Definition
A radio frequency channel operating at 978 megahertz, used by the Universal Access Transceiver (UAT) system to transmit and receive ADS-B traffic data, Flight Information Service-Broadcast (FIS-B) weather and aeronautical information, and Traffic Information Service-Broadcast (TIS-B) data between aircraft and ground stations. In the United States, the 978 MHz UAT link is approved for use only by aircraft operating below 18,000 feet MSL and is the channel that carries the free FIS-B weather products to suitably equipped aircraft.
Plain English
A specific radio channel that aircraft below 18,000 feet use to send their position and to receive free weather and traffic information from FAA ground stations.
Context Anchor
Seen when studying Flight Information Service-Broadcast equipment, cockpit weather displays, and the requirements for receiving broadcast weather information in flight.
Derivation
MHz stands for megahertz, a measure of radio frequency (millions of cycles per second). UAT stands for Universal Access Transceiver -- 'universal' because it carries multiple kinds of information (position, weather, traffic) on a single link. 'Data link' simply means a radio channel that carries digital data rather than voice.
Why Pilots Care
It delivers real-time weather and traffic information to the cockpit without voice radio or ground radar.
Analogy
It is like tuning a radio to the correct station. If the receiver is not built for that station, or the signal cannot reach you, you will not get the broadcast.
Intuition Check
Do not think of the 978 MHz UAT data link as the weather information itself. It is the radio path that carries the information to the aircraft.
Example Sentence 1
Because the aircraft was equipped with a 978 MHz UAT data link, the pilot received FIS-B weather updates throughout the cross-country flight.
Example Sentence 2
Aircraft in the traffic pattern shared position reports on the 978 MHz UAT data link.