Bearingless targets:
AIR Traffic (AT-1) devices receive not only ADS-B and FLARM® targets (exact 3D position) but also targets with "non-ADS-B" Mode-S transponders. These targets do not emit a position and are only detected in distance and altitude. A target bearing (i.e., the direction of the target from your own aircraft's view) is unknown. Therefore, these targets are commonly referred to as bearingless targets.
Theory of operation:
While the altitude is exact, the AT-1's distance estimation for Mode-S is rather coarse. As the AT-1 is a passive system without interrogation capabilities (which would at least quadruple its price and increase the installation complexity), AT-1 utilizes the received signal strength indication (RSSI) to determine a target's distance.
Discrepancies may happen depending on the own aircraft's antenna position (1090MHz receiving antenna of the AT-1), antenna cable attenuation, and the orientation of both antennas (the target's and the own antenna). Such discrepancies are filtered statistically.
Influence of the Installation:
However, in cases where the AT-1 antenna installation is unusual, for example, if it is particularly shielded or the orientation is not good, targets might be closer in reality than they appear to the AT-1. Please always ensure that you follow the guidelines in the AT-1 installation manual. All AT-1 antennas must be oriented vertically, and the cable attenuation must be within the specified values.
Display types:
The AT-1 is compatible with many AIR Avionics and third-party displays. You can display AT-1's traffic data on cockpit instruments, such as flight decks, apps, or dedicated cockpit displays of traffic information, like the ATD series from AIR Avionics.
Not all compatible display types can depict bearingless targets natively. In general, these display types are common:
| Display Type | Interface | 3D Targets | Bearingless targets | Bearingless targets shown |
| Modern CDTI like AIR Avionics ATD series | RS-232 AT Protocol | Up to 64 with advanced symbology | Ring Symbol with exact altitude | The closest target is always shown |
| FLARM LED Displays | RS232 FLARM Protocol | Usually up to 32 | Rotating LED, higher or lower indication | The closest target only in case of a warning |
| Flight Decks like GARMIN G3X or Dynon Skyview | RS232 GARMIN TIS Protocol | Up to 8 | Rotating symbols around the own ship symbol |
The closest target only in case of a proximity or a waring |
| Navigators/Flight Decks like GARMIN G1000 or GTN | Arinc429 | Up to 8 | Numerical info (Distance: xx nm, Altitude: +100ft) |
The closest target only in case of a warning |
The AT-1 installation manual holds a list of display types and their capabilities.
Warnings:
Traffic warnings are issued depending on the calculated time-to-impact (TTI) of an imminent collision. If a collision is unlikely or the TTI is longer than 25s, no warnings are produced. As some displays only show bearingless traffic if a waring is issued, it may seem that no traffic is received.
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