1. Radio Beacons

A radio beacon is a transmitter at a known location, which transmits a continuous or periodic radio signal with limited information content (for example its identification or location), on a specified radio frequency. Occasionally the beacon function is combined with some other transmission, like telemetry data or meteorological information.

Radio beacons have many applications, including air and sea navigation, propagation research, robotic mapping, radio-frequency identification (RFID) / Near Field Communication (NFC) and indoor guidance, as with real-time locating systems (RTLS) like Syledis or simultaneous localization and mapping (SLAM).

1.1. Radio Navigation Beacons

A most basic aviation radio navigational aid is the NDB or Non-directional Beacon. These are simple low frequency and medium frequency transmitters and they are used to locate airways intersections, airports and to conduct instrument approaches, with the use of a radio direction finder located on the aircraft. The aviation NDBs, especially the ones marking airways intersections, are gradually decommissioned, as they are replaced with other navigational aids based on newer technologies. Due to relatively low purchase, maintenance and calibration cost, they are still used to mark locations of smaller aerodromes and important helicopter landing sites.

There were^[1] also marine beacons, based on the same technology and installed at coastal areas, for use by ships at sea.^[2] Most of them, especially in the western world, are no longer in service, while some have been converted to telemetry transmitters for differential GPS.^[3]

In addition to dedicated radio beacons, any AM, VHF, or UHF radio station at a known location can also be used as a beacon with direction finding equipment.

1.2. ILS Marker Beacons

A marker beacon is a specialized beacon used in aviation in conjunction with an instrument landing system (ILS), to give pilots a means to determine distance to the runway. Marker beacons transmit on the dedicated frequency of 75 MHz. This type of beacon is slowly being phased-out and most new ILS installations have no marker beacons.

1.3. Amateur Radio Propagation Beacons

An amateur radio propagation beacon is specifically used to study the propagation of radio signals. Nearly all of them are part of the amateur radio service.

1.4. Single Letter HF Beacons

A group of radio beacons with single-letter identifiers ("C", "D", "M", "S", "P", etc.) transmitting in morse code have been regularly reported on various HF frequencies. There is no official information available about these transmitters and they are not registered with the ITU. Some investigators suggest that some of these beacons (the so-called "cluster beacons") are actually radio propagation beacons for naval use.

1.5. Space and Satellite Radio Beacons

Beacons are also used in both geostationary and inclined orbit satellites. Any satellite will emit one or more beacons (normally on a fixed frequency) whose purpose is twofold; as well as containing modulated station keeping information (telemetry), the beacon is also used to locate the satellite (determine its azimuth and elevation) in the sky.

A beacon was left on the moon by the last Apollo mission, transmitting FSK telemetry on 2276.0 MHz^[4]

1.6. Driftnet Buoy Radio Beacons

Driftnet radio buoys are extensively used by fishing boats operating in open seas and oceans.^[5] They are useful for collecting long fishing lines or fishing nets, with the assistance of a radio direction finder. According to product information released by manufacturer Kato Electronics Co, Ltd., these buoys transmit on 1600–2850 kHz with a power of 4-15 W.

Some types of driftnet buoys, called "SelCall buoys", answer only when they are called by their own ships. Using this technique the buoy prevents nets and fishing gears from being carried away by other ships, while the battery power consumption remains low.^[6]

1.7. Distress Radiobeacons

Distress radiobeacons, also collectively known as distress beacons, emergency beacons, or simply, beacons, are those tracking transmitters that operate as part of the international Cospas-Sarsat Search and Rescue satellite system. When activated, these beacons send out a distress signal that, when detected by non-geostationary satellites, can be located by triangulation. In the case of 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly (via GEOSAR), and furthermore, a GPS position can be encoded into the signal (thus providing both instantaneous identification & position.) Distress signals from the beacons are homed by Search and Rescue (SAR) aircraft and ground search parties who can in turn come to the aid of the concerned boat, aircraft, and/or persons.

There are three kinds of distress radiobeacons:

• EPIRBs (Emergency Position Indicating Radio Beacons) signal maritime distress,
• ELTs (Emergency Locator Transmitters) signal aircraft distress
• PLBs (Personal Locator Beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency response capabilities (i.e. 911)

The basic purpose of distress radiobeacons is to get people rescued within the so-called "golden day" [1] (the first 24 hours following a traumatic event) when the majority of survivors can still be saved.

1.8. Wi-Fi Beacons

In the field of Wi-Fi (wireless local area networks using the IEEE 802.11b and 802.11g specification), the term beacon signifies a specific data transmission from the wireless access point (AP), which carries the SSID, the channel number and security protocols such as Wired Equivalent Privacy (WEP) or Wi-Fi Protected Access (WPA). This transmission does not contain the link layer address of another Wi-Fi device, therefore it can be received by any LAN client.^[7]

1.9. AX.25 Packet Radio Beacons

Stations participating in packet radio networks based on the AX.25 link layer protocol also use beacon transmissions to identify themselves and broadcast brief information about operational status. The beacon transmissions use special UI or Unnumbered Information frames, which are not part of a connection and can be displayed by any station.^[8][9] Beacons in traditional AX.25 amateur packet radio networks contain free format information text, readable by human operators.

This mode of AX.25 operation, using a formal machine-readable beacon text specification developed by Bob Bruninga, WB4APR, became the basis of the APRS networks.

2. Infrared Beacon

An infrared beacon (IR beacon) transmits a modulated light beam in the infrared spectrum, which can be identified easily and positively. A line of sight clear of obstacles between the transmitter and the receiver is essential. IR beacons have a number of applications in robotics and in Combat Identification (CID).

Infrared beacons are the key infrastructure for the Universal Traffic Management System (UTMS) in Japan. They perform two-way communication with travelling vehicles based on highly directional infrared communication technology and have a vehicle detecting capability to provide more accurate traffic information.^[10]

A contemporary military use of an Infrared beacon is reported in Operation Acid Gambit.

3. Sonar Beacon

A sonar beacon is an underwater device which transmits sonic or ultrasonic signals for the purpose of providing bearing information. The most common type is that of a rugged watertight sonar transmitter attached to a submarine and capable of operating independently of the electrical system of the boat. It can be used in cases of emergencies to guide salvage vessels to the location of a disabled submarine.^[11]