For ADS-B Out equipment, which frequency must be used in Class A airspace?

The requirement for ADS-B Out equipment to operate at a frequency of 1090 MHz in Class A airspace is rooted in the specific role that ADS-B (Automatic Dependent Surveillance–Broadcast) serves in air traffic management. The 1090 MHz frequency is the same frequency used for Mode S transponders. This frequency enables the transmission of aircraft position, velocity, and other data to ground stations and other aircraft, thus providing necessary situational awareness and improving overall air traffic safety.

In Class A airspace, which extends from 18,000 feet up to 60,000 feet, there is a requirement for aircraft to have ADS-B Out capabilities operating on 1090 MHz to provide precise surveillance data to air traffic control and to other aircraft. This frequency allows for a broader range of coverage and is compatible with existing radar systems used in air traffic control, thus enhancing communication and tracking capabilities in high altitude environments.

The other frequencies listed do not serve the same function for ADS-B Out in Class A airspace. For instance, 978 MHz is primarily used for ADS-B In and for certain types of general aviation applications, while 123.45 MHz is a common frequency for air-to-air communications and not designated for surveillance. The 406