AIM-120 AMRAAM

The AIM-120 Advanced Medium-Range Air-to-Air Missile, or AMRAAM (pronounced am-ram), commonly known to air crews as the "Slammer," is a modern air-to-air missile (AAM).

Political and historical background
AMRAAM was developed as the result of an agreement, no longer in effect, among the United States and several other NATO nations to develop air-to-air missiles and to share production technology. Under this agreement the U.S. was to develop the next generation medium range missile (AMRAAM) and Europe would develop the next generation short range missile (ASRAAM). The breakdown in this agreement lead to Europe developing the MBDA Meteor, a competitor to AMRAAM and the U.S. pursuing upgrades of the AIM-9 Sidewinder. After protracted development, deployment of AMRAAM (AIM-120A) began in September 1991.

The eastern counterpart of AMRAAM is the very similar Russian R-77 AA-12 Adder, commonly known in the west as "Amraamski."

Operational Features Overview
AMRAAM has an all-weather, beyond-visual-range capability. It improves the aerial combat capabilities of U.S. and allied aircraft to meet the future threat of enemy air-to-air weapons. AMRAAM serves as a follow-on to the AIM-7 Sparrow missile series. The new missile is faster, smaller, and lighter, and has improved capabilities against low-altitude targets. It also incorporates an active radar in conjunction with an inertial reference unit and micro-computer system, which makes the missile less dependent upon the fire-control system of the aircraft.

Once the missile closes in on the target, its active radar guides it to intercept. This feature, called "fire and forget," frees the pilot from the need to continuously illuminate the missile's target with a radar lock, enabling the pilot to aim and fire several missiles simultaneously at multiple targets and perform evasive maneuvers while the missiles guide themselves to the targets.

Guidance system overview

Interception course stage
AMRAAM uses two-stage guidance when fired at long range. The aircraft passes data to the missile just before launch, giving it information about the location of the target aircraft from the launch point and its direction and speed. The missile uses this information to fly on an interception course to the target using its built in inertial navigation system (INS). This information is generally obtained using the launching aircraft's radar, although it could come from an infra-red search and tracking system (IRST), from a data link from another fighter aircraft, or from an AWACS aircraft.

If the firing aircraft or surrogate continues to track the target, periodic updates are sent to the missile telling it of any changes in the target's direction and speed, allowing it to adjust its course so that it is able to close to self-homing distance while keeping the target aircraft in the basket in which it will be able to find it.

Not all AMRAAM users have elected to purchase the mid-course update option, which limits AMRAAM's effectiveness in some scenarios. The RAF initially opted not to use mid-course update for its Tornado F3 force, only to discover that without it, testing proved the AMRAAM was less effective in BVR engagements than the older SARH-homing BAE Skyflash weapon–the AIM-120's own radar is necessarily of limited range and power compared to that of the launch aircraft.

Terminal stage and impact
Once the missile closes to self-homing distance, it turns on its active radar seeker and searches for the target aircraft. If the target is in or near the expected location, the missile will find it and guide itself to the target from this point. If the missile is fired at short range (typically, visual range), it can use its active seeker just after launch, making the missile truly fire-and-forget. At the point where an AMRAAM switches to autonomous self-guidance, the NATO brevity word "pitbull" would be called out on the radio to inform other pilots, just as "Fox Three" would be called out upon launch.

Kill probability and tactics

General considerations
Once in its terminal mode, the missile's advanced electronic-counter-counter-measure (ECCM) support and good maneuverability mean that the chance of it hitting or exploding close to the target is high (on the order of 90%), as long as it has enough remaining energy to maneuver with the target if it is evasive. The kill probability (PK) is determined by several factors, including aspect (is it a head-on interception, side-on or tail-chase scenario), altitude, the speed of the missile and the target, how hard the target can turn, etc. Typically, if the missile has a sufficient amount of energy during the terminal phase, which comes from being launched close enough to the target from an aircraft flying high and fast enough, it will have an excellent chance of success. This chance drops as the missile is fired at longer ranges as it runs out of overtake speed at long ranges, and if the target can force the missile to turn it might bleed off enough speed that it can no longer chase the target.

Lower-capability targets
This leads to two main engagement scenarios. If the target(s) is/are not armed or not armed with any medium or long-range fire-and-forget weapons, the aircraft firing the AMRAAM need only to get close enough to the target, depending upon whether the target is heading towards or away from the firing aircraft, and launch the missile(s) to have a reasonable chance of hitting. Especially against low-maneuverability targets, in this situation the missiles are unlikely to miss. If the target aircraft are approaching the launching aircraft, especially if they are moving fast, the missile can be launched at long range since the range will be closing fast. In this situation, even if the target(s) turn around, it is unlikely they can speed up and fly away fast enough to avoid being overtaken and hit by the missile(s) (as long as the missiles are not released too early). It is also unlikely they can outmaneuver the missiles since the closure rate will be so great. In a tail-on engagement, the firing aircraft might have to close to between one-half and one-quarter maximum range (or maybe even more for a very fast target) in order to give the missile sufficient energy to overtake the targets.

If the targets are armed with missiles, the fire-and-forget nature of the AMRAAM is invaluable, it enables the launching aircraft to fire missiles at the target and then turn and run away. Even if the targets have longer-range semi-active radar homing (SARH) missiles, they will have to chase the launching aircraft in order for the missiles to track them, effectively flying right into the AMRAAM. If the target aircraft fire missiles and then turn and runs away, their own missiles will not be able to hit. Of course, if the target aircraft have long range missiles, even if they are not fire-and-forget, the fact that they force the launching aircraft to turn and run reduces the kill probability, since it is possible that without the mid-course updates the missiles will not find the target aircraft. However the chance of success is still good and compared to the relative impunity the launching aircraft enjoy, this gives the AMRAAM-equipped aircraft a decisive edge. If one or more missiles fail to hit, the AMRAAM-equipped aircraft can turn and re-engage, although they will be at a disadvantage compared to the chasing aircraft due to the speed they lose in the turn, and would have to be careful that they're not being tracked with SARH missiles.

Similarly armed targets
The other main engagement scenario is against other aircraft with fire-and-forget missiles like the Vympel R-77 (NATO AA-12 "Adder") – perhaps MiG-29s, Su-27s or similar. In this case engagement is very much down to teamwork and could be described as "a game of chicken." Both flights of aircraft can fire their missiles at each other beyond visual range (BVR), but then face the problem that if they continue to track the target aircraft in order to provide mid-course updates for the missile's flight, they are also flying into their opponents' missiles. This is why teamwork is so important and advanced missiles with guidance systems with hand-off capability can help overcome this problem. The other main tactic would be to sneak up behind the enemy aircraft and launch missiles without them noticing, giving the launching aircraft sufficient time to leave the danger zone of the enemy after launching. Even if the enemy detects the launch and turns around, the speed and possibly altitude it loses during the turn puts its missiles at an energy disadvantage which may be sufficient for the other aircraft to defeat it successfully. This typically requires excellent ground-control intercept (GCI) or airborne radar (AWACS) facilities in order to be successful.

Variants and upgrades

Air-to-air missile versions
There are currently three variants of AMRAAM, all in service with the USAF and USN. The AIM-120A is no longer in production and shares the enlarged wings and fins with the successor AIM-120B currently in production. The AIM-120C has smaller "clipped" aerosurfaces to enable internal carriage on the USAF F/A-22 Raptor. AIM-120B deliveries began in 1994, and AIM-120C deliveries began in 1996.

The AIM-120C has been steadily upgraded since it was introduced. The AIM-120C-6 contained an improved fuze (Target Detection Device) compared to its predecessor. The AIM-120C-7 development begain in 1998 and included improvements in homing and greater range (actual amount of improvement unspecified). It was successfully tested in 2003 and is currently being introduced into active service (early 2005). It helps the U.S. Navy replace the F-14 Tomcats which are being retired and replaced with F/A-18E/F Super Hornets—the loss of the F-14's long-range AIM-54 Phoenix missiles can be partially offset with a longer-range AMRAAM.

The AIM-120D is a planned upgraded version of the AMRAAM with improvements in almost all areas, including 50% greater range and better guidance over its entire flight envelope yielding an improved kill probabiliy (PK). There are also plans for Raytheon to develop a Ramjet-powered deriviative of the AMRAAM called the FMRAAM. It is not known whether the FMRAAM will be produced since the British Ministry of Defence has chosen the Meteor missile over the FMRAAM as its preference for a BVR missile for the Eurofighter Typhoon aircraft.

Ground-launch systems
Raytheon successfully tested launching AMRAAM missiles from a five-missile carrier on an HMMWV (hum-vee). They receive their initial guidance information from a radar not mounted on the vehicle (probably the MPQ-64 Sentinel radar system or possibly a PATRIOT missile battery radar) and help to provide low-level, close-in defence while the PATRIOT system engages targets at higher altitudes and further ranges. The missile's range would be lower when launched from the ground, due to the lack of speed or altitude of the launch vehicle.

The Norwegian Advanced Surface-to-Air Missile System (NASAMS), developed by Kongsberg Defence & Aerospace, consists of a number of vehicle-pulled launch batteries (containing six AMRAAMs each) along with separate radar trucks and control station vehicles.

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