A (very) brief history of Anti-Radiation Missiles (ARMs) in US service.
The introduction of Soviet supplied SA-2 Guideline missiles into the air defenses of North Vietnam in 1965 found US Navy and Air Force tactical air power ill equipped to fight in the missile environment. This is somewhat surprising, as the Navy had a decade of experience with operations missile equipped cruisers and destroyers, and the Air Force had been training with Army Nike-Ajax sites for a similar length of time. When the North Vietnamese began using SAMs, US planes lacked radar warning receivers, and jamming equipment. Tactics and training to mitigate missile attacks were also lacking. Fortunately, one weapon was already under development to tackle the Guideline threat- the AGM-45 Shrike Anti-Radiation Missile. In this case “radiation” referred to radio-frequency radiation, not the glow-in-the-dark kind.
As soon as the semi-active radar homing technique for missile guidance was developed, designers also realized a weapon could be designed to home in on the enemy radars with a similar seeker. Using the AIM-7 Sparrow missile as a starting point in 1963, by 1965, the Shrike entered Navy service. It shared a similar configuration and size with the Sparrow, but was in fact an entirely new missile airframe. US Navy A-4 Skyhawks began carrying the Shrike in 1965, and soon it was carried aboard A-6A and later A-7 aircraft. The Air Force adopted the Shrike in 1966 on board F-105F and eventually F-105G jets.
The Shrike was a pretty good start, but it suffered from three pretty severe limitations. First, it had a pretty short range. In fact, its best range was less than that of the SA-2 missile that it was designed to defeat. That meant the attacking jet had to enter the engagement zone just to get a shot off. This relatively short range, about 15 miles, was a consequence of the choice of a relatively small missile body. That made it cheaper to buy, easier to carry, and easier to store aboard ship. But it still made life exciting for the crews using it.
Secondly, technology limitations of the time meant that the seeker head could only search for radars in very specific bands of the E/M spectrum. Since there are radars that operate on a wide variety of frequencies, that meant the services had to develop a wide variety of seeker heads for the missile. And if you were over the target and had the wrong seeker head, you were out of luck.
Third, the Shrike was a pretty dumb missile. It guided just fine as long as the radar site it was attacking kept radiating. But it didn’t take the North Vietnamese long to figure out that as soon as they saw a Shrike launch, they could just turn off the radar. The Shrike would go stupid and miss the target. Still, forcing the radar off the air meant it couldn’t guide missiles at the main strike group, and the minute or so it was shut down might be all the time needed for the strike to get in.
Still, something better was wanted. And so the Navy looked at adapting its SM-1 Standard missile (RIM-66) as an air-launched ARM. The “Standard ARM”, or AGM-78, was a BIG missile. Where a Shrike weighed just under 400 pounds, the STARM weighed in at a whopping 1370 pounds. A bigger missile meant a bigger motor, which meant much greater range. Whereas a Shrike had a range of about 15 miles, the STARM could reach out to 50 miles under optimum conditions, well outside the range of the SA-2. The bigger missile also had a larger, deadly warhead. Most importantly, the larger size meant there was considerably more room for electronics in the guidance section, so the STARM was equipped with a broad-band seeker head, giving the attacking flight crews much greater flexibility in engaging unexpected targets. Finally, the STARM also had a memory that allowed it to continue to the target even if the radar was shut down. Accuracy was degraded, to be sure, but it was better than nothing.
Now, these improvements sound great, but they came at a cost. The STARM cost much, much more than the Shrike. Further, while the Shrike could be fired from just about any attack plane in the Navy (the Air Force used dedicated F-105s for the SEAD mission), the longer range of the STARM meant more sophisticated detection and localization of the target was needed, so it had to be launched from a dedicated platform. The Navy modified a handful of A-6A Intruders to A-6B, which were used to carry the STARM. A typical A-6B might take off with a centerline fuel tank, two STARMs on the inboard wing pylons, and two Shrikes on the outboard wing pylons.
The Shrike and the STARM both served through the end of the Vietnam War and continued in service well into the 1980s until they were replaced by the current US anti-radiation missile, the HARM.
But as the 1970s wore on, the number and quality of Soviet radar guided missiles kept increasing. A better anti-radiation missile was needed. Development started on what became the HARM. The challenge was to fit the range, speed, broadband seeker and memory of the STARM into a Shrike sized package that could be carried by a wide variety of strike aircraft. In the end, the designers met the first half of the challenge, but had to settle for a missile considerably larger than the Shrike. Still, the AGM-88 HARM is a lot smaller than the STARM, weighing in at just under 800 pounds. While the Air Force uses it almost exclusively from F-16 jets equipped with the HARM Targeting System (HTS) pod, the Navy operates it from almost every F/A-18, as well as its fleet of EA-6B Prowlers. The “H” in HARM stands for “High speed.” With a speed of well over Mach 2, even at relatively low altitudes, the HARM has an operational range of about 50 miles. It’s broadband seeker can detect and track just about every radar source around.
The HARM has three operating modes- PB, TOO, and SP. PB, or Pre-Briefed is used when the geographic location and electronic characteristics of a target are known before launch. TOO, or Target Of Opportunity, uses the missile seeker before launch to display a track of active radars near the launching aircraft. The aircrew can select which target is the greatest threat to the strike package, and send a HARM to it. SP, or Self Protection, automatically targets the HARM at any enemy radar that locks on to the launching aircraft.
The HARM entered service in 1985, and as early as 1986 saw use in combat against Libya. It was widely used in Operation Desert Storm, the 1999 air war in the Balkans, and continues in service today. It has been widely exported to NATO and other friendly nations, and has been continuously updated.