system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do fighter pilots know when an enemy has them "locked on"?

Is that ominous buzz Hollywood or reality?
gpt
There are systems on the plane that can detect when a (potentially, but usually) hostile radar signal is frequently pulsing the plane directly. 

Other alerts are displayed/sounded if there are similar signals in the area, but not necessarily pulsing off of the plane directly.