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.
i thought the same but i do know of an aircraft that recently has a mode c transponder fitted. it had nothing before.
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.
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 :)
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
I don't doubt NCYankee's explanation because I know he knows the subject, but I don't think that is entirely how Mode S is operated in Europe.
Firstly, if one does a search of a GTX330 installation manual for "Elementary" or "Enhanced", one finds nothing. At least not in the IM I have. That's because these are European creations, done without reference to actual equipment that exists.
Elementary Mode S emits the 24 bit ID and also some sort of flag indicating the speed range of the aircraft, which i recall is configured in the GTX330 as being above or below 150kt or something like that.
Enhanced Mode S returns whatever you feed to the GTX330. There is no config in the GTX330 for elementary v. enhanced. It will just transmit on 1090ES everything connected to it via RS232 or ARINC429.
Also, a GTX330 won't radiate anything at all unless interrogated by SSR radar.
ADS-B has to be enabled separately and then it will radiate the various input data without being interrogated by SSR.
Is that incorrect?
Great explanation, NCYankee!
Mode S transponders don't reply with position information. The main difference between mode S transponders and mode A/C transponders is that the mode S transponder has a unique 24 bit address and can be interrogated individually. The mode S transponder can receive (1030 MHz) and transmit (1090 MHz) a digital message up to 54 bits or 108 bits. This permits exchanging more data than the squawk code and the altitude. Each transmission uses the aircraft ID, but after the first transmission, the parity is Xor'ed with the ID.
Mode S can also broadcast a message (1090 MHz) without being interrogated. The radar term for this is called a squitter. In the basic mode S, the squitter is used to broadcast the fact that it is nearby. A mode S radar will pick this up and realize that the transponder is in its airspace and therefore add it to its interrogation table. A full TCAS system will also receive the broadcast and is able to exchange information via the squitters between two aircraft that have TCAS. Because of mode S being individually addressable, the ground radar system can operate more efficiently and the capacity of the system is enhanced, particularly in congested airspace.
There are two levels now in use by the standard mode S, ELS (Elementary Surveillance) and EHS (Enhanced Surveillance). ELS only replies with the ID, altitude, ident status, and squawk code. EHS provides more information by adding selected altitude, roll angle, roll angle, true track angle, magnetic heading, ground speed, indicated airspeed, and baro climb/descent rate.
ADS-B does not use the reply messages, it only uses the squitters. Because it needs more data, it uses Extended Squitters (108 bits) and this includes position data and other data such as position integrity and accuracy codes. So, without ADS-B or 1090ES capability, the mode S transponder does not transmit its position.
Mode S is an interesting technology invented by the MITRE corporation to be backward compatible with the mode A/C transponders. To get the mode A/C transponders to ignore the longer mode S messages and to avoid interference from mode A/C transponders stepping on the mode S transmissions a technique known as side lobe suppression is employed. Mode A/C transponders receive the interrogations from the radar antenna as it sweeps by the azimuth of the aircraft, but it will also receive an interrogation when the antenna is pointed 90 degrees from the azimuth to the aircraft. This spurious signal is called a side lobe and is unwanted. If the mode A/C replies to the side lobe, then the radar will think it is in the direction that the antenna is pointed and will displace it 90 degrees. To avoid this, one pulse in the preamble to the inquiry is transmitted using a non directional antenna, but at a distinctly lower amplitude. The other pulse is sent only by the directional antenna. If a mode A/C transponder sees the non directional pulse and it is smaller in amplitude than the directional pulse, it assumes this is normal and replies. If instead it detects the two pulses similar in amplitude or the directional pulse being smaller, it assumes it is from the side lobe and goes to sleep for a short while. MITRE used this characteristic and encodes the mode S transmission with the directional pulse deliberately at a lower amplitude so that any mode A/C transponder will go to sleep for a while. It then continues on with a higher amplitude pulse in a later time slot so that the mode S transponder can check for the side lobe. The maximum length of the inquiry and reply messages are based on the time that the mode A/C transponders are required by the TSO to be asleep for the side lobe suppression. The side lobe suppression is checked every time the transponder is recertified. In the US this is every two years.
Please forgive my naivety but how do the integrated TCAS systems different from devices like the ZAON MRX type which detect radar interrogation bounces? I often see a number of aircraft, some quite close, and they arent detected by the MRX. The other aircraft cant all have their transponders switched off (well....) but I wondered if the integrated ones had some kind of radar as they seem to show much more traffic.
The ZAON and similar products are passive receivers. They rely on transponder emissions from targets which have been pinged by SSR radar. They have several problems
The "active" systems from e.g. Avidyne or Garmin transmit SSR-like pulses, uniformly in all directions, and then collect the responses from the targets. From the time delay they can determine the range pretty accurately, and using an array of (normally) four antennae mounted externally and the relative signal levels from them they can determine the azimuth (to maybe 10-20 degrees). And because they tap into into your transponder RF cable and also connect into your DME, they sidestep picking up your own transponder emissions that were triggered by SSR radars.
There is a x10 difference in cost 
In all cases the relative altitude is determined by comparing your own pressure altitude with the target's Mode C returned pressure altitude.
Integrated just detect the transponder positions.
