Transponder in GND mode... issue

[vlittle]

Last week, I received a call from ATC during my runup, asking me to squawk STBY on my SV transponder. The said it was showing up on their display.

It was in GND mode, and my understanding is that it would not reply to mode-a or mode-c interrogations in this mode.

The airport (Langley, BC) does not have it's own SSR, but may have a multilateration site.... but there is no info online that I can find to confirm this.

To be clear, Canada has not rolled out ADS-B in the southern regions of the country, so I assume we only have mode-a or mode-c coverage. I suspect that they may have mode-s equipment but have not rolled it out to ATC yet.

I report this because it **may** be a bug in the transponder, but it is just as likely to be a quirk in the Nav Canada SSR system.

 

[dynonsupport]

What SkyView software were you running?

We'll check with Trig to be 100% sure what behavior is expected in GND mode when in a Mode A/C environment.

 

[vlittle]

Running 11.1 special, as I recall. The special is the serial port fix patch for freq passthrough.

I have a call into Navcan about this to see if I can get more info.

 

[dynonsupport]

Vern,
We talked to Trig, and here's the responses in various modes:

SBY – The unit powers up but does not transmit or receive

GND – The unit will only reply to direct Mode S interrogations using the specific ICAO address. It will not reply to mode A, C and Mode S all-call interrogations.

ON – The unit will reply to all interrogations but omit altitude reporting. Usually only used on request from ATC if they think your encoder is faulty.

ALT – The unit will reply to all interrogations.

So, either you were actually directly interrogated via Mode-S, they have ADS-B receiving equipment (ADS-B spontaneously emits in GND mode), or you weren't actually in GND mode. 11.X and earlier would sometimes show in GND mode when the transponder was overriding and was actually in ALT, and this is fixed in 12.0. For instance, if you didn't have a GPS lock, you were in ALT as the GND mode needs a GPS signal to verify it's in GND.

Let us know if you've learned anything new.