[slavak]

We place them _both_ inside the aircraft and on the ground. While ATC does provide separation, conflicts still can and do happen. Aircraft have onboard systems to warn of conflicts and even to suggest corrective action, but the pilots must be the ones to make the correction.

[mrguyorama]

Indeed, I believe TCAS is one of the only places where planes and pilots override ATC, in that if ATC tells you to go down, and TCAS tells you to go up, you go up

[Tomte]

Unfortunately, the pilots at Lake Constance didn‘t really act this way.