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.

Darn! The cylinder radius should have been 0.4 nm, not 0.2 nm. It's a
pretty big thing...

-John

On Mar 10, 9:34 am, "jcarlyle" wrote:
(For ease of thinking, you can replace the paraboloid with a cylinder
of radius 0.2 nm - it extends from you to ATC.)

jcarlyle wrote:

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...

This seems like a plausible analysis, but it's not what Zaon shows in
their manual; instead, they talk about a "bubble of detection" around
your aircraft. A query to Zaon should be the next step, as it might get
an explanation of how their units deal with this situation.

There is a situation that elimanates this problem: multiple radars. This
could be a TCAS equipped airplane or another ATC ground radar.

--
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

The key lies in how they blank the receiver. Since there is no
connection to your own transponder, I think they simply blank for time
X when they get a signal of over Y watts. If they do, that will
produce the situation I described above.

Multiple radars will change the situation, but I believe there will be
blind spots there, too. I think you'll have multiple blind cylinders,
each pointing at the various radar sources.

And you're correct, it isn't what Zaon shows in the manual. It would
definitely be best to talk with them directly about happens in this
situation.

Eroc, I know I won't get any time this week to call them. Could you do
it, and post the results?

-John

On Mar 10, 10:24 am, Eric Greenwell wrote:
This seems like a plausible analysis, but it's not what Zaon shows in
their manual; instead, they talk about a "bubble of detection" around
your aircraft. A query to Zaon should be the next step, as it might get
an explanation of how their units deal with this situation.

There is a situation that elimanates this problem: multiple radars. This
could be a TCAS equipped airplane or another ATC ground radar.

jcarlyle wrote:
The key lies in how they blank the receiver. Since there is no
connection to your own transponder, I think they simply blank for time
X when they get a signal of over Y watts. If they do, that will
produce the situation I described above.

Multiple radars will change the situation, but I believe there will be
blind spots there, too. I think you'll have multiple blind cylinders,
each pointing at the various radar sources.

As long as they don't overlap, then maybe the unit can still pick out a
potential threat.

And you're correct, it isn't what Zaon shows in the manual. It would
definitely be best to talk with them directly about happens in this
situation.

Eroc, I know I won't get any time this week to call them. Could you do
it, and post the results?

Sure, I'll try to contact them Monday.


-John

On Mar 10, 10:24 am, Eric Greenwell wrote:
This seems like a plausible analysis, but it's not what Zaon shows in
their manual; instead, they talk about a "bubble of detection" around
your aircraft. A query to Zaon should be the next step, as it might get
an explanation of how their units deal with this situation.

There is a situation that elimanates this problem: multiple radars. This
could be a TCAS equipped airplane or another ATC ground radar.





--
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, sorry about the typo in your name in the message above.

Figuring out the geometry is involved, but some signal overlap is OK.
Explaining it in text is hard, but if you make timing diagrams on a
sheet of paper and translate that to geometry, you'll see the
solution. I think that if someone is in your ATC blind zone, and a
TPAS interrogates your transponder and his, and he's outside of the
TPAS blind zone, you'll see him.

Glad you'll try to call Zaon; this coming week is one I want to forget
before it even gets here!

-John

On Mar 10, 11:29 am, Eric Greenwell wrote:
As long as they don't overlap, then maybe the unit can still pick out a
potential threat.

Sure, I'll try to contact them Monday.

jcarlyle wrote:
Eric, sorry about the typo in your name in the message above.

Figuring out the geometry is involved, but some signal overlap is OK.
Explaining it in text is hard, but if you make timing diagrams on a
sheet of paper and translate that to geometry, you'll see the
solution. I think that if someone is in your ATC blind zone, and a
TPAS interrogates your transponder and his, and he's outside of the
TPAS blind zone, you'll see him.

Glad you'll try to call Zaon; this coming week is one I want to forget
before it even gets here!

-John

On Mar 10, 11:29 am, Eric Greenwell wrote:
As long as they don't overlap, then maybe the unit can still pick out a
potential threat.

Sure, I'll try to contact them Monday.

Don't forget, there is a finite delay in the transponder (3.0
microseconds by spec) between receiving the pulse from ATC and replying.
You're not looking for echoes here.

-Dave

ZL wrote:
jcarlyle wrote:
Eric, sorry about the typo in your name in the message above.

Figuring out the geometry is involved, but some signal overlap is OK.
Explaining it in text is hard, but if you make timing diagrams on a
sheet of paper and translate that to geometry, you'll see the
solution. I think that if someone is in your ATC blind zone, and a
TPAS interrogates your transponder and his, and he's outside of the
TPAS blind zone, you'll see him.

Glad you'll try to call Zaon; this coming week is one I want to forget
before it even gets here!

-John

On Mar 10, 11:29 am, Eric Greenwell wrote:
As long as they don't overlap, then maybe the unit can still pick out a
potential threat.

Sure, I'll try to contact them Monday.

Don't forget, there is a finite delay in the transponder (3.0
microseconds by spec) between receiving the pulse from ATC and replying.
You're not looking for echoes here.

-Dave

never mind, doesn't matter...

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.

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).

Darryl

On Mar 10, 8:29 am, Eric Greenwell wrote:
jcarlyle wrote:
The key lies in how they blank the receiver. Since there is no
connection to your own transponder, I think they simply blank for time
X when they get a signal of over Y watts. If they do, that will
produce the situation I described above.

Multiple radars will change the situation, but I believe there will be
blind spots there, too. I think you'll have multiple blind cylinders,
each pointing at the various radar sources.

As long as they don't overlap, then maybe the unit can still pick out a
potential threat.



And you're correct, it isn't what Zaon shows in the manual. It would
definitely be best to talk with them directly about happens in this
situation.

Eroc, I know I won't get any time this week to call them. Could you do
it, and post the results?

Sure, I'll try to contact them Monday.



-John

On Mar 10, 10:24 am, Eric Greenwell wrote:
This seems like a plausible analysis, but it's not what Zaon shows in
their manual; instead, they talk about a "bubble of detection" around
your aircraft. A query to Zaon should be the next step, as it might get
an explanation of how their units deal with this situation.

There is a situation that elimanates this problem: multiple radars. This
could be a TCAS equipped airplane or another ATC ground radar.

--
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" atwww.motorglider.org

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