points explain why the U.S. Navy much prefers, at least in the past has much preferred, graphics and text to voice communication. For example, in the 1920s the Navy had to decide how to pass torpedo-aiming instructions from the submarine control room, where the calculations were made, to the torpedo room, where torpedo gyros were set. As yet, there was no automatic link between the two. The simpler alternative was a voice pipe. It was rejected in favor of a visual indicator showing the correct gyro angle. Not only was the visual less likely to be misinterpreted, but it also remained, so that the torpedo angle, once set, could be checked against it.
HAIL uses acoustic digital spread spectrum techniques, applying a spreading code to a broadband carrier signal. This is similar to what is used in stealthy radars and radios. By comparing what it receives with a template embodying the code, a processing system can recover a weak signal buried in noise; Nautronix claims a processing gain of 20 decibels (a factor of 100). HAIL is now practicable because processors are more powerful than in the past. The weakness of the signal at any one frequency makes it difficult for anything but the appropriate processor to intercept. Given that transmission is stealthy, signals can be sent omnidirectionally. HAIL currently exploits a 12 kHz bandwidth in the 0-24 kHz range.
HAIL offers a data rate of about 200 bits per second at a range of up to 40 kilometers (km); to some extent, range can be traded off against data rate, so that less than 200 bits/sec can be sent to, say, 50 km (about 27 nautical miles). The 50 km figure is set by power limitation. However, because signals at higher frequencies do not travel nearly as far in water, the need for broad bandwidth limits range. In theory, a lower-frequency system operating over a narrower bandwidth at a lower data rate could reach farther. Nautronix thinks that the faster processors now becoming available may boost its data rate as high as 1,000 bits/sec, something less than half what Link 11 offers.
Nautronix currently offers HAIL as a stand-alone communications device. Imagine it instead as the carrier of an underwater data link signal, connecting the combat direction systems of submarines, surface ships, and perhaps also unmanned undersea vehicles (UUVs) on a computer-to-computer basis. That is what network-centric warfare demands. In this context, 200 bits/sec seems rather insignificant. It is less than a tenth the rate offered by Link 11, generally considered geriatric. That in turn is about a 25th of what a standard household modem can produce. To put this rate in perspective, however, remember that until quite recently the upper limit for high frequency radios was 75 bits/sec, and that Link 11 worked because it combined thirty such channels, not because it offered any better performance on one channel. Link 11 techniques cannot be used underwater because they require that a signal be sent on several frequencies in parallel; HAIL already uses all available frequencies for its single 200 bits/sec channel.
Probably even 200 bits/sec can be useful. Link 11 needs its 2,250 bits/sec because it handles a fast-moving air situation. It also needed considerable redundancy, because messages were often missed. There is current sentiment suggesting that a lower rate, something like 300 bits/sec, might suffice if all nodes in a system were intelligent enough to edit the message traffic they were passing.
Data link history may be relevant here. When Link 11 was invented, most ships lacked combat direction computers, hence could not make use of a digital data link. The solution was to translate Link 11 information into stereotyped radio teletype messages. They were good enough to handle the surface and antisubmarine warfare pictures. There were, for example, serious efforts to develop simple computers that could translate those 75 bit/sec messages back into graphic form so they could be used efficiently. For many years, the Japan Maritime SeIf-Defense Force used such a system on board its frigates. The situation as seen by several submarines would of course be similar to the frigates' surface picture. A few U.S. ships had similar systems.
The U.S. Navy is already investing in acoustic data links, but of a type different from HAIL. In the late 1990s, computers advanced to the point where it became possible to send elaborate graphics, such as periscope imagery, underwater. Ranges were comparable to those offered by HAIL, but the messages were anything but covert. Even so, the new generation of underwater links, now in service, inspired entirely new concepts of underwater warfare, such as the Naval Undersea Warfare Center's Manta. Obviously the few hundred bits per second offered by HAIL would be unable to send elaborate graphic messages. It would suffice only to pass enough information to help a few submarines (or perhaps UUVs) maintain a consistent picture of the surface and subsurface situation. It might also be able to transmit the sort of simple instructions included in the Link 11 set.
During the Cold War, the Soviets emphasized submarine-to-submarine communication. A recent history of their sonar shows that, from about 1960 on, not only were all submarines fitted with acoustic communications equipment, but such equipment also was integrated into successive submarine sonar suites. Limited published descriptions suggest that such systems operated by using limited vocabularies, just the sort that might be practicable with the Nautronix equipment. The Russians seem not to have published accounts of Cold War submarine tactics, but it would appear that their communications systems offered the means for coordinated action. They certainly included both IFF (identification friend or foe) and distance-measuring transponders, the latter offering submarines the ability to keep track of friendlies in the context of a complex underwater situation. The Soviet systems were hardly covert. Apparently none had been translated, but presumably analysis of Soviet exercises revealed that the appearance of particular signals indicated attacks were imminent.
One interesting twist in Soviet thinking was that they were willing to use underwater communication but, like the U.S. Navy, preferred passive to active sonar. They may have hoped to use links to support triangulation of target locations. It is not clear how well any of this worked in practice. Russian descriptions of Cold War Soviet research reveal that programs often stayed alive with little or nothing to show for the effort expended. Descriptions of actual practice suggest that systems often failed to function as conceived. Apparently the Soviets often were better at system engineering than at component engineering. That should not deter us from exploiting their more imaginative solutions to problems we face.
One interpretation of the Soviet use of underwater communications would be that they were interested in what we would call network-centric operations. In such operations, the shooter forms his tactical picture on the basis of information collected by other platforms and sensors in a network. Each platform in the network contributes to the net-generated picture, but each need not sense the target to be struck. One effect of network-centric practices, evident in recent air operations in both Afghanistan and Iraq, is that the target receives no real warning, because the sensor that spots it and the sensor that tracks it need not be on the platform that carries out the attack. The Soviets were fascinated by group operations, and it is difficult to imagine they failed to realize this possibility.
HAIL offers us what the Soviets sought, with the wonderful twist that it would be covert. Imagine the effect of operating something like HAIL in parallel to the existing high-capacity noncovert acoustic link. A UUV would covertly send processed video back to the submarine or another vehicle or UUV that operated it. From time to time there would be need for more detail, which the HAIL-like link could not transmit in any reasonable time. In that case, the covert link would transmit the order to use the overt link. Note the tactical picture supported by the HAIL-like link would let all parties in the net know where they were relative to each other. Given such information, noncovert transmission could be made directional, hence much more difficult for an enemy to intercept.
Right now HAIL provides range and rate information of the sort that would be used to build a tactical picture, but it does not feed into a submarine's combat direction system. Instead, it feeds a stand-alone text display. HAIL was first demonstrated in 1998, and as of 2004 it was in preproduction status. As of early 2004 it was on board all Australian submarines. In the Lungfish 2003 exercise, the Australian submarine Waller lost her underwater telephone, but was able to continue the exercise because she had HAIL on board. The Japan Maritime Self-Defense Force is expected to place HAIL aboard one submarine for RIMPAC 2004. The U.S. Navy leased sets for its Advanced SEAL Delivery System minisubmarine and for the submarine's support ship. It is currently being proposed for the U.S. Acoustic Rapid COTS Insertion submarine upgrade program.