Chopper Command is a great example of how Activision could take a game concept and improve it.  Chopper Command is similar in play to Atari's Defender, but I feel like I have much more control over the "chopper" in this game than of the spaceship in Defender.

The screen, like in Defender, has a "Radar" overview bar at the bottom which allows you to see all of the enemies and all of your convoy on any given level.  The game view is a side view in which you can move up or down and left or right.  You're goal is to shoot all of the enemy choppers and jets and to protect your convoy of 12 supply trucks that move from left to right at the bottom of the screen and avoid getting hit by jets or choppers or their missiles.  You score points for each enemy plane or chopper that you shoot down.  You also get bonus points at the end of each level for all of the remaining trucks in your convoy.