The
AGM-45 Shrike Antiradiation missile and the
AGM-78 Standard ARM carried the U.S. military through the Vietnam era. However, both had shortcomings. The Shrike was too limited in engagement envelope, and was easily visible when fired. Further, it was relatively poky by missile standards at
Mach 2, allowing enemy SAMs to confuse it by turning off their radars when their operators saw it launched. The AGM-78 was more capable in that it could be employed from a much more varied range of aircraft attitudes, which increased survivability of the firing platform; it also had a longer range. However, it was much, much more expensive than the AGM-45, and did not notably increase the speed of the attacking missile or decrease the visibility.
The U.S. Air Force and other SEAD arms (The Navy and the Marines) needed more capability. The next generation of ARM, the AGM-88 HARM, provided it. Its very name indicates one of its primary improvements; HARM is an acronym for High-speed AntiRadiation Missile. It is much faster than either the Shrike or the Standard ARM, although how fast is something that remains closeted - official sources just say 'supersonic.'1 Well, duh; both its predecessors were as well.
The HARM utilizes a low-smoke motor, as well, making it harder to see in flight. It was first deployed in 1984 as the basic AGM-88, but soon progress and the world situation forced upgrades. For example, Antiradiation missiles face one of the most quickly changing threat and targeting situations in the world. As SAM radars changed frequency, operating patterns and capability, the ARM was forced to keep up. The early models of the AGM-88 had what is called a 'fuzible link' to the memory systems in the missile. The result of this was that in order to accept software upgrades to counter these new threats, the missile (or at least, the missile's electronics) had to be returned to the manufacturer. Since the threat was changing quickly, later models (the AGM-88C and on) were fitted with improved guidance systems which could be upgraded in the field using their normal maintenance procedures - and sometimes, even from the firing systems themselves.
The early HARMs improved performance strictly based on speed, low observability and better software. The real problem, however, was that SAM operators were becoming adept at switching off radars during combat; indeed some systems were employing multiple radars and switching between them to confuse incoming ARMs. The HARM therefore received yet another upgrade with the AGM-88E in the form of an INS/GPS guidance system. This allowed the HARM to 'remember' where the enemy radars had been even when they switched off, or better yet be told where the radars it was aimed at were located by the firing platform. This reduced the ability of the SAM radar to confuse the HARM by using EMCON.
As the world became more complex after the Cold War, the threat environment did as well. Now, it was no longer a given that enemy units would be using unique radars, such as the initial Soviet-designed SAM systems. Indeed, it was now possible for both friendly forces as well as enemy forces to be using the same systems. During Desert Storm, for example, both Iraq and Coalition forces were using SA-2 Guidelines and MIM-23 Hawk SAM systems; the latter being a U.S.-deployed system! The HARM needed to be able to discriminate between friend and foe much more accurately (as the AGM-45 Shrike had discovered in the Gulf of Tonkin). Some of the safeguards put in place were procedural - Rules of Engagement were written to prevent HARM use when friendly radars were within the engagement envelope, for example. However, while that reduced friendly fire risk, it also reduced the usability of the HARM. Unlike most other air-to-ground weapons, the antiradiation missile is almost entirely on its own once launched and is a seeking weapon. So-called 'smart bombs' require guidance in the form of a continuous laser illumination of the target, or are navigated by the firing platform using TV links. Competent munitions don't care what's at the destination, but navigate directly to a point on the ground, so if that point is determined to be clear of friendly forces, they can be fired with impunity. Once a HARM is loosed, it will actively look for emissions sources to lock onto, and determine its own flightpath and targeting if the situation dictates.
The AGM-88 has been upgraded numerous times to cope with this problem. One thing we can assume without confirmation is that the HARM has been given the ability to discriminate between radar signals using some classified variance of the emitter - a form of IFF. In addition, its navigation systems mean that it is possible to give the HARM 'no go zones' where it will ignore targets which otherwise look viable.
Finally, the most recent version of the HARM has another trick up its sleeve. It has a terminal guidance system in the form of a millimeter-wave radar seeker, which looks at the shape of the target area and can hunt down radar systems even if they're not emitting. It won't work from very far out, but if the HARM can get close to the target based on its initial targeting data and navigation, even if the SAM isn't emitting, the HARM can try to attack it based on its radar signature (radars have to have nice flat reflective surfaces in order to work).
AGM-88 HARM
- Contractor: Raytheon Missiles and Space
- Length: 13 ft. 8 inches (4.14 m)
- Diameter: 10 inches (0.254 m)
- Range: 30 plus miles (48+ km), although the US Navy, with interesting math, says "Range: 80+ miles (57+ nautical miles/91+ km)."
- Speed: Some sources say Mach 2 but I'm betting higher, since the Shrike went Mach 2 and, after all, this is the High Speed version. :-)
- Propulsion: Morton Thiokol dual-thrust solid-fuel rocket with low smoke
- Warhead: 143.5 lb (65.09 kg) blast/fragmentation. Preformed fragments are included; the type and number have varied over the versions.
- Unit Cost:Between $285,000 and $320,000 (I think that's 1984 dollars).
- Utilized on: F-16C, EA-6B Prowler, EF-18G Growler, F/A-18 Hornet.
1 The US Navy says '760+ MPH' or ~1200 kph. GlobalSecurity.org says '2280 kph'. locke baron says open sources exist saying Mach 4+. I'm going to stick to 'definitely faster than Mach 2.'