Among several new items on display at this year’s Euronaval show in Paris were active-array radars. Active electronically steered radars (AESAs) have been available for some time; the new one this year was Raytheon’s S-band Air and Missile Defense Radar (AMDR-S) intended for the Flight III Arleigh Burke–class destroyers. It marks a considerable advance in AESA size and capability.
AMDR and earlier active arrays offer several advantages over passive phased arrays such as the SPY-1 that equips current Arleigh Burke–class destroyers. An active array can generate multiple beams simultaneously, because its transmitting elements can be formed into multiple subgroups. A passive array has only a single signal generator, so it produces just one beam at a time. An active array can also null out jammers to an extent impossible in a conventional phased-array radar (this technology has been applied to some communications satellites for exactly this reason). Active arrays are inherently modular because they consist of individual transmit-receive (Tx-Rx) modules. By way of contrast, a conventional phased-array radar may or may not be built of modular sections, but it has only one power tube, hence cannot really be modular or scalable. In the case of an active array, in practice the extent of scalability depends on how the elements are wired and built. Scalability can be spectacular. At the previous (2012) Euronaval, Israel Aircraft Industries displayed both its full-up Barak control radar, with four large faces, and a small-boat warning radar using a single module of the same radar.
Most existing active-array radars operate at shorter wavelengths (X band or above), meaning at relatively short ranges. For example, the German and Royal Netherlands navies use X-band active arrays to control SM-2 missiles on board their frigates. X-band means limited range; both of these navies use conventional rotating S-band radars to detect and track targets before engagement. The U.S. Navy uses S-band for its SPY-1 Aegis detection and tracking radar because it demands long-range detection. Replacement of SPY-1 by an active array has been discussed for many years, the barrier always being that S-band active-array operation was not yet efficient.
Great Potential
Compared to a conventional radar, an active array offers great simplicity. In place of the microwave plumbing at the rear of an SPY-1 antenna, it has wiring taking power up, and fiber optics carrying the signal up to the array and the received signal back down to the processor.
Modular active arrays offer other interesting potential. At present they are built like earlier passive arrays, all antenna elements being concentrated together. That is, from the outside AMDR-S will look much like the current SPY-1. However, it will have radically different survivability from the point of view of radar failure. SPY-1 relies on a single water-cooled power tube, which generates the powerful signal the radar emits. The radar can keep operating if some of its elements are destroyed, but not if the single tube fails.
It is difficult to imagine spreading parts of a passive array around a ship. However, the massive SPY-1 arrays are surely an obvious point of aim for an antiship missile, and some new missiles, such as the Norwegian NSM, are designed to attack a selectable aim-point. A truly modular radar such as AMDR-S offers a new possibility. Its elements might be spread around a ship in such a way that no selectable attack could destroy much of the ship’s radar capability. That would make modular active arrays not merely attractive but essential as a way of making a ship’s combat system survivable.
If active arrays offer so much, one might ask why any other kind of radar is still being made. The active array concentrates its weight at the antenna, which may present topweight problems. It also concentrates its power there. The array necessarily creates waste heat, which must be carried away. In a conventional radar, the waste heat is concentrated around the radar power tube deep in the ship, not in the antenna. For that matter, it has taken time to create Tx-Rx modules that can produce sufficient power to compete with the mature technology of radar power tubes (or, in some cases, solid-state power tubes).
AMDR-S is probably the largest active array currently in development. It is built out of modules (2 x 2 x 2 feet), assembled into arrays about the size of the SPY-1 arrays currently in service. Because the radar is modular, it can be expanded if so desired, at a cost in weight and in increased power demand. One special feature of the arrays is that sensitivity has been increased by a factor of about 30 compared with SPY-1. That improvement affects both required power and potential range. Raytheon claims that its GaN Tx modules are more efficient than those of any operational active-array radar, offering four times the power density at higher efficiency. Power density is the measure of how much power an array of a given size can emit.
Radar range is determined by how much power the radar receives back from a target. How much power comes back depends on the distance to the target. How much of what echoes back the radar receives, hence can use, depends on gain (in effect, the size of the antenna in wavelengths) and on sensitivity (the fraction of what is received that the radar can use). As a consequence, the radar designer can trade off sensitivity against power. Anything that increases sensitivity or power or gain by a factor of 30 (as Raytheon claims for AMDR) more than doubles radar range, all the other factors being equal. That is, whatever power the radar puts out is diminished by the fourth power of range by the time the radar is receiving an echo. Doubling the range reduces received power by a factor of 16. Tripling it reduces power by a factor of 81.
In addition to greater sensitivity, Raytheon claims its new Gallium Nitride (GaN) Tx-Rx modules are far more efficient than the usual Gallium Arsenide modules, and also that they are 34 percent less expensive. Greater efficiency would mean that the module turns more of the energy going into it into radar signal and less into waste heat, reducing the usual cooling problem associated with an active array. Plans for the new Flight III Burkes currently call for upgraded cooling (five 300- rather than 200-ton plants) and considerably more power (three 4 MW ship service turbogenerators rather than two 3 MW generators). The increase in power suggests considerably greater radar output, compared to the SPY-1D of the current Flight II. The footprint of the radar as installed should match that of the existing SPY-1. Indeed, one of the requirements of the RFP was that the new radar should be suitable for backfit into ships currently equipped with SPY-1.
At least in its initial form, AMDR-S will operate in tandem with the rotating X-band SPQ-9B radar, which has been in service for some time for horizon search and rapid warning and engagement of sea-skimming threats. SPQ-9B is a conventional radar; the contract envisages possible integration of a new X-band element with AMDR-S in some future installations. For now, integration is at the control or back end, both radars contributing to a common tactical picture of air threats. This common picture is fed into the ship’s combat system. On board an Arleigh Burke, SPQ-9B will be mounted on the foremast in place of the current SPS-67 surface search radar.
Threats from Above
At the lower end of the air-defense spectrum, the Israeli company Rafael displayed a naval version of its Iron Dome interceptor system, C-Dome. A ship is a radically different situation from the wide-area problem for which Iron Dome was designed. Dealing with mass rocket attacks, Iron Dome used a command system that filtered out all but threats to populated areas. That alone made Iron Dome affordable, because it converted mass attack by inexpensive rockets into a containable problem.
To handle even that scenario, Rafael had to produce an inexpensive but highly-maneuverable missile: Iron Dome never had very much time in which to intercept an incoming rocket. Rafael’s engineers reasoned that the missile itself might have other applications in which short range and very quick reaction were important. They focused on anti-sea-skimmer defense. The missile itself did not have to be modified, but its seeker software had to change to deal with low altitudes and signal reflection off the sea. Missile engagement range is shorter than that of most other point-defense weapons, but Rafael argues that longer ranges are unrealistic, given that a sea-skimmer would not be detected beyond the horizon say, more than 10 miles. The rejoinder might be that in effect Rafael is saying that longer range can become useful only if an incoming sea-skimmer can be detected well beyond the radar horizon. For example, the missile’s rising infrared plume might be visible well before the missile itself appeared.
At one time the U.S. Navy was interested in using HF radar to detect incoming missiles beyond the horizon, based on Doppler. This particular project died about 2000, but the widespread use of infrared search devices suggests that the plume offers real possibilities. Rafael offers its missile in a 2 x 2 vertical box launcher, which can be installed alongside IAI’s Barak 8. India has bought Barak 8, and it may be seen as a likely buyer for C-Dome.
The French company DCNI displayed a submarine buoy carrying electro-optic sensors. It associated the buoy with its ongoing work on a submarine-launched Mica missile as an antidote to antisubmarine aircraft and helicopters. At past shows, DCNI has shown a Mica in the torpedo-tube capsule originally developed for the Sub-Exocet antiship missile, without giving much indication of how the missile was to be targeted. At this show its representative pointed out that a hovering helicopter could be engaged on the basis of acoustic data (some in the helicopter community would say that modern helicopters are too well silenced to be vulnerable, but presumably DCNI had the pinging of the helicopter’s dipping sonar in mind). A maritime patrol airplane would be a different proposition. At the least, the submarine would want to be sure that its target actually was an attacker. Raising a periscope was a poor solution; better for the submarine to trail a buoy at the end of a fiber-optic cable. The buoy has been tested, but not yet from a submarine.
The buoy has far more interesting implications. The U.S. Navy’s Virginia class, among other submarines, employs an electro-optical periscope. Because there is no longer a tube connecting the CO with the lenses atop the periscope, the CO no longer has to be in a space directly under the sail. That reality has made for a much-enlarged control space, deeper in the submarine. Electro-optics also make it possible to reduce periscope exposure time, the sensors rapidly capturing a scene that the CO and others can examine at will. Even so, if the head of the periscope is detected, it gives away the position of the submarine.
The point of the DCNI buoy is that even this connection is not necessary. As long as the submarine command system knows where the buoy is relative to the submarine, the buoy can be towed at a distance. It may well be considerably larger than a periscope head, hence more visible, but as long as it does not betray the submarine’s position, that may not be a great problem. As for knowing where the buoy is, inexpensive modern inertial sensors offer a likely solution. Optics matter more rather than less for submarines operating in littoral areas; DCNI is dramatizing a solution to the vulnerability optics have always presented in the past. Given good medium-range underwater communications, the step beyond would be for the optics to be carried in an unmanned surface vehicle launched by (and retrieved by) the submarine.