Thanks for all the input, should probably moved the Audio/Mode-S/ADS-B/P3I onto a separate thread, but this is what I think we are coming to ....
Firstly the audio messages of 'above/level/below' indicate an angle to the horizon of
above = greater than +20degrees
level = between +20 and -20 degrees
below = less than -20 degrees
This can only be calculated with ADS-B/P3I - simple trigonometry, you need a distance and relative altitude.
For Mode-S, all you have is relative altitude, but no distance, therefore it is impossible to calculate the angle to the horizon, consider the angle to the horizon of +500ft at a distance of 100m and 1000m - completely different.
The altitude difference could be added for Mode-S altitude if it is valuable, but this would be of the form
"Notice, traffic, 500ft above", thus not indicating elevation to the horizon
Phew!
I think I am hearing two requirements for muting the audio :-
1. A manual mute: I like this idea with a configurable timeout, in other words mute for X minutes, then automatically re-enable, or mute permanantly (or pull out the plug!)
This could solve the issue of Taxi, Take off & Landing
2. Ground speed filter: if my speed is less than X Kns, then mute the audio.
This can only address the issue of Taxi & Take-off
These could both be config options, and in the case of Helicoptor, Paraglider, Hanglider, the speed option may not be a good choice.
Thx
Lee