Background information

• Airborne Collision Avoidance System (ACAS) II interrogates, within its range, all Mode S and Mode A/C SSR transponders squawking altitude. That includes ground-based transponders operated for testing or maintenance.
• If these transponders respond with an altitude report close to that of aircraft flying in the vicinity, their TCAS II Traffic Display will show a ‘ghost’ target and, more seriously, could generate TAs/RAs against such targets.
• Recently, events have been reported in which RAs were generated by transponders that were being tested on the ground.
• These unnecessary RAs were disruptive to the flight crew and air traffic control.

EUROCONTROL Mode S & ACAS programme advice

• TAs/RAs due to transponder testing on the ground are disruptive and potentially hazardous and must be prevented.
• To avoid these TAs/RAs special caution and appropriate procedures are required during transponder testing and maintenance.