No problem bookworm.
BTW, I’ll send you PM on another subject shortly.
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.
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.
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.
boscomantico wrote:
What I was getting that is that an aircraft is not “Mode C” if it doesn’t have an encoder. A Mode C capable transponder is not enough.
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 didn’t say zero pressure altitude, I said 0000 as in binary 000000000000
0 pressure altitude is:-
Gillham:- 000011010000
Octal:- 0620
See here:-
Any avionics shop person who has a Mode C transponder tester can answer this one. Maybe there are pullups which drive the value to FL600 or some such?
TAS equipment definitely does not filter out a zero pressure altitude; I see planes sitting at the EGKA (elevation 12ft) holding point when I am on final and get proximity warnings off them, and if QNH is 1013 they will be radiating a zero pressure altitude. And regardless of QNH they will be radiating something close to mine when I am nearly on the ground.
But on top of that I reckon ATC radars might be filtering out obviously wrong stuff.
I presume it is that can that with no encoder connected, it in effect is transmitting 0000 which doesn’t resolve to any altitude.
I am well aware of that. What I was getting that is that an aircraft is not “Mode C” if it doesn’t have an encoder. A Mode C capable transponder is not enough.
boscomantico wrote:
Hein? Then how can it squawk Mode C?
Because Mode C is the interrogation mode, not the output data. A transponder without an altitude encoder (or a transponder switched to ON not ALT) will respond to a Mode C all-call interrogation used by TCAS, but will return an unknown altitude.
As was discussed earlier in the thread, classic transponders are all “Mode A/C” transponders, regardless of whether they have an altitude encoder attached.
