transponder mode question
 

ATC asked a military flight to try their second tx. They didn't have a second tx. The aircraft then asked if ATC could read mode 1 or 2. ATC said mode 3 only.

What are these modes?

This page might help too.

Mode 3 is civilian Mode A.

From what I remember, mode 3 gives squawk, altitude and speed, while the others are for mission use which filters what info is sent depending on what "source" is asking.

If I'm wrong then appologies, its been a while.

IIRC, Mode 1 is similar to the original WW2 IFF (Identification Friend or Foe)which has 2 digits and therefore only 64 codes. It works on the same frequency as ATC transponders. Mode 2 has 4 digits and 4096 codes and is almost identical to ATC but uses a different frequency. Mode 3 is the same as ATC transponfers and has th ability to carry extra information such as Mode C - or altitude reporting. Mode 4 is encrypted and the codes can only be loaded with a data loader.

Typically, mode 3 will be used as the ATC transponder. Modes 1 and 2 are used for IFF purposes in tactical situations with possibly the squawk changing at regular intervals. Mode 4 is for tactical purposes and can give far more information than the other modes.

And a bit of SSR trivia - the wartime codename for IFF was "Parrot". Which is why when interrogated, it "Squawks".

Often wondered why, now I know.
Makes perfect sense.
Thanks.

Which reminds me of a phrase which I haven't heard for a very long time and that is,

"Strangle your parrot"

To turn your IFF to OFF.

Regards,
BH.

well I was pretending when they say Mode C they are referring to the version capable of altitude reporting. but the other day the Mode S with different features (ident, level, with level or ident) grabbed my attention, then what is Modes S or what are the differences between mode C Mode S?

mode c is altitude reporting.
mode s is a more sophisticated method of interrogation and reporting useful in crowded airspace.

The biggest difference between mode C and mode S would be the way transponders are interrogated. Mode C is interrogated in a general manner- i.e. the radar head asks all transponders to reply with their 4 digit code and altitude. The radar head then waits for the responses to get the distance to them. Mode S can send out a general roll-call like that, but it can also interrogate an individual transponder to get what information it needs (a rather large list of things potentially). The biggest advantage of mode S is that individual addressability. Mode C systems can get responses from transponders that were requested by another radar head- leading to them being in different places than expected.