Analysis from a JHU CS Prof

Thu Sep 13 21:24:25 UTC 2001

Also sprach Leigh Anne Chisholm
>I despise posting off-topic, but I want to say two things...

>1.  If a transponder is turned off, it doesn't mean that you don't show
>up on radar--a blip appears on the radar screen as long as you're high
>enough to be detected.  If however you fly low enough, you can fly
>below the radar's detection capability.  I don't offhand recall what
>height that is--it's been years since I was active as a pilot and
>prospective Air Traffic Controller.

That would depend on the distance from the transmitter...the farther
from the transmitter, the higher you could fly and not be detected.

Another issue is the efficiency of the transmitter and receiver at
picking up the reflected energy.  From what I understand, air traffic
control radars are, in this day and age, really quite pathetic as true
radars.  They largely depend on transponder responses to be able to pick
up aircraft, as even fairly large aircraft, at some distance from the
transmitter, may only give a secondary (transponder) return, and not a
primary (reflected energy) return.
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