So now I understand why there is immediate PROLONG after AS off in your case - the Mach 3 SR-71 jumps out of the static beam from the RPC virtualy immediately. Even if it is still pointing in the right direction - you have to think in 3D - beam directs from the ground level up under some angle (which is constantly increasing as the target is coming close to the RPC). By the way, look what elevation marker does when you track such a high speed closing target.