Originally Posted by
wiedehopf
This is incorrect. The altitude is always broadcast independent of aircraft altimeter setting.
Why would you design the system to potentially transmit a bogus altitude because the pilots set the altimeter wrong?
Wikipedia can be a bad source so google "mode c pressure altitude" because that is what is transmitted, the pressure altitude.
And the pressure altitude is independent of the local pressure. Then the ATC system corrects for local pressure and voila you get a correct readout.
Now the number on the planes altitude indicator may be different from the one displayed on the ATC system because the pilot already set standard pressure. But that's not a problem is it?
OK we're going a little off topic here so I won't prolong the thread too much. Let's look at it the other way around. Example: Two aircraft operating below the transition layer, both indicating 7000 by altimeter, regardless of whether cruising or climbing/descending. One has QNH set, the other 1013.2. The one operating on QNH will correctly display 7000 to ATC, the one operating on 1013.2 will not, due to the QNH correction applied by the ATC system. ATC allows a 200FT tolerance to verified levels. In other words, if a pilot reports at 7000, then so long as the display to ATC shows 6800 to 7200, then the report will be accepted as correct.
So coming back on to topic if there is other traffic at 8000 or 6000 on QNH above or below the aircraft operating on 1013.2 at 7000 and the pressure difference between QNH and 1013.2 is > 6HPa the wrong way then the aircraft on 1013.2 will be deemed to be in vertical conflict (assuming a 1000FT vert separation standard is applicable). Similar applies to a climbing flight that sets 1013.2 while still below the transition layer. That is the reason why climbing flights should not set 1013.2 while still below the transition layer.
Oh, and let's not get hung up about Mode C (or even Mode S, for that matter). Radar is a dying technology, rapidly being replaced by ADS-B and with accelerating pace of technology change who knows what next.