From the USCG Navigation Center
406 MHz EPIRBs
The 406 MHz EPIRB was designed to operate with satellites. The signal frequency (406 MHz) has been designated internationally for use only for distress. Other communications and interference, such as on 121.5 MHz, is not allowed on this frequency. Its signal allows a satellite local user terminal to accurately locate the EPIRB (much more accurately -- 2 to 5 km vice 25 km -- than 121.5/243 MHz devices), and identify the vessel (the signal is encoded with the vessel's identity) anywhere in the world (there is no range limitation). These devices are detectable not only by COSPAS-SARSAT satellites which are polar orbiting, but also by geostationary GOES weather satellites. EPIRBs detected by the GEOSTAR system, consisting of GOES and other geostationary satellites, send rescue authorities an instant alert, but without location information unless the EPIRB is equipped with an integral GPS receiver. EPIRBs detected by COSPAS-SARSAT (e.g. TIROS N) satellites provide rescue authorities location of distress, but location and sometimes alerting may be delayed as much as an hour or two. These EPIRBs also include a 121.5 MHz homing signal, allowing aircraft and rescue craft to quickly find the vessel in distress. These are the only type of EPIRB which must be certified by Coast Guard approved independent laboratories before they can be sold in the United States.
A new type of 406 MHz EPIRB, having an integral GPS navigation receiver, became available in 1998. This EPIRB will send accurate location as well as identification information to rescue authorities immediately upon activation through both geostationary (GEOSAR) and polar orbiting satellites. These types of EPIRB are the best you can buy.
406 MHz emergency locating transmitters (ELTs) for aircraft are currently available. 406 MHz personnel locating beacons (PLBs) are available.
The Coast Guard recommends you purchase a 406 MHz EPIRB, preferably one with an integral GPS navigation receiver. A Cat I EPIRB should be purchased if it can be installed properly.
Triggering a 406MHz EPIRB will get the SAR ball rolling, but it will not provide rescuers with your location unless it has an onboard GPS. It can take several passes in order to get a good fix. Having an EPIRB with a 121.5MHz beacon or a VHF radio will help them localize your position. That could still equate to a long time in the water. Getting an EPIRB with an onboard GPS can get rescuers heading your way in just a few minutes. That might be worth the extra $400 - especially if you're 17.5 miles offshore and you've been catching blues and makos all day
http://www.acrelectronics.com/resqfix/resqfix.htm
http://www.acrelectronics.com/aquafix/aquafix.htm
BTW: The Coast Guard's new communications system, called Rescue 21, has the ability to DF on VHF radios. If you're in range of the high sites you have a good chance of them receiving your Mayday and plotting your position. Another good reason to have a VHF Marine Band radio with you. Just make sure you broadcast your Mayday on Ch. 16.
I'm not sure if Sector San Diego has received Rescue 21 yet, but they had a SPAWAR system that DF's from the high sites. The local high sites are at Pt. Loma, Mt. Soledad and on one of the peaks on San Clemente.
Here's a video about the use of Rescue 21 in a rescue
http://www.gdc4s.com/content/816a4a1...ideo_popup.cfm