Enter the following button presses during gameplay, but not while the game is paused.

80087c1e0300 Infinite armor
800855ba0004 Reloader
800855b40004 Air support

Debug menu: At the title screen, quickly press Up, Down, Left, Right, Down, Down, Right, Right, Square. If you did the trick correctly, you hear a chime and a debug menu screen appears. Among the options are a level select, CD/sound test, and credits.

Invincibility: Start a game, and press Start and Select. Choose to abort the mission to quit the game. Then, at the title screen, press Up, Up, Up, Down, Down, Down, Right, Right, Triangle. If you did the trick correctly, you hear a chime. Start a new game, and your tank is now invincible.