On Saturday, November 3, 2012 2:29:03 PM UTC-7, Mike C wrote:
On Nov 3, 8:59*am, Dave Nadler wrote:

On Friday, November 2, 2012 8:45:43 PM UTC-4, Mike C wrote:

I am wondering if using Oshkosh as an example applicable to a few

sailplanes, in a thermal, is not a bit of a strawman argument.



Oshkosh was used as an example of where Mode A/C cannot work.



A separate point, it is my impression that *traffic is so congested

there that the controllers are overwhelmed, not that the signals are

garbled and unreadable.



"Your impression" ? The controllers are not overwhelmed, they use

appropriate tools and manage the highest traffic density anywhere,

very professionally. As you would know if you've flown into

Oshkosh (or in the airshow ;-)



Mode C is valid, safe and not the worthless tired technology

that it seems to be portrayed as.



Sure, which is why Mode C was phased out in Germany years ago,

with the rest of the world following...



For new installations, get Mode S and we'll all be happier

with the results. And it won't have to be replaced if/when

FAA catches up with the rest of the world (not holding my

breath).



Mode C does work in many cases, but...



Dave,



Are you sure that the reason that formation pilots only have one

transponder on, is not because of collision alerts with aircraft in

close proximity of each other? That is what I was told. Also told the

reason transponders are turned off is because of alerts in heavy

traffic



Mike.

No, the reason for all but the formation leader in going squawk standby in formations is 'garbling' with all the Mode A/C transponders otherwise replying to the same interrogation and the SSR *possibly* being unable to decode the signal from each aircraft. Modern SSR systems also have pretty advanced decorrelators that can untangle several overlapping replies. This is something that dates back to the earliest days of SSR radar and is discussed in the FAA AIM, ATC Procedures section.

PCAS type systems may also be affected by garbling, but this is not the reason ATC cares about it. You can sometimes see what I expect are the garbling effects with Mode A/C transponders with when you formation fly with an aircraft and it dissapears intermittently.

But to Eric's points, just don't worry about all this wonky geek crap. If any of this was really worth worrying about you'd have the ATC folks who work closely with the glider community in busy traffic places raising it as an issue. And AFAIK they have never done that--all the discussions I have with those folks, both civil and military is they just want to see transponder adoption near their airspace, Mode C, Mode S they don't care.

And fly the glider. I'd hate to be in gaggles where folks are coordinating who's turning off transponders, then you have to remember to turn them back on again.

OTOH if you are flying in large gaggles near a busy ATC facility you have lots of very real reasons to be talking to your friendly ATC folks, not there are far more important things to talk about than the possibility of garbling. And I certainly expect those ATC folks to notice the presence of large gaggles even with severe garbling.


Darryl