Interception course stage
Grumman F-14 Tomcat carrying an AMRAAM during a 1982 test
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 track system, from a
data link from another fighter aircraft, or from an
AWACS aircraft.
After launch, if the firing aircraft or surrogate continues to track the target, periodic updates—such as changes in the target's direction and speed—are sent from the launch aircraft to the missile, allowing the missile to adjust its course, via actuation of the rear fins, so that it is able to close to a self-homing distance where it will be close enough to "catch" the target aircraft in the
basket (the missile's radar field of view in which it will be able to lock onto the target aircraft, unassisted by the launch aircraft).
Not all armed services using the AMRAAM 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 beyond visual range (BVR) engagements than the older
semi-active radar 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, within visual range (WVR) or the near BVR, it can use its active seeker just after launch, making the missile truly "fire and forget".[
citation needed]
Boresight mode
Apart from the slave mode, there is a free guidance mode, called boresight. This mode is radar guidance-free, the missile just fires and locks the first thing it sees. This mode can be used for defensive shot, i.e. when the enemy has numerical superiority.