TCAS advisories: collision-avoidance alerts in broadcast data

TCAS, the traffic alert and collision avoidance system, watches the transponders of nearby aircraft and warns the crew when one gets close. When the geometry gets tight it issues a resolution advisory, a spoken instruction such as climb, descend or level off, which the crew is expected to follow promptly. Advisories are issued automatically and happen in routine traffic as well as in close encounters.

Transponders that support it report an active advisory in their broadcasts. The site decodes those reports, needs at least two active messages within 30 seconds while the aircraft is airborne, and matches the aircraft the advisory names with its own track. It then measures how the vertical rate changed after the advisory and how close the two aircraft came. Advisories decoded from Mode S replies instead of ADS-B broadcasts are treated as experimental and held back.

An advisory does not establish a near miss or a loss of separation. TCAS works from its own short-term predictions, and many advisories happen at distances that air traffic control treats as normal. The data cannot show what the crews heard or said, and the spacing figures come from broadcast positions, which have errors of their own.