AIR Traffic (AT-1) devices receive ADS-B and FLARM® targets (exact 3D position) as well as targets with "normal" Mode-S transponders, which do not emit a 3D position. Such targets without a position are detected in range and altitude but without a bearing. These are also referred to as "bearing less" targets.
The distance is estimated by measuring the field strength, the altitude is calculated from the altitude signal of the other transponder.
Displays fully supporting bearing less targets
In most displays, the current nearest bearing less target is symbolized by a circle around the own ship position, together with an altitude indication.
This circle is always present if a bearing less target enters a certain area, roughly at a distance of 2 to 3 miles and +/- 1000ft or closer. Only one circle is shown which depicts the nearest bearing less target.
These displays support bearing less targets natively:
- AIR Traffic Displays
- Most apps including SkyDemon and Foreflight
Some Displays, for example EFIS systems using the RS232 TIS Protocol (a.k.a. GARMIN TIS), are not able to transfer such bearing less targets natively. They can process and display only up to eight targets at a time. Normally, these eight targets are used for eight targets with a 3D position and no bearing less targets are shown.
However, If a bearing less targets gets too close, six of the eight targets are used to symbolize a circle around the own ship symbol and to generate a warning.
In conclusion, some displays using the RS232 TIS protocol show bearing less targets only in the case of a traffic warning.