'airborne' bit

[moshe-braner]

The 'airborne' bit in outgoing 'legacy' radio packets is based on any nonzero velocity. This may be causing unnecessary alarms in nearby FLARMs when SoftRF device is stationary, due to GPS location noise.

Should investigate possible fixes:

• set airborne bit to always false, then experiment. If nearby FLARMs still give traffic warnings, restore simple airborne bit setting.
• find out how own 'velocity' is computed, try and smooth it out over a longer period.
• try and base airborne bit on longer-term 3D velocity or movement.
• at the least, require velocity > 1 rather than > 0.

[moshe-braner]

Modified source code to:

• wait 30 seconds after first valid GNSS fix, then
• store initial location and altitude
• look for significant movement away from the initial location
• then if significant velocity call it "airborne"
• Also if zero speed for about a minute call it a landing.

Affected files:

• SoftRF.ino - added GNSSTimeMarker global variable, initialized in GNSS loop
• Legacy.cpp - declare and process static vars for initial location and airborne status

[moshe-braner]

Found in FLARM's configuration manual: The THRE parameter, with a default value that "depends on aircraft type", "Sets speed threshold in m/sec [1-10] below which the aircraft is treated as "on ground". In this mode, no warnings are given and other aircraft are not warned about own aircraft." Thus it seems important to send out the appropriate data in the "airborne" bit!