Two corrections to my previous post.
1) The message sizes are 56 bits and 112 bits.
2) The squitters are used in a mode S transponder just to alert nearby aircraft with TCAS installed and are not used to alert the ground radar site, although they will see them as well. They are the same format as an "all call" interrogation.
The following is quoted from RTCA DO-181C - Mode S MOPS:
1.2.5.2 Surveillance The primary function of Mode S is surveillance. For the Mode S transponder, this function can be accomplished by use of "short" (56-bit) transmissions in both directions. In these transmissions, the aircraft reports its altitude or ATCRBS 4096 code as well as its flight status (airborne, on the ground, alert, Special Position Identification [SPI], etc.). There are two types of squitter transmissions, i.e., transmissions spontaneously generated by the transponder. The short (56-bit) squitter has the format of an All-Call reply (DF=11) and is transmitted by a transponder approximately once every second. This squitter is received and used by aircraft equipped with TCAS to detect the presence and 24-bit address of Mode S equipped aircraft within signal range. The extended (112-bit) squitter (DF=17) contains the same fields as the short squitter, plus a 56-bit message field that is used to broadcast Automatic Dependent Surveillance (ADS) data approximately four times per second. The extended squitter is used by TCAS or other air-air applications, and ground ATC users for passive air and surface surveillance.
“Special surveillance” interrogations from airborne collision avoidance systems are addressed to Mode S-equipped aircraft based upon the address extracted from squitter signals. These interrogations are used for Mode S target tracking and collision threat assessment
To respond to to the initial question which I assume is aimed at UK installations, adding an altitude encoder to an existing transponder is not changing the transponder so an upgrade from A to C is perfectly legal. For the record it is also legal to replace a mode A/C transponder in the UK with another of the same make and model if one fails. I checked this with our local avionics people as we have a working KT76A on the shelf after an upgrade.
Steve :)
My original post might have sounded somewhat tongue in cheek but I was really wondering why people flying with a Mode A don't replace with with a Mode C. For the benefits (a much better ATC service, and visibility to TCAS equipped aircraft) it seems a no-brainer.
But I have heard it claimed that this is illegal, and that any replacement has to be with a Mode S, which obviously costs a bit more money.
i thought the same but i do know of an aircraft that recently has a mode c transponder fitted. it had nothing before.
As said in my post above, for most people it won't be replacing a Mode A with a Mode C unit, but merely plumbing in and connecting an encoder.
I don't think I've seen a transponder fitted in the last 20 years which was incapable of being ModeC.
I believe there was a rule introduced in the UK that mandated that all new transponder fits should be Mode S, but to be honest, since I got my Mode S fitted over four years ago, I haven't paid much attention, so it may have gone away.
