Guidance system overview[edit]
Interception course stage[edit]
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
infrared search and tracking system (
IRST), 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 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 BVR engagements than the older semiactive radar homingBAE Skyflash weapon—the AIM-120's own radar is necessarily of limited range and power compared to that of the launch aircraft.