The two systems complement each other:-


Cospas-Sarsat satellites can work out the position of a standard 406MHz EPIRB, but may not be able to download it to an earth station until they happen to pass over one. Geosar satellites cannot work out the position of an EPIRB unless the signal includes position information, but can download the distress alert, identity, and position to an earth station immediately.


Doppler positioning


In everyday life, the “Doppler effect” is probably most noticeable when you’re standing by the side of a busy road: it makes the sound of each car change in pitch at the moment it passes you. The same thing happens to the EPIRB transmissions as they are received by a satellite: when the satellite is flying towards the EPIRB, it receives a slightly higher frequency than when it is flying away from it. Only at the instant at which the satellite is closest to the EPIRB does it receive a signal at the exact frequency that was transmitted. By analysing how quickly the frequency changes, the satellite can also work out how far from the EPIRB it passed.


EPIRBs


EPIRBs vary in their design and operation, but can be divided into two groups:-


Category 1 EPIRBs are stowed in brackets from which they are released and activated automatically if immersed to a depth of 2-4 metres.


 


Category 2 EPIRBs have to be manually removed from their brackets. Depending on the make and model, they may be activated by turning the right way up, by dropping into sea water, or by a manual switch.

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