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.