jcarlyle wrote:

Now suppose the other transponder is between you and ATC, on the line
that connects you and ATC. Guess what? You'll never detect him! The
ATC pulse reaches him first, firing his transponder, and both pulses
reach your ship at essentially the same time. Your transponder then
fires, blanking the ZAON. By the time it unblanks, his pulse and your
pulse have propagated far beyond the ZAON antenna - they aren't around
for detection.

All this suggests the shape of the dead zone - it consists of a
flattened hemisphere sitting on a paraboloid. Your glider is at the
center of the flattened hemisphere, with the larger radius
perpendicular to the line connecting you and ATC, and the smaller
radius heading away from ATC and parallel to the line connecting you
and ATC. The curving end of the paraboloid sits right on ATC, while
the open end of the paraboloid connects to the flattened hemisphere.
(For ease of thinking, you can replace the paraboloid with a cylinder
of radius 0.2 nm - it extends from you to ATC.)

So, if you fly with a TPAS in a glider equipped with a transponder,
keep an especially careful lookout for planes coming at you from the
direction of ATC, . They will NOT be detected by your TPAS, unless you
periodically turn your transponder off! Then you'll detect everything
right up to oh too close...

This seems like a plausible analysis, but it's not what Zaon shows in
their manual; instead, they talk about a "bubble of detection" around
your aircraft. A query to Zaon should be the next step, as it might get
an explanation of how their units deal with this situation.

There is a situation that elimanates this problem: multiple radars. This
could be a TCAS equipped airplane or another ATC ground radar.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org