If you don't have radar systems detecting them long before the border guards hear them, then you can't do anything about it anyway. Accurate detection is the ke

If you don't have radar systems detecting them long before the border guards hear them, then you can't do anything about it anyway. Accurate detection is the key part of any anti-air system - if you're sitting at the controls of an anti-air system and your first notification of an enemy jet plane is a call from someone who literally heard it, then you still can't fire anything at useful at it, and the only thing that you can do is walk away before your (apparently useless) anti-air system gets bombed.