In the following illustration we see two unmanned aerial systems (UAS) on a collision course. Neither aircraft sees the other because both aircraft are outside the field of view of the other aircraft.
At 22 seconds into the video, we reenact the encounter with both aircraft presumed to have ADS-B Lite. With only a one watt transmitter, detection may occur up to 12 miles away. For the sake of brevity detection is shown 12 seconds before arrival at the intersection. For large aircraft the transmit and receive antennas can be spaced widely apart. This is called antenna diversity. Small UAS may have to use a single antenna. This means neither aircraft can receive if both are transmitting at the same time. ADS-B Lite solves this problem by synchronizing the measurements to GPS and then delaying data transmission to a selected broadcast period. Latency is removed by discarding the subseconds after the second from the receive time (i.e. the broadcast delay). You see this in the video as alternating blue and yellow broadcasts. Unlike standards compliant ADS-B which transmits location and velocity data twice per second, ADS-B Lite only transmits once per second and groups all data in a single broadcast period instead of broadcasting data piece-meal over multiple broadcast periods. This gives more time to listen without mutual interference. Known as time division multiple access (TDMA) means up to 1024 emitters can be tracked concurrently. Together with low power ADS-B Lite’s approach to TDMA solves a major problem with standards compliant ADS-B: frequency saturation.
With latency removed a single broadcast is sufficient to accurately predict, the time, place and altitude of an encounter. When the first blue wavefront from the fixed wing aircraft crosses the quad-copter, the quad copter is alerted to the presence of the fixed-wing aircraft. Within a thousandth of a second the quad-copter displays conflict awareness by changing its color to red; by placing a red triangle at the intersection and by issuing the directive to climb along with a counter indicating 12 seconds remain until minimum separation. When the first yellow wavefront from the quad-copter crosses the fixed wing aircraft, the fixed wing aircraft is alerted to the presence of the quad-copter and it too changes color; places a red triangle at the intersection; issues a directive to descend and indicates 11 seconds remain until minimum separation.
Part of the economy of scale for UAS beyond visual line of sight (BVLoS) operations is that a single pilot commands multiple aircraft concurrently relying upon automation to handle simple tasks like maintaining attitude, course, airspeed and altitude. In the simulation neither pilot has responded by eight seconds to go, so an audio alarm is sounded to attract their attention. At four seconds to go no action has been taken so the automatic systems override the pilots by commanding the fixed wing aircraft to descend and commanding the quad-copter to climb. Each aircraft’s maneuver is determined from an encounter model so that the actions are coordinated. The change of altitude displays at the left and right edges of the simulation and in the views from the display. With 75 feet of separation, the quad-copter passes over the fixed wing aircraft and the collision is avoided. After the all clear signals the encounter is over, the alarms are turned off and the aircraft return to their original altitudes.
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