Useful hints:
 To kill the robot at the end of every second level, 
 fly at 2 1/2 units (in other words, with two blocks 
 of your altimeter filled in and one at halfway). 
 Shoot 6 shots at the robot's missile, and you will 
 kill it quickly. If you miss, either dodge the missile, 
 or keep firing and you can still blow up the missile. 
 However, you get less points for hitting the missile 
 while it's in the air. 
 Flying at an altitude of 1 1/2 will allow you to hit 
 ground targets while making their shots go harmlessly 
 under you. 
 
 Shoot airplanes on the ground: it will reduce the number 
 you need to kill in the space scene. If you shoot all the 
 airplanes in space you will get a bonus. 
 On the levels in outer space, start up the highest point 
 on the upper left and start shooting immediately. You 
 should be able to hit the satellite right away. If you 
 miss, keep moving to the left and keep firing. This works 
 for the second satellite also. This is important in later 
 levels because it is possible to run out of fuel in space: 
 if you hit the satellite, you will get some back. 
 
 There are only two levels that repeat. The first air fortress 
 level never gets much harder: use the same pattern for it. 
 On the other levels, only the gaps become narrower. 
 Always keep firing! This way you can see if you will make 
 it through the gaps, and you can adjust your height as needed. 
 On the outer space levels, you can tell if you're at the 
 same height as the enemy planes by watching the color of 
 their canopies. If the canopy turns white, you're at the same 
 altitude. This makes it much easier to grab the 1,000 point 
 bonus for destroying 20 enemy planes, as well as to avoid their 
 shots. 
 On later levels you will lose fuel quickly. Make sure you hit 
 as many fuel tanks as possible. If you run out of fuel you will 
 lose a ship.