TPAS and Transponder - Blind Spot

jcarlyle wrote:
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.

Not quite: 0.4 mi
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.

John's analysis seemed plausible, so I contacted Zaon about it.
Summarizing a fairly technical reply:

"The situation described can produce a dead zone. Generally it is not a
problem in practice, because there are many other interrogation sources
(mainly other ATC radars and TCAS systems) that interrogate transponders
besides the ATC radar in line with your glider and the other aircraft.
These replies will not be masked, so range and altitude can be
determined by the MRX."

I'd add that it's unlikely an aircraft could stay directly between you
and the ATC radar for very long. If it was climbing, you'd have to climb
even faster, and vice versa. If it was coming straight out from the
radar, you'd have to be flying directly towards or away on the sloped
line going to or from the radar, and so on.

So, it's an event with a low probability in the first place, and a high
probability of mitigation by interrogations from other sources. Further,
if it's an airliner, it has TCAS (he'll see you, you'll see him); if
it's a GA aircraft, it might be in contact with ATC and warned of your
presence, or it might have a PCAS unit and detect your transponder; and
finally, you might visually detect each other! My opinion: the dead zone
risk is insignificant.

--
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