Jim is fairly correct in the terminology he is using.

There are Mode A and Mode C and Mode S interrogations and what a transponder replies with depends on that interrogations (and if it support that type of interrogation, a "Mode S Transponder" Supports Mode A, C, and S interrogations. A "Mode C" transponder supports Mode A and C interrogations)

Lots of people use "Mode C" or "Mode C interrogation" to mean any altitude interrogation even though most Mode S transponders will usually be interrogated via Mode S type interrogation to return altitude data. They'll respond perfectly like a Mode C transponder for legacy Mode C interrogators.

Legacy SSR issues interleaved Mode A and C interrogations, modern SSR adds Mode S. TCAS does Mode C and Mode S only. Sone GA PCAS systems do legacy mode C only.

Mode S interrogators (SSR and TCAS) utilize a trick (using side lobe suppression) to prevent Mode S capable transponders replying to their Mode A or C legacy interrogations.


On Monday, May 9, 2016 at 9:26:43 AM UTC-7, David Kinsell wrote:
On Mon, 09 May 2016 08:32:58 -0700, JS wrote:

On Monday, May 9, 2016 at 8:16:44 AM UTC-7, David Kinsell wrote:
The bigger question is how well your PF is handling your Mode C
transponder? It has trouble filtering out your own transmissions, may
want to consider upgrading to Mode S for best results. I assume you
have Aggressive filtering selected, and you are receiving Mode C?

Using aggressive filtering and a Trig TT22 that's being interrogated by
Mode C. The flashing blue dot is boring.
Until checking the "use TXP altitude" box, every pull-up brought a
collision warning.
This is my second PowerFLARM used in conjunction with Mode S. The
previous installation of PF with Trig TT21 wasn't a problem. But that
might have been using better FLARM firmware?
Jim

Mode C doesn't interrogate anything, it responds to ground radar or TCAS.
With no Mode C in your ship, can't imagine Aggressive filtering would be
needed. Probably best not to use it.

-Dave