To kill Zaxxon, 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 Zaxxon'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 platform : 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.
The white canopy only indicates that the enemy plane is at the highest altitude. At any altitude, when an enemy is in your line of fire you will hear a tone and a white X appears in front of your plane. 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.