In February, Secretary of Defense Ashton B. Carter announced that the U.S. Navy would soon field a version of the SM-6 antiaircraft missile as a supersonic (Mach 3.5) antiship weapon. The SM-6 differs from earlier Standard Missiles in that it has an active (rather than semi-active) radar seeker; it is designed to attack enemy missiles beyond the horizon of the firing ship. It is already designed to maneuver violently at low altitudes to do so. What is interesting in the announcement is the way in which an existing proven missile can be quickly re-tasked. That is a consequence of the character of modern radars and control systems rather than of the specifics of the SM-6, and it is probably a harbinger of further conversions of existing weapons for new purposes.
The Navy is already showing much greater interest in antiship weapons as it recognizes China and its People’s Liberation Army Navy as peer competitors somewhat analogous to the Soviet Navy of the Cold War. Thus there was considerable interest in buying a new supersonic antiship missile for several years before the SM-6 announcement. The new missile may well still materialize, but experience strongly suggests that development will take a long time. Modification of an existing missile is much faster, and in the case of the SM-6 it almost certainly will be.
Why should that be? The answer is that missile-seeker operation and missile control are now largely run by software. Nearly all modern radars use computer-generated waveforms amplified by power tubes (or transmitted via Tx-Rx modules). That contrasts with a past in which a radar signal was generated by a power tube, such as a magnetron, with more or less fixed parameters. The radar was in effect hardwired for a particular function, such as air-to-ground ranging. Computer-generated waveforms became practical in the 1970s; among the very first radars to use them was the AWG-9 in the F-14 Tomcat. Given computer waveform generation, the radar in an F/A-18 Hornet can switch easily between air-to-air and air-to-surface modes. The consequence of this change was graphically demonstrated during Operation Desert Storm when a pair of F/A-18s encountered Iraqi fighters while on a bombing run. They switched their radars to air-to-air mode, shot down the Iraqis, and then switched back to resume the original mission. That the F/A-18 radar could do what the much more massive radar in an F-14 could do a few years earlier reflected the rapid development of computer technology under Moore’s Law: Computer power doubles (or size halves) about every 18 months. In the years since the F/A-18 radar was conceived, much greater computer power has shrunk to the point where it can fit inside a missile.
Multi-Mode Capabilities
Software-controlled radar has had many other consequences, perhaps most dramatically in the decline of electronic countermeasures. The old hardwired radars could be recognized by their more or less fixed waveforms, whose parameters were listed in the electronic libraries of countermeasures equipment. When an Iraqi airplane attacked the USS Stark (FFG-31) in 1987, her electronic-warfare operator exhibited an early version of the problem: He thought the attacker’s air-to-surface radar was that of an Iranian F-14 operating in one of its many modes. At that time, and for years afterward, nearly all radars were still hardwired. The Soviets went so far as to build radars with alternative (war-reserve mode) magnetrons, the idea being that they would switch modes when war was imminent, making their radars unrecognizable. During the run-up to the 1991 Gulf War, coalition aircraft made dummy runs toward Iraqi airspace specifically to force the Iraqis (who had Soviet equipment) to turn on their radars in war mode, so that the key parameters of the waveforms associated with these special modes could be tabulated for use during the air campaign. Coalition jammers were programmed accordingly. It is still possible to associate emissions with individual radars (just as the markings on a bullet are associated with a particular gun), but it is difficult to imagine accumulating a catalog of all the world’s radars on a set-by-set basis.
The SM-6 is a more benign application of the same idea of software-generated radar signals. Given the dramatic shrinkage of computers, it should not be a surprise that a missile can now accommodate a multipurpose radar, or that the computer memory involved can accommodate software to generate several different radar modes. The SM-6 is designed to intercept targets at a wide variety of altitudes, so it probably already can switch between multiple air-to-air modes. Adding an antiship mode is not a great leap. Much the same can probably be said of the missile’s programmable autopilot. The autopilot has been a feature of Standard Missiles since the advent of Aegis; normally it is commanded by the Aegis combat-direction system. To make the SM-6 an effective over-the-horizon missile doubtless required that the autopilot be commandable by other platforms, or perhaps via a data-link net (Link 16 is usually associated with such commandable missiles, including Tomahawk).
The idea of using the commandable Standard against surface targets is not entirely new. Some time ago a version of the missile (SM-4) was proposed as a means of attacking land targets. The autopilot offered great precision, as the firing ship could command the missile to strike as needed. This particular version of Standard Missile was rejected, presumably because the missiles were badly needed for fleet air defense. Note that it was not an alternative use of existing missiles; at that time the change to a commanded attack on a particular point required a different version of the Standard Missile. (Previous semi-active versions of the missile, back to SM-1, all had an inherent antiship capability, but only to the radar horizon; some foreign navies bought the SM-1 as an antiship missile. The U.S. Navy also had a ship-launched version of the Standard ARM missile, which could home on shipboard radars, and it tested an active-seeker version as an interim alternative to Harpoon; this latter program was not pursued.)
‘Present in the Missile Memory’
Using a special version of the Standard for surface attack raised a particular problem. Standards, as well as Tomahawks and Enhanced Sea Sparrows, are all fired from the same vertical launchers. Ships have limited numbers of vertical-launch tubes, and they cannot be replenished at sea, at least not outside calm water. Even then, replenishment is a lengthy business. The choice of missiles at the outset of an operation must be difficult. Incidentally, this is much the same problem as that of choosing submarine load-outs when torpedo tubes can accommodate alternative weapons, such as swim-out missiles and torpedoes. There is a reason the U.S. Navy has a single type of heavy torpedo for both surface and undersea targets. The Royal Navy made much the same choice. That required considerable effort, because surface and undersea targets require very different kinds of homing and other features, but it was considered worthwhile (the Soviets were unable to produce a dual-purpose torpedo, so their load-outs for any one type of target were more limited than those of Western submarines).
It seems likely that the computer capacity of the new version of the SM-6 solves this kind of problem. If the missile’s radar can switch modes as required, the modification is to the software, not to the hardware of the missile. The alternative modes are always present in the missile memory, and a command at firing time (or even after) determines which mode the missile turns on as it flies. In much the same way, the missile’s autopilot is software-controlled (or software-defined), and both antiair and antiship versions can be resident in its memory at all times. Reportedly the antiship version of the SM-6 will incorporate a GPS receiver, so that the missile can be sent to an over-horizon target, but the missile is already intended to attack air targets beyond the horizon. That suggests that it already has some form of navigation like GPS.
Most likely the main change in the antiship (or dual-purpose) version of the SM-6 is in its warhead. There really is a difference between a warhead intended to destroy a fast air target and one intended to damage a ship. Even here the advance of computer power may be a solution. For some years, lists of naval-research projects have included multipurpose warheads that could be set off in different modes as required. Presumably the details of detonation would be controlled by the missile computer, possibly on the basis of whatever its active seeker sees.
The result would be an SM-6 that could be switched between antiair and antiship modes at the time of use, not before. No commander would have to guess in advance whether more antiship or antiaircraft missiles were needed, because there would be no distinction. All the SM-6 missiles in a ship’s vertical launchers could be used for either function, as needed. Moreover, because the SM-6 already exists and has been tested successfully, its antiship version can be fielded extremely quickly, far sooner than any alternative.
Dr. Friedman is the author of The Naval Institute Guide to World Naval Weapon Systems, Fifth Edition, and Network-centric Warfare: How Navies Learned to Fight Smarter through Three World Wars, available from the Naval Institute Press at www.usni.org.