TPAS and Transponder - Blind Spot

A few days ago, Bumper mentioned his TPAS "went deaf" in his
transponder equipped glider while flying near other transponders. Jim
S asked about the blanking distance, and Eric Greenwell found in the
manual for the Zaon MRX that this was about 0.4 miles.

But the shape of the dead zone is very important, and we must consider
it. Because of the time difference in the way the ATC radar pulse hits
both transponders, the propagation times of your transponder's pulse
and the other transponder's pulse, the length of the transponder
pulses, the saturation dead time of the ZAON, and the signal
recognition time of the ZAON, there is a funny looking dead zone
around your aircraft where planes can be coming straight at you
without you ever detecting them!

Consider first your glider and the other transponder ship being equal
distances away from the ATC radar. Both transponders fire at the same
time, the ZAON overloads and blanks off for a certain time, then wakes
up. If the other transponder is far enough away from you that the
propagation time for his pulse (plus a little more time for the ZAON
receiver to recognize that it sees a pulse) is greater than the ZAON
blanking time, then you'll detect him. I think this is the 0.4 nm
distance that Eric found in the manual.

Now consider the other transponder being farther away from ATC than
you, on the line that connects you and ATC. Your transponder fires
first and blanks the ZAON. The ATC pulse has to propagate to him for
his transponder to fire, then his transponder pulse has to propagate
back to you to be detected by the ZAON. The net result is that he can
be as close as 0.2 nm for you to detect him.

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...

-John

Note: the 0.4 and 0.2 nnm distances are for a ZAON MRX. These
distances could very well be different for other manufacturers and
models of TPAS devices.