The ones that rely solely on the 121.5 frequency are being phased out and you probably cannot get them anymore. I have one that will communicate with coastal satellites and emit the locator beacon on 121.5 so if I'm in trouble, the satellite gives the coast guard a general area and the homing beacon brings them to me. However, the satellite capabilities of this unit are limited and satellite transmitting power is weak - it only works in coastal US areas due to the low orbit geo-syncronous satellites that it communicates with. I bought it three months ago and I'm not sure it's available anymore due to this technology being discontinued in the next few years. The 406Mhz EPIRBs are now the norm and the signal communicating with the sattelites is much stronger so that it can reach higher altitude orbiting satellites that cross oceans. If these satellites receive a signal, they record the unit ID and the location and beam it back down to earth as soon as they get over a land monitoring station. I hope I never need to use mine.