Proper test procedures to prevent uninhibited transmissions

Effectivity: Challenger and Global aircraft 

ATA 34-54 

By: Camelia Constantin – Technical Services

Unintended transmission from Transponders/ADS-B OUT systems were detected when operating under test conditions. The testing of transponder and ADS-B OUT equipment with the associated broadcast of altitude and position information can present some safety concerns in the local airspace as well as in air traffic control functions. 

 As stated in the FAA Safety Alert for Operators (SAFO) 17002, in cases where transmission lines are not attached directly to test equipment, antenna shielding must be used to prevent propagation of test signals that might interfere with Air Traffic Control (ATC) operations or Traffic Alert and Collision Avoidance System (TCAS) equipped aircraft operating in the area.  

By using an antenna coupler over transponder/TCAS antenna and using the antenna shield for dual antenna installations, you can avoid false intruder information and fully test Automatic Dependant Surveillance Broadcast (ADS-B) system components.  

Since Radio Frequency (RF) interference may be picked up by nearby aircraft or an ADS-B ground station and rebroadcasted as a false intruder, it is the responsibility of the user to ensure minimal leakage due to the variations in aircraft configurations/test setup, service facility location and ADS-B ground station location.  

 When performing Aircraft Maintenance Manual/Publication (AMM/AMP) tasks that require ADS-B Out function, test aircraft could be monitored using an ADS-B flight tracking website (e.g. https://globe.adsbexchange.com) to make sure it is not rebroadcasted as a false intruder by an ADS-B ground station. 

Service facilities that are located near ADS-B ground stations can also pull GPS breakers on ADS-B out equipped aircraft. This will prevent ADS-B ground stations from rebroadcasting signals when performing some tasks like the Functional Test of the Air Data Computer (ADC) that require the transponder to be transmitting but does not require the ADS-B Out function itself. 

 VIAVI, the IFR-6000/TC-201A test equipment manufacturer, issued the application note “Reducing RF Leakage Using VIAVI Solutions Antenna Couplers” which explains the best practices for attenuating the RF signal from the transponder antennas to avoid interference with ATC and nearby aircraft.  

FAA recommends maintenance personnel adhere to proper test procedures to prevent uninhibited transmissions. It is recommended that the technicians familiarize themselves with both the FAA SAFO and the VIAVI Solutions document. 

In addition, to learn more about how to prevent propagation of test signals with potential to interfere with ATC operations or TCAS-equipped aircraft operating in the area, please refer to the Bombardier Reference Instruction Letter (RIL) BA-0004, Rev. 2 “IFR-6000 transponder & TCAS testing with GSE 34-54-04 & 34-54-05 Antenna Couplers” which can be found on Bombardier’s Customer Portal.