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Comment by gok | Hacker News Reader

It's kind of baffling to me that there's a system for detecting collisions with other airplanes in the sky (TCAS) and a system for detecting collisions with the ground (EGPWS) but not system for detecting collisions with other airplanes on the ground. This easily could have killed a thousand people.

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TCAS alerts about "conflicts" with other aircraft on take-off and landing all the time. It's just that those are almost always false positives, so they get ignored.

There are many aircraft on the ground at airports. Given the high approach speeds, this is probably a non-trivial problem.

SFO has ground radar systems (ASDE-X/ASSC) [1] which allows ground and tower controllers to see potential conflicts on taxiways and runways. Pure speculation, but I don't think that these systems' conflict logic take into account an aircraft trying to land on a taxiway.

[1] https://en.wikipedia.org/wiki/Airport_Surface_Surveillance_C...

Yeah the report mentions that the systems for detecting conflicts and mis-alignments don't really talk to each other.

Different problems, no? Trying to pick out an airplane against the ground is different from detecting an object in the air, which doesn't generally reflect radio back, and detecting the ground, which does reflect. The problem you're describing requires you to differentiate between two things that both reflect signals back to you. It's obviously do-able since the military does it all the time for targeting. However, I doubt it's cheap and there might be cheaper ways to do it that don't leverage the two systems you've mentioned.

TCAS uses active transponders, not radar reflection. https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...

TCAS works via the transponders of other aircraft, which they actively transmit.

I think this is really just a case of flying into the ground. It just happened to be in close proximity to other aircraft. There are lots of systems that are available to avoid that (ILS, GPS etc.) which were not used in this instance. And a system for avoiding collisions for taxing aircraft is probably not much use at such high speeds.

Some airports do have a system [0] designed to track taxing aircraft and allow collision risks to be detected.

https://en.wikipedia.org/wiki/ASDE-X

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