TPAS and Transponder - Blind Spot

John

If you are realy interested I hope this gives you a few key words to
look up if nothing else: Overlapping pulse trains in SSR/transponders
is called garbling, and systems to handle that perform de-garbling.
Specifically you are discussing syncronous garbling where the garbling
is syncronised by the radar interrogation. Systems like TCAS that are
more unidirectional than SSR radar use techniques including wisper-
shout and directional antenas to try to de-garble their signals. A
funky little summary on this stuff is at http://www.radartutorial.eu/13.ssr/sr01.en.html
(see brief mention at http://www.radartutorial.eu/13.ssr/sr15.en.html
for a de-garbling algorithm). If you have access to IEEE there are
papers available there on SSR, collision avoidance etc.

As for general UHF/microwave signal processing, you can do pretty
amazing stuff with very low noise / high dynamic range front-ends,
possibly more than you would expect if your background is ultrasonic
signal processing. And in the case being dicussed the closer the
target gets you have less of a signal dynamic range problem.

But who knows exactly what Zaon does. I'd be suprised if they ever got
into details. Again all I was doing was cautioning is it probably
won't be signal blanking, not at least as initilaly described. None of
this stuff is my area/background, A very long time ago I did research
on ultra-low phase noise microwave sources and some exotic
applciations of those and have just been curious in the past about how
SSR worked.

---

BTW I had not poked around the Zaon website in a while and I now
noticed that they have an installation guide what talks about a panel
install kit and "audio enabled" MRX modules that give audio out. Also
they talk about multi-antenna installs. They definitly are not afraid
of getting the MRX antenna too close to the transponder antenna, they
spec only a few feet minimum distance between externally mounted MRX
and tranponder antennas. So I might have to take back my previous
concern about transponder antennas being really close to the MRX
antenna.

See http://www.zaonflight.com/component/...id,8/Itemid,43

Cheers


Darryl

On Mar 10, 5:37 pm, "jcarlyle" wrote:
Darryl, I admit I oversimplified things. One of the reasons was that I
deal with analog ultrasonic signals produced by nature, not digital
pulse trains from transponders, so it made a first cut analysis easier
for me.

Hopefully Zaon will be willing to tell Eric what they're really doing
inside the box. Since I own a MRX and am planning on installing a
transponder, I'd be more than happy if my analysis at the start of
this thread is wrong!

Meanwhile, it sounds like you understand transponders, RF and digital
processing. Can you refer me to something on the web that would
explain the basics of how partially overlapped pulse trains are
differentiated? Using a stand-alone detector on analog signals of
similar frequency and fairly similar shape, I know I can't detect a
second signal 20-30 dB below an overlapping signal - so the
possibility you hint of for digital signals is outside of my
knowledge. Thanks!

-John

On Mar 10, 1:34 pm, "
wrote:

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.

Darryl