You say.....
I had set the Mode C/S separation to +/- 2000 feet and was was flying along at circa 1500 feet which is fairly normal, but found I kept on getting audio alerts for aircraft that were 1300 feet below me, 1100 feet below, etc.
...so they could quite clearly have just taken off and were potentially climbing towards you.
This type of alert should only occur with 'bearingless' Mode C or S aircraft, where the alerts are based on received signal strength as well as relative altitude, because their position and distance are otherwise unknown. Whilst your suggestion for an 'automatically variable' relative altitude filter certainly has some merit, in order to effectively provide the 'variable' relative altitude filtering you are suggesting, PAW would have to take account not only of your current altitude, but the height of the ground you are flying over, which we cannot possibly do for the whole of the UK, let alone further afield. Without this information, rising ground would rapidly bring aircraft on the ground back into an 'alert' situation and falling ground would drop climbing aircraft out of your chosen alert bracket without you even being aware this was happening.
Hi Peter, thanks for your reply, glad that my idea potentially has merit
The alerts I received (and there were a few of them as I was flying) were bearingless CS targets because all I was warned of was the target altitude. I think I had the mode CS range set to the recommended setting of 'Short', but I will check this.
You're right, as I never saw any of the warning aircraft I am surmising that they were aircraft on the ground based upon my altitude and their relative altitude, and they could have been aircraft that were taking off and so were becoming a danger. With some kind of offset altitude alert +2000/-1000 or smarter logic like I suggested based upon my own altitude I feel that I would have had less false positives which has to be a good thing. The last thing I want is to become complacent and start ignoring alerts because I am receiving too many of them.
I do agree, having a database of altitude information within PAW would be impracticable, and so I thought of the smart range logic as being an achievable alternative.
I have also just been looking at your post over in the 'Altitude Filter' thread. You seem to have at least 27 contacts showing on your XC Soar screen at the same time, with nothing inside 6 miles from you, so can I ask how you know the aircraft were all on the ground?
This is the other issue I faced, that XCsoar doesn't offer any ability to filter out traffic so is I believe showing
everything that PAW is detecting, regardless of range or height, and is plotting them on the 6 mile+ ring which makes for a display that is impossible to use. When I click on some of these individual aircraft I could see that in several cases they were jets flying thousands of feet above me, but with so many aircraft plotted and XCSoar not offering any ability to filter based upon range or height, I can't separate the proverbial wheat from the chaff.
I guess this comes from XCSoar being originally developed for gliders where the low power FLARM would only collect alerts from fairly near aircraft. PAW picks up stuff miles away and so the display is now overloaded.