aircraft. Using its built-in inertial navigation system (INS), the missile uses the information provided to it pre-launch to fly on an interception course toward the target After launch, if the firing aircraft or surrogate continues to track the target, periodic updates, e.g. 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 (a development of the Sparrow), since the AIM-120's own radar is necessarily of lesser range and power as 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 to guide it to intercept. Boresight Visual mode Apart from the radar-slaved mode, there is a free guidance mode, called "Visual". This mode is host-aircraft radar guidance-free—the missile just fires and locks onto the first thing it sees. This mode can be used for defensive shots, i.e. when the enemy has numerical superiority. Variants and upgrades Air-to-air missile versions There are currently four main variants of AMRAAM, all in service with the United States Air Force, United States Navy, and the United States Marine Corps. The AIM-120A is no longer in production and shares the enlarged wings and fins with the successor AIM-120B. The AIM-120C has smaller "clipped" aerosurfaces to enable internal carriage on the USAF F-22 Raptor. AIM-120B deliveries