Radio beacon

Lore & Background

The most basic radio-navigational aid in aviation is the non-directional beacon (NDB), a simple low- and medium-frequency transmitter used to locate airway intersections and airports and to conduct instrument approaches. Aviation NDBs, especially those marking airway intersections, are gradually being decommissioned and replaced with newer technologies, though they remain in use at smaller aerodromes and helicopter landing sites due to low cost. Marine beacons based on the same technology have also been used by ships; most in the Western world are no longer in service, while some have been converted to telemetry transmitters for differential GPS. Marker beacons, a specialized type used with instrument landing systems (ILS), transmit on 75 MHz to give pilots distance to the runway; this type is slowly being phased out. Amateur radio propagation beacons are used specifically to study radio signal propagation and are part of the amateur radio service. A group of single-letter high-frequency beacons transmitting in Morse code have been regularly reported, with no official information or ITU registration; some investigators suggest they may be naval propagation beacons. Satellites in geostationary and inclined orbits emit beacons on fixed frequencies for station-keeping telemetry and to locate the satellite in the sky. The Apollo 17 crew left a beacon on the Moon transmitting FSK telemetry on 2276.0 MHz. Driftnet radio buoys, used by fishing boats, transmit on 1600–2850 kHz with 4–15 W; some types, called SelCall buoys, answer only when called by their own ship to prevent theft and conserve battery. Distress radio beacons include EPIRBs (maritime), ELTs (aircraft), and PLBs (personal use), and the largest system is the international Cospas-Sarsat satellite network, which locates 406 MHz beacons to within about 2 km, while newer GPS-equipped beacons transmit location to within about 100 meters. In Wi-Fi, a beacon is a data transmission from a wireless access point carrying the SSID, channel number, and security protocols. In AX.25 packet radio networks, beacon transmissions use Unnumbered Information frames to identify stations and broadcast operational status; this mode became the basis of APRS networks.

Reader's Guide

Radio beacons have served as a foundational technology for navigation and emergency response across multiple domains. In aviation, NDBs and marker beacons provided pilots with essential location and approach guidance for decades, though many are being phased out in favor of satellite-based systems. Marine beacons similarly guided coastal navigation before differential GPS took over. The use of beacons for propagation research by amateur radio operators continues to contribute to understanding of radio wave behavior. Distress beacons, particularly the Cospas-Sarsat system, have become a critical global search-and-rescue infrastructure, enabling rapid location of emergencies on land, at sea, and in the air. The evolution from first-generation beacons with 2 km accuracy to GPS-integrated beacons with 100 m accuracy has dramatically improved rescue effectiveness. In modern wireless networking, Wi-Fi beacons enable client devices to discover and connect to access points, while AX.25 packet radio beacons support amateur radio data networks and APRS. The versatility of the radio beacon concept—from lunar telemetry beacons to driftnet buoy markers—demonstrates its enduring utility in both specialized and everyday applications.

Did You Know?

• Some single-letter high-frequency beacons transmitting in Morse code are suspected to be naval propagation beacons, but no official information exists about them.
• The Cospas-Sarsat system uses about 67 Earth-orbiting satellites operated by 45 member nations to locate distress beacons on 406 MHz.