TPAS and Transponder - Blind Spot

wrote:
Eric what are you reading in the manual that says 0.4 nm? The only
reference I saw to 0.4 nm is the discusison that if you see false
targets of less than 0.4 nm range then you may need to clean the
transponder antenna, etc.

The remark was in a different thread, and it was based page 10,
"Resolution and Accuracy". The remark was...

"My MRX manual doesn't directly answer this question, but my reading is
you have range down to at least 0.4 nm, which is 2400'. It's been giving
you advisories and alerts from 5 nm, so you should have spotted the
threat by the time it's that close."

I'm sure the situation is more complicated than John Carlyle's analysis
suggests, as you point out below (and there must be other things we
haven't thought of), and that's why Zaon should have the chance to
answer the question about a dead zone.


Also it is worth remembering the Zaon PCAs devices are not just
"blanking" the receiver during the local transponder reply. The Zaons
are reading and doing an altitude decode of the local transponder
signal and using that if possible for the altitude reference rather
than the built in altimeter. How good their RF front end and post RF
digital processing is will determine how well they can differentiate
partially overlapping pulse trains from the local and other
transponders. And you better believe they have to do this since the
most nieve approach of "blanking" during the entire ~20us transponder
pulse train (ignoring the ident pulse) would give a dead zone of
~6km. I'd love to see a schematic.. :-)

Like other posters I suspect this not much of an issue in practice
because of multipe illuminations from SSR, TCAS etc. However one thing
with some of the funkier glider tranponder antenna installs is that
the PCAS may be seeing much more RF power from the local transponder
than the designer expected, especially for situations like with RF
transparent fiberglass fueslages and maybe a less than great ground
plane betwen the PCAS and antenna, tranponder antennas mounted in the
cockpit etc. In which case maybe the dead zone is larger because of
the Zaon's reduced ability to detect overlapping pulse trains.

---

The Zaon MRX works amazingly well in my experience and is a great
suppliment for transponders in gliders, but especially for seperation
from heavy iron lets keep the focus on getting transponders in gliders
in heavy traffic areas. Transponders absolutely work -- with no effort
from me except turning on my transponder I often notice traffic
vectored around my glider when flying near Reno (I hear Reno arivals/
departures telling traffic I'm there).


--
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