Hi Doc
I think I managed to outwit the ModeC/S + filter last night. I was night flying in VMC on top of an overcast layer, and I am sure there was not a soul about, futher East Midlands Radar were giving me a traffic service and they were quiet, and nothing seen.
I started a descent from 3500 to 3000, at around 500fpm, and within a few seconds got a ModeC alert, red ring @ -100ft. I swiftly hit the altitude hold and as quickly as it appeared it then disappeared. I had a good look out and was so convinced it was my own ModeC transmission I continued descent. On doing so the same thing happened but within around 2 seconds the warning vanished again.
What I think is happening is that the cabin pressure is lagging a few hPa behind the actual barometric pressure as the autopilot starts the descent, then equalises again. So the transponder is reporting a slightly higher pressure.
I found I can sometimes get the same short spurious -100 below by opening my DV window!!
Not sure if this helps in development. On the whole though, the filter is very helpful.
I think you are right - you have outwitted my filter
I have had a few reports about rapid ascent/descent, but opening the window is a new one, but I can see what is happening.
The Filter is using a number of heuristics to remove YOU (ie, Mode-C you) from its list of threats.
I have identified that there is a 10 second window, and if you ascend descend quickly enough it no longer thinks that your own Mode-C transponder is you - basically it moves outside the filter window.
So at 500fpm, that is 83ft over 10 seconds, add on the coarse granularity of 100ft altitude encoding for Mode-C and there you have it!
I had not thought about the window opening, but of course, this could have the same effect of altering the internal pressure, and fooling the pressure sensor temporarily, but that is something I cannot address.
Its at this point where I put on my jobs-worth hat and say - hey the Mode-C is BETA
We think we have a fix for this particular situation of rapid ascent/descent, which we hope to test out in the air as soon as the weather improves, and if this provides better behavior under these circumstances, it will become part of the release.
Thanks for the feedback Doc- this is useful information
Thx
Lee