There's 21 Campaign missions and you must get an "A" rating in all of them. Getting the "A" rating can be quite hard, as sometimes it can be completely random (you can get many many points and still get a "B" rating). Try to be precise, kill as much enemy fighters as you can to get more points, be quick, (finish under par time), try to get the secondary or secret objectives, and the "A" rating should be attained.
(c) Linkinito @ PSTO