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.
In order to prevent the transmission of a virtual altitude which could then be mistakenly used by airborne systems, the followings steps are recommended:
Use effective screening or absorption devices on the antennas or physically connect the ramp test set to the antenna system.
Where possible, perform the testing inside a hangar to take advantage of any shielding properties it may provide.
Manually set the altitude to a high value (e.g. over 60,000 feet) or unrealistically low (e.g. negative 2000).
Select the transponder(s) to ‘OFF’ or ‘Standby’ when testing is complete.
The simulation of TCAS operations must not be carried out by the radiation from an antenna located on, or remotely based from a workshop.