Also known as: ICAO, International Civil Aviation Organization
ICAO (the International Civil Aviation Organization) is the United Nations agency that standardises international civil aviation, including the ADS-B surveillance system and Mode S transponders.1
Overview
ICAO was established by the Chicago Convention of 1944 and is the UN specialised agency for civil aviation, with more than 190 member states and headquarters in Montréal. It sets worldwide Standards and Recommended Practices (SARPs), published as Annexes to the convention, that make international air travel safe and interoperable: everything from runway markings and airspace procedures to the radio systems aircraft use to be seen and to communicate. The technical radio standards live chiefly in Annex 10 (Aeronautical Telecommunications), which defines the secondary surveillance radar transponder modes and the data formats that ride on them.
For an SDR listener the most important of these is the Mode S transponder and the ADS-B (Automatic Dependent Surveillance – Broadcast) system built on top of it. ICAO assigns every aircraft a unique 24-bit address (the ICAO hex code) that identifies it in every Mode S and ADS-B message, and it defines the 1090 MHz Extended Squitter format in which aircraft broadcast their identity, position, altitude, and velocity. Position is encoded using Compact Position Reporting, a clever scheme ICAO specifies to squeeze a global latitude/longitude into few bits. ICAO sets the what — the message formats, the address allocations, the required performance — while the detailed how (the Minimum Operational Performance Standards that avionics must pass) is written by regional bodies: RTCA’s DO-260 series in the United States and its harmonised counterpart, EUROCAE’s ED-102, in Europe. The radio spectrum those systems occupy is allocated internationally through the ITU.
Relevance to SDR
ICAO standards define the 1090 MHz signals an SDR receives when tracking aircraft, and that
makes aircraft tracking one of the most accessible and rewarding SDR activities: because ADS-B
is an open, unencrypted broadcast standard designed for interoperability, any receiver can
decode it. When a tool like dump1090 or a similar decoder shows you an aircraft’s flight,
altitude, and position on a map, it is parsing exactly the Annex 10 Extended Squitter format,
resolving the ICAO 24-bit address, and running the Compact Position Reporting algorithm ICAO
specified. The global uniqueness of the 24-bit address is what lets independent ground receivers
worldwide — and crowdsourced networks — stitch together a coherent picture of air traffic.
Aircraft surveillance sits outside GopherTrunk’s land-mobile trunking focus, so GopherTrunk does not itself decode ADS-B; dedicated tools handle 1090 MHz. But the reference stands as context for the wider RF landscape an SDR user explores, and it explains why aviation signals are so consistent across the world: a single UN body harmonised them. See the other signals you’ll meet lesson for where ADS-B fits among the non-trunking signals you can receive.
Sources
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International Civil Aviation Organization — Wikipedia, for ICAO’s history, structure, and standards role. ↩