If the cheat is entered correctly, the screen will flash white briefly.
God Mode (invulnerable):
Up, Down, A, B, Left, Right, L, L, R, R, B, A
Unlock Flight History (all missions available from Flight History terminal):
Up, Up, L, R, Down, Down, A, A, B, A
Unlock alien ships in tactical database:
R, L, A, A, A, B, Up, Left, Down, Right, Up, A