121,5 MHz

The Cospas-Sarsat 121.5 MHz system is composed of:

121.5 MHz distress beacons;
the polar-orbiting satellites of the Cospas-Sarsat LEOSAR system; and
the associated ground receiving stations of the LEOSAR system (LEOLUTs).

The frequency 121.5 MHz is an aeronautical emergency frequency. 121.5 MHz radiobeacons were developed in the mid-seventies for installation on aircraft, as Emergency Locator Transmitters (ELTs). However, they can also be used on board ship as Emergency Position-Indicating Radio Beacons (EPIRBs) or as Personal Locator Beacons (PLBs). These beacons transmit signals that are relayed by Cospas-Sarsat LEOSAR satellites to Cospas-Sarsat LEOLUTs which process the signals to determine the beacon location. The alert which consists of the computed position of the transmitter is then relayed, via a Mission Control Centre (MCC), to the appropriate Search and Rescue Point Of Contact (SPOC) or Rescue Coordination Centre (RCC).

Doppler location (using the relative movement between the spacecraft and the beacon) is the means used to locate these very simple devices. To optimise Doppler performance, satellites in a low-altitude near-polar orbit are used, however, the location accuracy of 121.5 MHz beacons is not as good as the accuracy achieved with 406 MHz beacons because of the relatively poorer frequency stability performance of these older generation beacons. The low satellite altitude results in a low uplink power requirement, a pronounced Doppler shift, and short intervals between successive satellite passes. The near-polar orbit could provide full global coverage, but 121.5 MHz alerts are produced only if the satellite "can see" both the beacon and the LEOLUT simultaneously for a period of time. This constraint of the 121.5 MHz system limits the useful coverage to a geographic area of about 3,000 km radius around each LUT. Furthermore, the Doppler processing technique provides two positions for each beacon: the true position and its mirror image relative to the satellite ground track. In the case of 121.5 MHz beacons, normally, a second satellite pass is required to resolve this ambiguity.

The diagram depicts a LEOSAR satellite orbiting the Earth and its instantaneous field of view is indicated by the red circle. In this example the beacon located in the Northern Atlantic is within the local coverage area of the LEOLUT located at the north west portion of Africa, whereas, the beacons located on the southern tips of South America and Africa are not.
The local coverage mode at 121.5 MHz has proven very effective in providing quick alert and location data for the large number of existing 121.5 MHz distress beacons. However, the transmission characteristics of these beacons do not permit satellite on-board processing of the signal, therefore, global coverage is not possible. Furthermore, the system cannot generally distinguish between a 121.5 MHz distress beacon transmission and any other 121.5 MHz signal. This limitation results in a large number of false alerts in the form of Doppler locations which do not correspond to distress beacons.