That could be the case, however... The MRX PCAS uses an internal
barometric sensor in the absence of an own ship's transponder. I've
observed its startup and noted that it initially reports a transponder
code of 3707 or some other nonsense, after my transponder is turned on
and begun replying to interrogations, the PCAS with show a TX code of
1200 and, finally, 1202 to which my Trig is set to reply.

I don't know what the required Mode C/S accuracy is below FL180, but if
it's +/- 200 feet, then you could occasionally see a 400' error. That or
my PCAS lost lock with my Trig, or it's just sloppy. Next time I see an
aircraft on which I've received an alert, I'll check the PCAS for the
transponder code it's using for its calculations. If I gain some
insight, I'll report it here.

On 7/19/2015 11:43 AM, Ramy wrote:
A note about transponder altitude inaccuracy. I also noticed significant inaccuracy when I used the Zaon MRX but once I switched to PowerFlarm I found mode C reported altitudes much more accurate, so I suspect the significant inaccuracy that Dan noted is due to MRX error, not necessarily transponder error.

Ramy

--
Dan Marotta