Definition
The radio frequency (1090 megahertz) used by aircraft transponders to transmit replies to secondary surveillance radar interrogations and to broadcast ADS-B Out position reports under the 1090ES (Extended Squitter) standard. It is one of two ADS-B Out broadcast frequencies authorized in U.S. airspace, the other being 978 MHz (UAT).
Plain English
1090 MHz is the radio channel that aircraft transponders use to talk to air traffic control radar and, on equipped aircraft, to broadcast their position to other traffic. It is one of the two frequencies the FAA allows for ADS-B Out.
Context Anchor
Seen in transponder, ADS-B Out, traffic display, and aircraft equipment requirement discussions.
Derivation
MHz stands for megahertz, meaning one million cycles per second. The number 1090 is simply the assigned frequency. Naming the system by its frequency (rather than a brand or acronym) is common in aviation radio because the frequency itself is the technical fingerprint of the equipment.
Why Pilots Care
Using this frequency ensures ATC can accurately track the aircraft and that collision avoidance systems receive reliable data from nearby traffic.
Analogy
It is like a radio station frequency, but for aircraft equipment instead of music or voice. The equipment has to transmit and receive on the expected frequency for the system to work.
Grounding Statement
When the transponder is interrogated or when ADS-B is active, the aircraft sends its data packets on this one specific frequency so receivers on the ground or in other aircraft can pick them up.
Intuition Check
Do not read 1090 MHz as a procedure or an altitude. It is a radio frequency used by aircraft electronic equipment.
Example Sentence 1
The aircraft is equipped with a 1090 MHz Extended Squitter transponder, so it meets ADS-B Out requirements for flight above 18,000 feet.
Example Sentence 2
ADS-B Out equipment must transmit on 1090 MHz to satisfy equipage rules in Class A and B airspace.