Military Emergency Beacons

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Overview Military Emergency Beacons
By Dr Thomas Withington Last updated: September 9th, 2022

Emergency beacons are used by armed forces worldwide to locate military personnel and vehicles in distress. Ruggedized locator beacons are typically MIL-STD-810G certified, can be automatically activated on impact or immersion and are available in fixed and portable variants.

Rugged Emergency Beacons for Military Use

Emergency beacons use radio transmissions to send a signal notifying that the person or vehicle equipped with the beacon is in distress. The beacons transmit two signals, one of which is transmitted to a satellite constellation. The other is broadcast to provide emergency services a means of locating the beacon, preferably within the first 24 hours. This one-day period is when the vast majority of those in distress are rescued. Some beacons are activated automatically after a catastrophic event by impact or water immersion, for example. This is useful if individuals in a crashed aircraft or sinking vessel are incapacitated and incapable of activating the beacon.

Satellite Emergency Beacons

When the beacon is activated, it transmits a radio signal on a frequency of 406 megahertz/MHz. If the beacon is registered, this 406MHz transmission will include information. Examples include emergency contact details for the person or organization using the beacon, a description of the person or vehicle and where they are located. An unregistered beacon’s transmission will carry less data, but typically includes information on the beacon’s manufacturer and serial number. It may also include the identifying number of the vehicle and/or an international maritime telephone number for the person using the beacon.

The 406MHz frequency is reserved by the International Telecommunications Union (ITU) for use by these beacons. This ensures the frequency remains clear and devoid of interference. The radio transmission is detected in space by satellites in the Cospas-SARSAT constellation in a line-of-sight range with the beacon. The constellation comprises over 40 satellites capable of determining the beacon’s location within two kilometers (one nautical mile). The satellites then broadcast the beacon’s area back to Earth. First responders closest to the beacon then initiate a rescue based on the area determined by the satellites. Once in the area rescuers can detect the 121.5MHz signal broadcast allowing them to pinpoint its source.

Once activated the beacons transmit a half-second stream of data to the satellites every 50 seconds. The precise time the beacon transmits varies across a two-and-a-half second window. This prevents multiple beacons transmitting at the same time and causing interference on the 406MHz frequency. Likewise, the 121.50MHz is a guard frequency kept free of non-emergency traffic. Organizations like the US Federal Aviation Administration may require pilots to monitor the 121.50MHz frequency whenever possible. This can help rescuers locate the source of the beacon’s transmission.

The Cospas-SARSAT System

The international Cospas-SARSAT constellation was established in 1988. A quartet of countries, Canada, France, the Soviet Union and the United States established the International Cospas-SARSAT Programme as a non-profit organization. Satellites supporting Cospas-SARSAT include the American, Canadian and French SARSAT, Russia’s Cospas, the US GOES, the European MSG, India’s INSAT and Russia’s ELEKTRO/LUCH constellations. When satellites detect a transmission from a radio beacon, the information is sent to over 200 nations. Governments receiving the information and users of the beacons do so for free. The entire process from the transmission being made to the satellite, its detection by the satellite and the forwarding of information regarding the transmission, takes less than one minute.

Some beacons now routinely carry a built-in Global Navigation Satellite System (GNSS) receiver. This improves the precision of the beacon’s location by the satellites as GNSS information can now be transmitted alongside the other information listed above on the 406MHz signal. This helps provide more detailed information on the location of the transmitting beacon.