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Yes, it's the TCAS. It sees in all directions. It's secondary radar so it only sees planes with a transponder, but that's usually all of them. https://en.wikip

• tialaramex 6y ago
  > It's secondary radar so it only sees planes with a transponder, but that's usually all of them. In particular we're talking here about aeroplanes on an approach to a major airport (if they aren't on approach then by definition they aren't choosing between ILS and a visual clearance, and if it isn't a major airport why are we trying to bunch them up more?). All US major airports have a "Mode C veil" which means there's a regulation requiring aircraft near those airports to have transponders ("Mode C" is a transponder mode in which the aircraft reports its own assessment of its altitude based on air pressure as well as a four octal identifier).
  • bencoder 6y ago
    Since January 1st 2020 the requirement is for ADS-B out. So they will be reporting their GPS position also, now.