The EPIRB (Emergency Position Indicating Radio Beacon) is a device normally located on the wing of the bridge in a bracket, sometimes on both wings. It could also be placed on the bulkhead on the wheel house. The device is small enough to carry to the life boat. Otherwise the unit can be left on the bulkhead and the ARM (automatic release mechanism) will release the EPIRB if the ship sinks.
GMDSS utilizes the COSPAS-SARSAT satellite system to intercept and locate signals from EPIRBs, Emergency Locating Transmitters (ELTs), and Personal Locating Beacons (PLBs). COSPAS in Russian language means Cosmicheskaya Systyema Poiska Avariynyich Sudov (Space System for Search of Distressed Vessels).
SARSA T is the acronym for: Search And Rescue Satellite Aided Tracking. Together COSPAS-SARSAT is a joint system established by organizations in the United States, Russia, Canada and France to locate EPIRBS operating on 406 and 121.5 MHZ.
Some SARSAT satellites are able to relay certain beacons that transmit on 243 MHZ (military), but not all COSPAS-SARSA T ground station (LUTs) are equipped with equipment for this frequency.
Let's look at the Space Segment of the system.
COSPAS-SARSA T relies on 6 satellites in low earth orbit (LEO) and geostationary satellites for the relay of distress alerts. COSPAS satellites orbit at an altitude of approx 1000 Km. And they complete an orbit about every 105 minutes. These SARSAT satellites orbit at an altitude of approx 850 Km and complete and orbit about every 100 minutes. (And passes from horizon to horizon in about 15 minutes.) The maximum delay a distressed vessel should experience would be a wait of about 90 minutes for the satellite to be in range. The current COSPAS-SARSAT constellation is comprised of 3 COSPAS and 3 SARSAT satellites. With 6 satellites, the delay time in relaying distress alerts is reduced to an average of 90 minutes.
The COSPAS-SARSAT system operates in two basic coverage modes:
a) Real time Mode; and
b) Store and forward mode.
The121.5 MHZ beacons operate in the real-time mode, the 121.5 MHz signal is transmitting continuously. The signal is sent so that any passing aircraft will receive the signal on the emergency receiver in each aircraft. The satellite can calculate the position of the 121.5 MHz beacon but may require two passes.
There are other satellites that have 121.5 MHz receivers such as the NOM weather satellites. These satellites can receive the signal but have no way of knowing where the signal is located. They will notify the ground station that there is a signal but that is the only information, there is no position information on the signal.
The 406 MHZ portion of the EPIRB is sending a signal with a message indicating the registration number of the EPIRB and the satellite will compute the location by the use of the doppler effect. This signal is sent in bursts with digital information contained in the burst. If the Satellite is not in view of the EPI RB then nothing will happen. As the satellites are in polar orbit and there are only six it will be a little time before the signal is received .by the satellite.
Of the signals from the 406 MHz EPIRB the satellite can fix the position within about 2.5 miles. By comparison the 121.5 MHz beacon the fix is about 9 Miles and it may take two passes to obtain the fix.
As the satellite approached the EPIRB position, the carrier frequency transmitted by the EPIRB will increase. As the Satellite moves away from the EPJRB position, the carrier frequency will decrease. As this frequency is changed by the motion of the Satellite it is calculated and will give approximate location of the EPIRB.
