Base Tower is a level in Syphon Filter that continues from Base Bunker. Lian is captured and apparently killed by Rhoemer while Gabe must destroy a gunship that assaults him.
Disable radar tracking
You are now standing on the tower roof. There is a large radar dish in the center. Go around it and climb to its control panel. Press Triangle or shoot it to disable the radar (objective completed/checkpoint).
Shoot down attack helicopter
The next fight is a tough one. The chopper has machine guns that can kill you in a matter of seconds. The PK-102 is the best weapon to use against it and you will find ammunition for it on each side of the dish (as well as flak jackets). During the first phase of the fight, the chopper will circle above you. You can avoid its machine gun by running in diagonal (arrow + R2 or L2). As soon as it goes over your head, turn around and shoot it as long as you can. Use the lock-on targeting mode and keep shooting as long as you see green in your target gauge.
After a few times, you will enter the second phase. This time the chopper will rise frome below on each side of the tower. He then repeats the manoeuvre on the opposite side and so on. You should position yourself on the same side of the dish. Wait for it to rise, then when he starts shooting, run in diagonal to one of the corner. When the chopper has gone past you, turn around and shoot like previously.
There is an alternate strategy which would consist of waiting for the chopper on the opposite side of the dish. This way you are easily protected from its machine gun. The downside however is that the chopper then have time to drop one or two soldiers on the tower. In this case you would constantly have to deal with enemies while trying to shoot the chopper at the time. I strongly suggest that you use my strategy above which is more effective not really more dangerous.
Finally, the third phase combines the two first one. Be prepare to react accordingly and repeat until the level ends (objective completed).