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International Satellite System For Search and Rescue |
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121.5 MHz Beacons
It is estimated that there are almost 500,000 121.5 MHz beacons in use world-wide. The list below gives typical 121.5 MHz beacon signal characteristics. Most of these units are used aboard aircraft and are required to meet national specifications based on ICAO standards. Transmission characteristics of 121.5 MHz beacons are included in ITU Recommendation ITU-R M.690-1. The initial ICAO standards were not established with the aim of satellite reception of 121.5 MHz signals. The 121.5 MHz Cospas-Sarsat system was designed to serve the existing type of beacons, even though system performance is constrained by their characteristics. Parameters such as system capacity (number of simultaneous transmissions in the field of view of the satellite which can be processed by LEOLUTs) and location accuracy are limited. No information is usually provided about the operator's identity, although a morse coding of the signal is included in some models; however, these data are not processed automatically by Cospas-Sarsat LEOLUTs. Despite the limitations described above, the efficiency of 121.5 MHz beacons has been greatly enhanced by the use of satellite detection and Doppler location techniques. 121.5 MHz beacons carried aboard aircraft can usually be activated both manually and automatically by shock (using a crash sensor or G switch). This latter feature has led to numerous false alerts when a beacon is mounted in an aircraft with insufficient care or when an aircraft makes a "hard landing". By providing the location of transmitting beacons, Cospas Sarsat can be instrumental in the quick processing of false alerts.
Typical 121.5 MHz Beacon CharacteristicsRF Signal Transmitted
Modulation
* Peak Effective Radiated Power relative to a 1/4 wavelength monopole mounted on a ground plane
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Email: mail@cospas-sarsat.int |