Wow! That was an education on how the system works and I certainly need to
study some more.

Regarding PCAS and how I use it in *my* environment, it's not so much a
bearing/distance/height pointer showing me where to look, as it is watching
the distance/delta height and noting whether it's closing, matching, or
diverging from my flight path.

I think you nailed it in your last sentence: "Owners/pilots should be
evaluating what is best for their needs, and that may be to get both."


"Darryl Ramm" wrote in message
...
On Saturday, June 22, 2013 9:10:18 AM UTC-7, Dan Marotta wrote:
To be accurate, it's Mode A (reply code) you're talking about, not Mode C

(pressure altitude). And why would Flarm users filter out signals from

possible threat aircraft? Or did I misunderstand the statement about

turning warnings down/off?



Most of the US air space is covered by ATC radar and so my transponder is

pinging away in reply so I expect Flarm equipped aircraft would see me if

they came into my area. Flying out of Moriarty yesterday, I was one of
only

two gliders to stay up and, in 2.1 hours did not get a single blip on my

PCAS. I verified that it was reading my transponder replies by changing
my

code and testgin again. I don't think Flarm could have improved my

situation any.



I know I should have simply stayed out of this but... Fire away.

There seems to be some confusion here.

To be accurate and correct :-O ...

PCAS typically has *nothing* to do with Mode A replies and is all about Mode
C replies. Mode A encodes the squawk code and Mode C encodes the altitude. A
traditional Mode A/C transponder when being interrogated by an SSR system
responds to interleaved Mode A and Mode C interrogation pulses (the targets
are interrogated multiple times in both Mode A and C each rotation as the
SSR interrogator beam sweeps past the aircraft) and responds with whatever
its being asked for. PCAS systems, including in the PowerFLARM just listen
to those replies and try to guess which ones are mode A or Mode C (the
replies don't explicitly say what type they are, traditionally the
interrogating SSR knows what it asked for). The PCAS first job is to
typically discard any replies it things are mode A and then only using the
Mode C replies, work out the altitude of the threat and whether it should
display that/issue an alert etc. How well PCAS systems work given they are
effectively blind and are have very little data, have to make guesses about
Mode C replies and have to infer range from the RF power etc. With Mode S
threats PCAS systems can be smarter, e.g. they can absolutely know that a
squawk code and altitude transmission comes from the same aircraft etc.
There was one older PCAS system that did display threat aircraft squawk
codes (e.g. it decoded both Mode A and C) with the claim that its nice to
know if a threat is VFR or under ATC control etc. But to me that is all just
more info you have to display/let users understand etc. and likely not worth
it.

One of the benefits of lots of the special Mode A codes like 1200, 1202,
7700, 7600 is that they do not encode as valid Mode C altitudes but lots of
other altitude and squawk codes can alias with each other. This is not an
issue when dealing with Mode S transponders being interrogated by a Mode S
interrogator.

PCAS clearly cares about Mode C replies because it wants to know the
altitude of any threat so it can work out what to do with it, including just
ignore it. Mode A does not help much, and with pure Mode A and C
transmission in dense traffic environments the PCAS cannot ever tell how to
match up Mode C and A replies from the same threat aircraft.

And to complicate things further TCAS and TCAD systems never issue a Mode A
interrogation, another reason PCAS never want to rely on anything Mode A, if
they just listen to mode A and you were in a TCAS interrogator only
environment you would just never ever see the threat. There are some complex
second order things that PCAS systems could try to do with Mode A
transmissions, but I suspect none of them do.

And to be clear on products here... Flarm equipped aircraft never see any
PCAS threats. *PowerFLARM* does see Flarm and PCAS, at least in the USA
where all PowerFLARM come with this capability, in other geographies things
may be different, for example in Europe "PowerFLARM Pure" does not include
PCAS reception capability.

Lots of USA airspace where gliders fly, especially close to
terrain/mountains etc. is not covered by SSR and so transponders will not be
interrogated by SSR. the obvious pointer here is that controlled airspace
has SSR ground interrogator coverage and uncontrolled airspace may not. That
will change to some extent as ADS-B rolls out, but hats a who other hairy
topic. The savior there frequently is airborne TCAS interrogators, there are
lots of airline, private jet and military aircraft flying around issuing
mode C (and Mode S, but not Mode A) interrogations and that can be an
important factor here to make out PCAS systems work.

Especially in mountainous regions you have to be cautious about PCAS, it
might appear to work fine and then suddenly not work as you fly behind an
obstruction that shields an SSR ground interrogator or as the overhead TCAS
interrogator flies away. Or it might appear to work fine and then as you
descent down to pattern altitude you can lose the interrogator yet be
exposed to the more dangerous traffic density around an airport.

I'm not sure exactly what point you are making on you PCAS observation out
of Moriarty. Assumption about whether you would see other gliders depends
firstly on if those other gliders are Mode C or Mode S transponder equipped.
And what the local interrogator coverage is, at Moriarty especially when low
near the airport, which I assume does rely on TCAS interrogators. Or if you
wanted to compare seeing them with PowerFLARM then you need to know if they
are PowerFLARM equipped. What exactly was your point here and what are all
the other gliders in the area equipped with?

PowerFLARM is clearly superior to PCAS in high-density glider on glider
scenarios like contests because of the much more accurate threat
information, because more gliders in the fleet are likely to be PowerFLARM
equipped than transponder equipped and because there is no issues with
needing the Flarm system to be interrogated. Transponders and PCAS have huge
benefit when mixing with other traffic (especially gliders carrying
transponders where there are high density TCAS equipped fast/commercial jet
traffic) and working with ATC etc. Its simply a false paradox to make forced
comparisons between transponders vs. PowerFLARM. Owners/pilots should be
evaluating what is best for their needs, and that may be to get both.