Yo Hans

I do not think Eric is overselling PCAS.

Like all traffic awareness technology PCAS had limitations and people
really should try to understand those, but before mentioning some of
those let's clarify one thing... Yes PCAS only works if the threat
aircraft transponder is being interrogated. Both the interrogation to
the transponder from somewhere and transmission from the transponder
are ~1GHz so are absolutely line of sight. Interrogators include SSR
ground based radar, including civil and military terminal radar
(~60nm) as well as area radar (~250nm). Obviously this is line of
site, degrades as you get low and since the radar signal only fans up
in a wedge shape you also may also not get proper interrogation
overhead of the main radar beam. However in many situations your
transponder is being interrogated by several radars, but even more
usefully in many situation your transponder is also being interrogated
by multiple airborne interrogators such as TCAS and TCAD systems (like
the Garmin 800 series or L3 Skywatch). And effectively all airliners,
many corporate jets, many military transport, etc. have TCAS.

It is often suprising how you can be in some remote valley and yet see
the transponder being interrogated. That is airborne interrogators at
work.

Also lets clear up one misconception that blinky light or LCD thing on
most transponders that shows the transponder is being interrogated is
really not showing you what is going on. Yes if it is blinking your
transponder is being interrogated. But it is just saying it's talking,
it does not show you the actual interrogation rate (which could be
hundreds of times per second in busy airspace). So don't rely on the
blink rate to tell you you are in an area of high interrogation or
not. Obviously no blinking says no interrogations at all, which is not
good.

So for a sampling of some PCAS limitations....

PCAS relies on the user reading the manual, there are settings for
alert ranges, altitudes etc. on most units that the user should be
aware of. Also basic alerts, beeps etc. may be confusing unless the
person has read the manual and spent time playing with the unit.

PCAS relies on the other guy having a Mode C or Mode S transponder.

PCAS relies on the other guy turning on his transponder (I'll spot
friends who forgot to turn on the trasponder and let them know over
the radio).

PCAS relies on the other guy having his transponder correctly set to
Mode C (or "ALT") not accidentally put on Mode A.

PCAS relies on the threat aircraft transponder being interrogated. If
its not you get no warning. While it is amazing how many places do see
interrogations from radar, TCAS, TCAD etc. there are clearly places,
especially down low on ridges or in valleys etc. where relying on this
would not be a good idea.

PCAS relies on line of sight (or partially reflected off the airframe
etc.) signal from the transponder antenna to the PCAS antenna. So for
example if you are flying higher than the threat aircraft and you PCAS
antenna is partially blocked from below by the instruments and maybe
carbon fuselage and the threat glider has a transponder antenna
underneath... you get the picture... PCAS may not see the threat
glider or may assume it is far distant (becasue of the simple radio
radio power based guess that PCAS uses to approximate distance).
However you will often at least get some warning, amazingly the RF
signals do have ways of reflecting off aircraft surfaces etc. But
there are clearly cases where you don't see somebody you should and
this may explain many of those. More sophisticated PCAS installs might
have external or dual antennas (top and bottom), and more
sophisticated transponder installs might have two antennas as well but
that's a bit too much for most gliders.

PCAS relies on a vertically polarized signal. So things tend to work
best with transponder antennas pointing as much as possible up and
down and PCAS antennas doing the same. Both with a good view of the
sky.

PCAS uses relative altitude as the primary threat determinant and that
has issues. PCAS relies on the altitude difference a lot because the
range to the threat aircraft is guesstimated from the received power
and so the range is pretty inaccurate (but very useful to show a
closing threat). By contrast TCAS and TCAD use interrogation-response
times to much more accurately determine distance to the threat
aircraft. The relative altitude between you and the threat is
determined from the altitude encoded in the threat's transponder
signal and either the PCAS unit's internal altimeter or the altitude
it reads from your own transponder signal. One problem with this is
that the PCAS unit cannot generally tell what is a Mode C altitude
transmission and what is a Mode A transmission (ie. it's squawk code)
from the same transponder. (I can hear the next question... a Mode C
transponder has to both answer Mode C (altitude) and Mode A (squawk
code) interrogations). But the PCAS just sees a stream of bits and
does not know whether the transponder was asked by the interrogator to
send Altitude or Ident. The PCAS can do some smart guessing and things
like the Zaon MRX seem to do this very well, *but* if it get this
wrong it can think the threat aircraft is at some completely bogus
altitude far off from where it is. Since relative altitude is the
primary threat determinant the PCAS system might just ignore that
target. In practice this problem seem very very rare. BTW Mode S
transponders being interrogated Mode S don't have this "alaising"
problem.

Since PCAS systems uses altitude as the primary threat determinant a
relative altitude error of several hundred feet or so may make the
difference between a PCAS unit alarming on a threat or not. Check the
transponder altitudes in both your own and the threat aircraft are
reporting correctly and also if it has one check the internal
altimeter within the PCAS unit. You can usually get to this via a
menu. With a Zaon MRX for example it will try to read your local
transponders encoder altitude and base its relative altitude
measurements between the aircraft on that. If however it's internal
altimeter (measuring cockpit ambient pressure) seems off from that I
believe it can revert to using the internal altimeter. If you are
seeing strange or unreliable behavior that might be altitude reading
related I would check with Zaon.

---

So no technology is infalible. PCAS can be a temendous tool, but it
helps to understand how it works. I have flown with my MRX for about 5
years and think it was a great safety investment.

One of the major benefits I see with PCAS is once people fly with one
a little a response like "holy !@#$ I had no idea there was so much
stuff out there" is not uncommon. PCAS units acts both as as a useful
alert device and an effective reminder to keep a healthy visual scan
going, make radio calls, etc.

Did that help? (and Hans try to stay awake in my next transponder
presentation :-))


Darryl


On Feb 11, 3:08 pm, weersch wrote:
Hi Eric
Very good overview article. Thanks for that.
I think you are maybe a bit too optimistic about the PCAS systems.
Please don't get me wrong.
I would still recommend everybody to get one.
But everyone needs to understand the limitations.
I don't have any pertinent data, but this is what I sense:
PCAS is not "watertight".
I hear continuous stories about missed threats.
Last weekend my (transponder-on) threat did not show up on my friend's
PCAS.
You already indicate that it works "well", not perfect.
To stay with your phrasing and adjusting it a bit.
If you fly in Reno area and have 10 surpises a year, the PCAS will
probably indicate 8 out if those 10.
But not eliminate all 10 out of 10 surpises.
My understanding is also that if a Mode C transponder does not get
interrogated (e.g. out of radar range), no threat will show up on your
PCAS.
For these remaining cases and all the other stuff without transponder,
you still very much need to keep your eyes out there.

All these tools are great to significantly reduce risks, but they do
not eliminate all risk.
The worst scenario is to replace the reduced risk with complacency,
assuming that everything is safe now with your fully tooled up glider.
According to Murphy's law, the remaining risk will come and bite you.

Hans Van Weersch
With "always-on, radio-checked" radio, "always-on, tested"
transponder, "always-on" instruments, 25Ah of battery (going to 31.5
Ah) and getting an MRX-A soon.