The GPIRB, has a built in GPS, that broadcasts its own location. This shortens the time required to get an accurate fix on the beacon location and saving time at the beginning of an SAR (Search And Rescue) operation.
GPIRBs have a float-free bracket that releases it if it is submersed as in a sinking. There is a manual mode to turn the unit on manually and a test mode which should be used on a frequent basis to test the operation. It has a minimum 48 hours operating life, 8-channel internal GPS and comes with a lithium battery.
The position of a 406 MHz EPIRB is determined by calculations using the Doppler shift in the beacon's distress signal which occurs as satellites approach and recede in overhead orbits. The accuracy of the calculations is determined by the number of signal bursts received by the satellites. Accuracy is enhanced when a satellite passes directly overhead, because the satellite receives the greatest number of signal bursts. The only real problem with the system is that it takes time for an accurate fix to be acquired.
The GPIRB determines its own position. When activated, its internal GPS finds its own position, just like an onboard GPS. Having located itself, it broadcasts its identity and position on 406MHz. It will then shut down for 20 minutes to conserve power, and repeat the process of locating itself and rebroadcasting. It will continue to update its position every 20 minutes as long as it is active. The advantage of a GPIRB is that an accurate fix is almost instant. Its frequent update allows rescuers to compute drift accurately, and direct SAR teams directly to you, which is difficult to do with the time delays of an EPIRB.
