I’m curious: is a Mode C altitude reply basically identical (in message format terms) to the Mode A reply, in other words, it’s basically a 4 digit octal number, and the way the Mode A reply (squawk entered into the transponder by the pilot) and Mode C reply (the altitude, a coded 4 digit octal value) really only differentiated by timing? I guess it makes a lot of sense – when Mode A and Mode C was designed, it had to be something extremely simple for the box on the plane to generate, given the technology of the time, so building the same message format with a different timing was a simple way of adding altitude information without having to make the transponder much more complex and retaining compatibility with existing radar stations.
The ground station interrogates in a Mode A/C interlace – it alternates each interrogation at either 8 microseconds for Mode A or 21 microseconds for Mode C and the airborne transponder responds accordingly. The reply consists of 2 framing pulses 20.3 microseconds apart with the data between the framing pulses. Mode C Altitude data is encoded in the same format as a Mode A reply – a 4 digit slip-binary. The last available pulse (D4) doesn’t appear until 30700 ft.
bookworm wrote:
So lemme get this straight:A “Mode C aircraft” is one that replies to a Mode C interrogation with altitude information.
A “Mode A aircraft” is one that replies to a Mode C interrogation without altitude information.If you find that terminology helpful, feel free. Just don’t expect me to use it.
I apologise for the tone of my post above bosco, I was having a rather frustrating day. Your point is a good one.
No problem bookworm.
BTW, I’ll send you PM on another subject shortly.
