Yes. I am astonished that you do not already know that.
On 9-11, the combined maximal efforts of civilian and military aviation were unable to locate four airliners because their transponders were turned off. This occurred near the New York and DC metropolitan areas, some of the most carefully controlled air space in the world.
Without a transponder, an aircraft is just a raw radar return among, potentially, hundreds or thousands of other returns from civilian and military aircraft, weather, birds, balloons etc. Add to that glitches, ghosts and anomalies in busy air space and it is not surprising that a large aircraft without a transponder can hide in plain sight.
There was no real problem locating the four airliners hijacked on 9/11. In fact, one of them (United 175) didn't even turn its transponder off. The other three were adequately tracked with primary radar. The problems with the airliners that day were deciding what to do about them (e.g. intercept them) and then actually doing it (e.g. no procedures were in place to quickly scramble fighters in a case like this).
Primary radar is difficult and inconvenient but it's not nearly as difficult as you make it sound.
This is not true; the difficulty in using raw radar returns to manage and defend the domestic airspace was the primary reason the FAA grounded all civilian aircraft on 9/11--to clean up the scopes.
Andrews scrambled two (unarmed) F-16s to intercept flight 93; they just followed the Potomac north because no one knew where that plane was. Even after the PA crash was confirmed, those fighters spent hours contacting radar returns to check them out and get them to land.
That just shows that primary radar is difficult to use, and that it's hard to track a plane on radar after it's crashed. I'm not disputing that. Perhaps "no real problem" was bad phrasing, what I meant was that they were able to track them and they were not "unable to locate" the planes.
On 9-11, the combined maximal efforts of civilian and military aviation were unable to locate four airliners because their transponders were turned off. This occurred near the New York and DC metropolitan areas, some of the most carefully controlled air space in the world.
Without a transponder, an aircraft is just a raw radar return among, potentially, hundreds or thousands of other returns from civilian and military aircraft, weather, birds, balloons etc. Add to that glitches, ghosts and anomalies in busy air space and it is not surprising that a large aircraft without a transponder can hide in plain sight.
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