In May, details of a hitherto secret U.S. battlefield reconnaissance aircraft developed during the 1980s, Northrop’s Tacit Blue, were revealed. Unlike the F-117 and B-2, Tacit Blue resembles nothing so much as a slightly rounded brick provided with wings and a V-shaped tail. The air intake is a tunnel leading back from its flat upper surface, so that a ground-based radar cannot look into the compressor of the jet engine, normally a major contributor to radar cross-section.
Tacit Blue was built to fly its big side-looking radar deep into enemy territory to detect forces advancing toward the front from hundreds of miles away. Once located, they could be attacked by long-range weapons such as the Army Tactical Missile System (ATACMS). Tacit Blue, probably part of the Assault Breaker program of the 1980s, was seen as a complement to the E-8A Joint Surveillance Target Attack and Recognition System (JSTARS), which carries its own side-looking radar but is hardly stealthy. Planners feared that the aircraft would never get close enough to deep- echelon forces to locate them.
As it happened, although a fly-by-wire control system helped keep Tacit Blue in the air, its grossly unaerodynamic form caused severe problems, and performance was less than sparkling. In the end, it probably proved easier to improve the radar performance of the more conventional JSTARS, allowing it to locate deep forces from stand-off ranges outside enemy territory. More likely with the end of the Cold War came the perceived end of very mobile long-range Soviet surface-to-air missiles capable of dealing with JSTARS, hence the end of any attempt to cure the obvious problems of Tacit Blue.
Tacit Blue is, however, still interesting because of the implications it has for the connection between radar antenna design and stealth. The problem was that a side-looking radar requires a slab-like antenna, precisely the sort of shape most antithetical to what might be considered conventional stealth thinking. Apparently, the designers thought more deeply about exactly what a radar does: a phased array forms a beam in an arbitrary direction, and it also can reflect an incoming beam in any desired direction, because in effect the phase shifters convert a flat surface Pointing in one direction to an electrical analog of the same surface pointing elsewhere.
In this sense, Tacit Blue was a different application of the same stealth idea embodied in the F-117—rather than try to absorb radar energy, reflect it away from the enemy trying to receive its echo. The difference is that the F-l 17 does its reflection by shaping (as do missiles advertised as stealthy); Tacit Blue does much the same thing electronically. One might speculate that Tacit Blue had to have flat sides, because curvature would have made the necessary fast beam-forming calculations (made as the radar operated) too difficult.
This concept of electrically steering a reflected beam already has been exploited in some Russian phased arrays. Western systems feed energy into numerous small wide-angle radiators, whose relative phasing is computer-controlled. The backside of a SPY-1 antenna, for example, is a maze of microwave plumbing, leading radar energy into the face of the antenna. Some Russian radars work the same way. Others, such as the SA-N-9’s Cross Swords and the SA-N-7’s Front Dome, use a single feed to illuminate an array. Phase shifting of the array surface is used to move the radar beam electronically. The Russian system is very much like a more conventional dish radar, except that the dish is now electronically active to move the beam around. Its advantage is that the phase shifters need not deal with high-powered signals. On the other hand, the single rather massive feed blocks some of the antenna surface, and thus must carry a cost in radar gain.
Presumably, the Tacit Blue radar designers used phasing to reflect unwanted radar signals. One might speculate that the radar could operate over a large but tuned radar-frequency range, so that it could transmit and scan in one frequency while reflecting signals at a different frequency away from their transmitter, hence away from their intended receiver.
More generally, a shift from conventional reflector antennas to various kinds of phased arrays may drastically reduce the radar cross sections associated with those antennas, if only because they are no longer made of highly reflective material. The
French Sea Tiger target-designation radar may be a case in point. The conventional version is a classic paraboloid antenna with a horn feed. Although in theory the paraboloid directs all radiation approaching from head-on into the horn, in fact it is a large radar target from any other direction. In the French Navy’s stealthy La Fayette-class frigates, the reflector is replaced by a flat planar antenna, which must be a two-dimensional phased array. The flat portion of the antenna carries a pair of end plates. In this case, phase-shifters are used mainly to form the radar beam, but they may be buried in a largely non-reflective structure. That alone should reduce radar cross-section dramatically.
In a missile or an airplane, the reflecting radar antenna in the nose clearly has an enormous radar cross-section. The larger the dish, the greater the gain—which means both better radar performance and, usually, a larger target for enemy radars. For example, after the USS Vincennes (CG-49) shot down the Iranian Airbus, many commentators asked how a big airliner could possibly have been mistaken for a fighter. Surely the sheer magnitude of the radar return (in effect, the target’s radar-cross section) would have tipped off those on board the ship. The answer was no, partly because the big radar of the fighter’s AWG-9 system requires a big radar-cross section (the two big engines would have helped considerably).
This is nothing new. One way to slash radar cross-section, used on some of the large Russian antiship missiles, was to stow the antenna edge-on when it was not in use. The missile would cruise at high altitude with its radar turned off, then switch it on when its fire-control system predicted that the target was coming into view. It would store target locations, switch off the radar, then dive to low altitude. The radar would come on again only as the missile approached the horizon of the target group. Each time the radar was switched off, the antenna was stowed to cut the missile’s radar cross-section. Any failure in the stowing mechanism would have caused the missile to fail. Also, in some cases, the radar must remain in operation continuously.
Tuned radomes are another approach. If the radome is opaque except over a very narrow band of frequencies, chances are that the opposition (using a different band) will not see the antenna behind it. This idea is being applied to the next-generation Swedish missile corvette, and reportedly it has been applied to the Exocet Block II missiles now in service.
Much depends on the width of the usable frequency band. The missile gives itself away not merely because it reflects radar signals, but also because it generates such signals. Radars can be made stealthier, generally by spreading the energy of each pulse over the widest possible band. The simplest example is a pulse-compression radar, which often uses “chirped” pulses (frequency modulation [FM] slides). Even though peak power may be quite low (hence hard for an intercept receiver to detect), a sustained chirped pulse is equivalent to a substantial conventional one. The longer the pulse, the higher the equivalent peak power. Making a pulse longer, however, generally means using a broader range of frequencies. The ultimate radar of this type is Signaal’s Scout FM radar, in which pulses are so long that they merge together; they are distinguished by frequency.
Clearly a tuned radome limits bandwidth, hence—apparently—the stealthiness of the signal passing through it. The most likely solution is phase coding for pulse compression, which is used in some Italian (Alenia) air-search radars. A phase-coded pulse is constructed of multiple sub-pulses, all at about the same frequency, but differing in phase (in effect, in timing). This pulse can be compressed on its return just like a chirp, but by using more complex electronics.
The cheap modern phase-shifters that have made possible radars like the SPY-1 also make practical much simpler phased-array radars, like Sea Tiger. They also may reduce drastically the price almost every ship and airplane pays in passive stealth for its radars and other electronics.
In theory, a phased array can handle numerous simultaneous radar signals independently, where a conventional dish sees mainly the signals striking it head-on. That is, after all, why it associates its direction with the direction to the target. Any signals it receives from other directions are merely noise.
Existing phased arrays typically form a single beam pointing in a particular direction. Pulses are sent down the beam, and their echoes are received from the same direction. After dwelling on that direction for a time, the beam is turned off and reformed in another direction. That corresponds closely to the action of a conventional radar, which looks in one direction at a time.
A beam-preformed radar could, in theory, look in all directions simultaneously. For example, it might use an omnidirectional transmitter. It could also continuously detect emissions from other radars with radar-like directional precision. Alternatively, it might be able to form several transmitting beams (in different directions) simultaneously, using separate electronics, receiving signals from all of them without mutual interference. In theory, beam preforming on a radar scale is a possible direct consequence of the shift toward phased arrays.
In theory, beam preforming can solve what has been until now a fundamental problem in warship design: too many radars competing for too little centerline and mast space. To date, the closest approach to solving this problem has been the Italian SPY-790 (Empar), a rotating single-face phased array associated with the family of antiaircraft missile systems (FAAMS) program. The problem was to combine horizon search (against low fliers) with volume search (for longer range, mainly against conventional aircraft) in a single antenna located at the optimum position, high on the foremast.
The solution emerged because the phased array could move the beam across the antenna face, not merely up and down (as is common in phased arrays). On some scans, the radar beam was centered in the face of the array, which scanned rapidly over the horizon. Alternatively, the beam scanned backwards as the antenna turned rapidly. That gave it a lower scanning rate, so it could dwell longer to pick up more distant targets. The phased array could also pick the appropriate beam shape (in both dimensions) for each task. Critics of Empar argue that because it interweaves the two kinds of scans, it can never devote enough time and energy to either.
Beam preforming, if it could be applied, might help solve the problem by providing a pair of transmitting beams whose scan rates are not connected to those of the receiving beam. For example, a relatively low-powered omnidirectional horizon transmission would provide an unusual combination of, in effect, long dwell times for good detection and narrow receiving beams for good directional resolution. The volume search transmission would not have to be nearly as directional as the receiver, so it might use a smaller and lighter antenna.
One can go further. Many ships have slab-sided superstructures, which are major radar reflectors. In newer designs, hulls and superstructures have been redesigned with angled surfaces like those of the F-117 specifically to reduce the signal reflected back to an approaching radar. If computer costs continue to crash, and speeds continue to increase, it is at least conceivable that at some point it will be possible to cover portions of a ship with some sort of printed radar-like array, its reflecting beams adjustable by computer. The beams actually emerging front the ship would be quite weak, hence quite safe for those on board.
One advantage of such a ship would be simplicity of design (those slab-sided structures were often there because they offered more internal volume). Of course, the structure would also function as a rather precise radar warner. It is also true that the point at which all of this becomes reasonably inexpensive and practical could be so far in the future that it is invisible; Tacit Blue, however, which reflected 1980s (or even late 1970s) technology, suggests otherwise.