New Mode:
By completing all 18 missions (finish the game)save and load. Now you new mode like

Eject from plane:
Hold L1 R1 L2 R2 all at the same time, there for letting you escape before you are shot down.

SP. CONTINUE:
Start over from mission one with all Aircraft, Weaponry and $$$ obtained previously.

FREE MISSION MODE:
Play any mission from the game.

TRIAL MISSION MODE:
A new mode which let you play Score Attack or Time Attack mode.

Also, beat the game once and go to option screen, you'll now have new goodies like
Scene Viewer> let you watch any scene from the game. Music Player> listen to all 36 tracks.

Unlock X-02 MULTIROLE:
Get S-RANKING on all 18 mission on normal mode.

New aircraft paint:
On normal level and above, shoot down aircraft with that aircraft's pilot's name on it. On radar it will show up as red symbol and it is harder to shoot down. Should appear away from the main combat zone.

Expert mode:
To get expert mode complete all missions on Very Easy, Easy, Normal and Hard.

Ace mission mode:
To unlock ace mission mode, beat expert mode. It's very hard, as one hit from a missile will kill you.