Except for ADS-B, traffic received on 1090 MHz (Mode-C/S transponder equipped aircraft) is non-directional. This means that these aircraft only send a coded signal (squawk code, altitude, etc.) but not their position. Based on the power of the received signal (signal strength), as well as changes of the signal strength, an approximate distance to the aircraft and coarse collision risk can be calculated by the FLARM algorithms. However, the relative bearing to the aircraft cannot be shown, since this is not known.
Certified FLARM Compatible displays will indicate the distance and angle (or altitude difference) to non-directional aircraft. On LCD-based displays, the distance is usually visualized with a distance ring around ownship. LED-based displays show a specific LED pattern, to distinguish the warning from directional (FLARM and ADS-B Out) targets.
Displays connected using the Garmin TIS protocol cannot indicate non-directional targets. The FLARM device will instead simulate these targets by transmitting 8 ghost targets around ownship, at the approximate calculated distance. This is done on a best-effort bases, without any guarantees. Also note that it is insufficient to connect a FLARM device to a displays via the Garmin TIS protocol. As a primary means of collision avoidance, the FLARM device must be connected to a certified FLARM Compatible displays (as explained here).
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