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The U.S. Navy is using vacuum systems hooked to 400-foot-long four-inch hoses with special Kennett nozzles to clean bilges that require a 60- to 70-foot vertical lift. Units at the Trident Facility, Bangor, Washington, include a chemical flushing unit.
HF Radar Shows Promise
Several U.S. companies competing for a U.S. Navy contract to build a shipboard high-frequency (HF) surface-wave radar displayed their designs at the annual Washington, D.C., Navy League show in April. The Navy wants a system that can detect reliably a small sea-skimming missile at 20 nautical mdes—and an airplane-size target at 40 nautical miles.
Compared to conventional radars that operate at much higher frequencies, an HF surface-wave set would be much more likely to overcome low-level multipath problems, which are caused by reflection off the sea surface, and surface clutter. The HF surface wave actually extends well beyond lhe horizon, offering a platform the earliest pos- s,ble warning of a missile’s approach. Large shore-based sys- terns, for example, can detect surface ships at ranges of 180 nautical miles. The much shorter ranges the Navy wants should be attainable with small shipboard sets.
HF technology is hardly new, although it has been used mainly for communication. Its long-range capability stems from the sky waves that bounce off the ionosphere; the surface wave—tied to the surface of the sea—carries for much shorter distances, but still extends well beyond the horizon. HF direction finding ("huff- duff’) was a decisive Allied counter against 'Vorld War II German U-boats, which had to communicate with their shore-based commanders to operate effectively in wolf packs. Most warships today carry some form of HF direction-finder.
Perhaps the most intriguing technical point ,s that an effective HF direction-finder can use a very small antenna to detect long-wavelength signals; in contrast, radar antennas are generally many wavelengths wide. Compact shipboard antennas suffer from very limited gain, which is why they generally pick up signals only at relatively short ranges.
An HF radar probably would combine a non-directional (floodlight) transmitter with a receiving antenna much like that of an HF direction-finder. Existing direction-finders can localize signals within about 1° in bearing, a performance quite comparable to that of good radars. One proposal described at the show paired such antennas to improve target localization by triangulating, using the baseline between the antennas.
Floodlighting has the advantage that the radar is always open to objects crossing its horizon and there is no delay while the radar beam moves into position. On the other hand, typical shipboard direction-finding antennas generally cannot distinguish several signals of the same frequency coming from different directions. The HF radar, then, might find it difficult to handle salvoes of missiles timed to arrive nearly simultaneously from different directions. The Watkins-Johnson Company approach promises solutions, but it appears that something rather more elaborate than a masthead antenna would be needed. It might be provided in the form of small antennas spread along the hull, as in the U.S. Navy’s Classic Outboard high-frequency signals exploitation system, which was devised largely to assist in Tomahawk missile targeting.
In the absence of any form of triangulation, range probably could not be determined very accurately. Targets would be detected using the Doppler effect, i.e., by their velocity towards the ship, which would eliminate most sea clutter. At higher radar frequencies, that usually means a pulse-Doppler radar putting
Vacuum Systems Keep Navy Green
out tens of thousands of pulses per second to capture Doppler shifts of tens of thousands of cycles per second. At lower frequencies, the Doppler shift is much smaller, and the associated pulse-Doppler radar would emit many fewer pulses per second. It would not put enough power into any one pulse to ensure that the target would be detected.
An HF radar probably would emit continuously to detect an incoming target by its Doppler shift, indicating its closing rate. Varying the radar frequency up and down would change the observed Doppler shift and also would measure approximate target position. One great advantage of continuous emission would be very low emitted power; such a radar could not be detected easily by shore stations or by nearby ships. The great drawback of HF systems has been that they are so easy to detect and track at great distances.
Aside from seeing beyond the usual radar horizon, HF radar might enjoy a considerable advantage when dealing with stealthy aircraft and missiles, whose radar-absorbing coverings probably are ineffective at the radar’s very low frequency and long wavelength. Stealthy shapes might also become irrelevant, since the radar signal would resonate with the overall size of the target, not with target details such as special shapes. This argument suggests that HF radar may prove the antidote to low- observable aircraft.
Stealth aircraft advocates may reply that conditions at sea are rather special. Because the sea conducts electricity, it can carry the HF surface wave efficiently; there is no comparable HF surface wave on land. While it is possible to build an HF radar to work over land, it would need a massive antenna to form a beam and to focus its energy (the conductive sea much simplifies beam-forming).
If all this seems too obvious to be new, the reader should note that the Royal Navy tried a simple HF surface-wave radar in the oiler Grey Rover in 1988, and that several companies, including Marconi, have tried to market surface-wave coastal-defense radars. Several U.S. companies have developed coastal surface-wave radars for use over the sea—e.g., to detect oil spills—and the technology has existed in one form or another for about two decades. Because of its primarily scientific application—and because of the success of conventional radars— it apparently attracted little naval interest. Now, however, the short-range antimissile problem has come to the fore; a compact HF surface wave radar may be just the sensor needed to trigger shipboard defenses.
Variations on conventional radars also were displayed at the show. Westinghouse exhibited an SPQ-9B, an antimissile variation on the existing SPQ-9 track-while-scan surface- search radar that is part of the standard Mk 86 fire-control system. The SPQ-9 scans the horizon at 60 revolutions per minute, updating its picture of a target’s position every second. If the target does not move very fast, such updates suffice to track and even to direct gunfire.
That data rate cannot suffice for a missile-defense system. Nor is the conventional radar signal processor likely to distinguish a very small incoming target from the surrounding sea clutter. The SPQ-9B solution superimposes a pulse-Doppler radar and receiver on an existing radar (the operator can choose either radar picture). With its very high pulse rate, the Doppler radar pours out enough energy to detect even a very small target, and its signal processor can distinguish it from the surrounding clutter. In this particular case, the pulse rate is the highest for which a high subsonic target, such as an Exocet missile, is clearly distinguishable (the pulse rate imposes a maximum unambiguous target speed on the radar, just as it limits the maximum unambiguous range the radar can reach).
Pulse Doppler is a very conventional solution. Data rate is a trickier question. If the radar rotates faster, it can react more quickly to a target popping over the horizon, but it then spends less time pointing in any one direction, pouring less energy into that direction—and is thus less likely to pick up a small target. Slow rotational rates improve a radar’s chance of picking up a small target. If the radar happens to be pointing away from a target when it pops over the horizon, however, the target can close to a much shorter range before it is detected. Moreover, what matters is target position on a series of scans, since target movement normally can only be measured that way.
The Westinghouse solution is clever. The SPO-9B has three separate radar feeds, spaced next to each other. As the radar turns, each stares in sequence at the same place on the horizon, tripling the amount of radar energy poured out—the scan rate is halved, when compared to the SPQ-9.
Cleverness goes much further. Normally, a pulse-Doppler radar must shift pulse rate in order to measure range accurately;
that takes time. In the SPQ-9B, each of the three feeds probably operates at a slightly different pulse rate and the system computer compares the three to find target range. On a single scan, then, the radar locates the target and measures its speed (or rather the speed component pointed towards the ship). Any other kind of radar would require at least two scans to obtain the same two quantities, both of which are needed if the ship is to react effectively.
The U.S. Navy also is seeking an infrared search-and-track system (IRST), having abandoned work on the U.S.-Canadian SAR-8 some years ago. An IRST offers over-the-horizon Per' formance in that it can detect the rising plume from a missile motor. On the other hand, it cannot measure the range to the plume. Moreover, it takes time for the plume to rise above the horizon. If the missile maneuvers violently on its approach, the position of the plume may not be a good indication of the missile’s course, or a good cue to other sensors that must pick up the weapon once it has come over the horizon.
Infrared technology is quite mature; the main issue in developing an IRST is automatic detection in the face of numerous false targets such as clouds reflecting sunlight. The planned U.S. IRST would operate on one of the two IR bands. A future system might overcome clutter by comparing the intensity of the IR signal from the same object in both bands, in effect measuring the object’s temperature.
More Thunder Out of China
More evidence of oil under the Spratly Islands in the South China Sea is heating up the Far East. Seven countries claim these previously unattractive islands: Brunei, China, Indonesia, Malaysia, the Philippines, Taiwan, and Vietnam. China has recently published maps showing what it refers to as an historic claim to a large area of the South China Sea, including the islands. A Philippine force recently destroyed Chinese markers in the area, in effect claiming the islands. As for Vietnam, where in the past Vietnamese maps emphasized Cambodia (which had been the main target of Vietnamese expansionism), now Cambodia is usually masked by the standard block of type captioning the map, whereas the Spratlys are prominently described as part of Vietnam.
China is clearly the most powerful state in the region, but its fleet cannot yet project power far from home. Very few Chinese surface combatants have either modern combat-direction systems or surface-to-air missile systems. Such ships probably would fall easy victims to modern jet aircraft, particularly if armed with stand-off missiles. China has no aircraft carriers and the Spratlys are barely within range of air bases in southern China. It is unlikely that aircraft flying from the southern bases could support a Chinese occupation. On the other hand, none of the other claimants can project much naval power either.
The United States conceivably would have no obvious interest in whoever owns the Spratlys, as long as the owner develops their resources and places them on the world market. Given the claimants, that seems almost inevitable.
The region’s nations do have one other way of quickly changing the local balance of naval power. They can buy the only current pool of surplus naval assets—that held by the Russians. Navies cannot be built in a day, and once built, they require vast numbers of experts to operate and maintain them- The implication would seem to be that Russians would have to be hired to operate the ships, at least at first.
Nothing like that seems to have happened, but the crisis is still young. Too, everyone in the region may imagine that China will be paralyzed for several years following the death of Deng Hsiao-Ping, which is probably imminent. Once the crisis matures into imminent disaster, will we be wise enough to ensure that we are the ones to decide whether to intervene?