Transponders don't transmit unless interrogated, typically by either ground
radar or other aircraft equipped with TCAS.

In areas with no radar coverage, passive devices would be less useful
although TCAS equipped aircraft may interrogate nearby aircraft. Then if
they are in range, either could show up on passive devices. The risk of
collision is greater near airports and in busier areas and these most often
have radar coverage.

bumper




"Eric Greenwell" wrote in message
...
Marc Ramsey wrote:

Tim Mara wrote:

as far as I know transponders do transmit even when not being
interogated.they just don't reply with as strong a signal and not as
often


I don't believe they transmit at all, unless they receive a radar
signal. Perhaps Eric or others more knowledgeable than I, know...

I've never read that or heard anyone suggest that before, so I really
doubt it. They were designed long before the passive "alerter" units
became available, which are the only instruments I know of that might
benefit from it. Perhaps someone with a TPAS, Surecheck, etc could tell
us if their unit gets a signal from their airplane when it's in the
hangar and the transponder clearly isn't getting hit by radar?

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Eric Greenwell
Washington State
USA