TPAS and Transponder - Blind Spot

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

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

Darryl, thanks very much! The links were exactly what I was hoping
for; there's good stuff in there.

This is a bear of a week for me, and I don't have time to dig into the
ramifications of what you've provided.. But I see what you're driving
at, now, and it's clear that I let my ultrasonic work color my
analysis.

Funny thing, I was thinking before you replied about using correlation
on overlapping signals, and sure enough, that's what they're using for
the de-garbling. Don't think I ever saw a hardware correlator,
although I did see a hardware device to do the FFT butterfly once. Do
they still use such things in this computer age?

-John

On Mar 12, 5:53 pm, "
wrote:
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 athttp://www.radartutorial.eu/13.ssr/sr01.en.html
(see brief mention athttp://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.

Seehttp://www.zaonflight.com/component/option,com_docman/task,cat_view/g...

FYI, all MRX units have an audio for alert, the new units have an audio
output for use with headsets or intercoms added (optional)
tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

wrote in message
oups.com...
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

Tim

Your back - you must have finished throwing out all those transponders
you are not going to sell :-)

Thanks for degarbling my post, yes I meant an electrical audio out for
an headset/audio panel

Do you know if current Zaon MRX systems (few months old) can be
upgraded to have the headset/audio panel audio out? Is the volume of
that output independently adjustable?

Thanks


Darryl

--

BTW I also misstyped unidirectional when I meant omnidirectional when
talking about TCAS.


On Mar 13, 9:00 am, "Tim Mara" wrote:
FYI, all MRX units have an audio for alert, the new units have an audio
output for use with headsets or intercoms added (optional)
tim
Please visit the Wings & Wheels website atwww.wingsandwheels.com

wrote in message

oups.com...

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/...ask,cat_view/g...

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

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

"Eric Greenwell" wrote in message
news:nt1Kh.5445$0W5.1788@trndny05...

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


Having flown with a TPAS for about 5 years now, my experience backs up what
Eric is saying. While I've noticed the "blind spot" often, mostly while
flying with other gliders, I've always had visual contact on the other
aircraft. I think this is because the blind spot doesn't extend all that far
from my aircraft.

bumper

Eric,

Thanks so much for following up with Zaon. Kudos to Zaon for being
forthcoming with you, too. I love my MRX, and the attitude Zaon
expressed in answering you makes me like the company that much more.

By the way, I think there's a typo in your reply regarding minimum
detectable distance - I think you meant to say 0.1 mi, not 0.4 mi.

-John

On Mar 14, 8:39 pm, Eric Greenwell wrote:

Eric Greenwell found in the
manual for the Zaon MRX that this was about 0.4 miles.

Not quite: 0.4 mi

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."