Portable ADS-B Out systems, also known as "suitcase" units, should not be operated (transmitting) aboard any aircraft. While marketing associated with these units may imply approval for use by way of an FCC license, the FAA prohibits their use for the following reasons:
- The positioning of portable, suction-cup GPS antennas associated with these units often require they be affixed to front or side windows or glare shield to obtain a usable signal. Such antenna placement obstructs the pilot's view. Connecting wiring also interferes with aircraft controls and instruments.
- ADS-B Out avionics require a valid Mode S code to be transmitted to operate properly with ATC automation and other ADS-B aircraft. Mode S codes, also known as the ICAO code, are assigned to an aircraft during registration and then programmed into transponders and ADS-B Out avionics. Mode S codes remain static until a change in aircraft registration or identification (N-number) occurs. Portable units require users to input the Mode S code assigned to each aircraft flown. A high number of Mode S code entry errors have occurred with this procedure, which prevent proper target correlation within ATC automation systems (target drops/traffic conflict alerts), which have resulted in increased workload and unnecessary distractions for pilots and controllers.
- The positioning of ADS-B antenna is also vital in the quality of the signal that is transmitted, and if capable, received by the ADS-B device. There have been a number of aircraft identified using portable ADS-B devices that result in degraded performance due solely to poor antenna location.