You have to capture the SAM sites. Controlled airspace belongs to the side who holds them, so hang on to them -- build militia in them.

Thanks. I did read the manual and didn't remember this, if it was in there.

The SAM site is not located on Drassen, have you rescued Skyrider yet? He's chopper pilot, if you had then he will tells you the location of each SAM site, i believe you might need to assign some merc to the heli and attend to move them, then he will pop up and tells you the SAM site location. Or you can just go to the SAM site directly, the Drassen SAM site in 2 sector west from the southernmost Drassen town sector. There are I believe 4 SAM Site, 1 in West of Drassen, 1 South of Chitzena, 1 South of Cambria and 1 in Meduna.

The Drassen SAM-site is 2 sectors east of Drassen mine, not west.
You need Skyrider first. Talk to some of the people with a name over there head in the Drassen airport sector.
They'll tell you about Skyrider. He's in the swamp near Drassen. Either east or west.
Travel through the sectors around Drassen. If you find him, you´ll get a message you've noticed someone in the sector.
Go to the sector, travel to the house in the center of it and talk to Skyrider.
Escort him to Drassen and you got yourself a chopper pilot.

IIRC after some time (1 or 2 in-game days I think) he´ll tell you the location of the SAM site near Drassen.
Conquer it and he´ll tell you the locations of the other sites.
Tip: once conquered, train militia to the max amount.

Uhm yeah east not west, my mistakes