What is "Below Radar" and how does it work?

Solution 1:

It's altitude based, not terrain based. Just because you're skimming low over a mountain cliff doesn't necessarily mean you're under the radar barrier altitude. Keep the map's geography in mind. When you want to be below radar you must be near sea level.

Solution 2:

Well, I have it unlocked, and even way outside the range of all ground based anti-air, with only a jet on my tail and flying so close to the ground a slight bump could've caused me to crash, I'm still being locked on to. So, it's not a passive unlock that needs to be equipped to work. If so, then Air Radar, which is also classified as a "Permanent Upgrade", would also have to be passive — but it still has to be equipped in order to work.

Solution 3:

It does work, but only for air-based AA. Ground-based AA (both infantry and vehicles) are not affected.