I have a fair amount of experience with the various types of locators. There are some rumors floating round that do not exactly hold water. Some may have been started by people selling various devices. Here is the basic overview as I currently understand it –
PLBs – these are basically small EPIRBs. I am going to leave out the shortcomings of the older versions that needed two low flying satellites to pass over you before getting a location fix. I am only going to talk about the modern GPS type. When these things activate and a satellite is in range, a signal is sent that indicates the serial number of the unit & the GPS location of the activation. This activation message goes back to a central monitoring station. This works from ANYWHERE in the world that a satellite can be seen. The monitoring station then looks up the contact info from the serial number. They then call the in-case-of-emergency number that you listed on your profile & if someone answers, they ask if you are really out in the area where the distress signal was sent from. If that person says yes, then they notify the local authorities. In some countries like the US, the Coast Guard then mounts a rescue attempt. If you were off the coast of some place like Somalia, good luck getting a useful response from their navy. If your contact person does not answer the call from the rescue agency, then they try a secondary person, if you have one listed. If they still don’t get an answer, then they try back in a few hours. If nobody answers at all ever, the rescue info can be delayed by a day or more in being sent out. Most of these gizmos need to go back to the factory for a battery change every 5 years, or after an activation, which ever comes first.
AIS locators – these are little short range transmitters that talk directly to AIS receivers in nearby vessels that have them. Range is usually a couple of miles, but varies with conditions. This little thing indicates your GPS position directly to those around you, but does not talk to the satellites directly. AIS comes in 3 flavors. One is receive only. This is available on a lot of modern VHF radios. The next is send & receive. This talks to nearby ships directly. The third & most expensive version, reports to satellites. These are usually only on large ships. If a short range local transmitter reaches a ship with a satellite link, then the position of the short range transmitter may be reported to the satellite as well. Positions that get reported to the satellites will normally show up on websites like vessel finder & marine traffic. AIS does not report to the same rescue agency that the PLBs report to.
DSC – this is an accessory function that was added to most modern VHF radios. It can be used for a few different things, but for the sake of this discussion, I’ll only mention the little red button. If your unit is set up correctly & you hit the little red button, then a distress signal with GPS location is sent directly to all DSC equipped units within range. A boat mounted VHF will usually send a signal 20-30 miles but sometimes can go 50 or more miles under good conditions, with a good antenna. Hand held units usually go a couple of miles if you are in the water & maybe 5-10 miles if you are standing on deck.DSC does not report to the PLB monitoring stations, but the USCG does monitor DSC. Most recreational boaters who receive a DSC distress call don't understand what it is & just hit the dismiss button to get their radio to work normally again.
MMSI – this is an identification number that you need to program into your AIS, DSC, etc. Without this number, the unit will not work. In the US, you can either get the number from the FCC or Boat US. The FCC number is valid in other countries. The Boat US number is not. The boat US number can have it’s associated vessel info modified if you get a new boat or want to change the designation to a personal unit. The FCC number can’t be edited at all. The Boat US number can be gotten quickly. The FCC number takes a little longer.
Spot Tracker or In Reach - these are inexpensive subscription transmitters that ping your position every 10 minutes or so, depending on how you have the settings configured. The breadcrumb trail is posted to a website that your buddies can watch. Some of these also have basic text message capabilities. These talk directly to satellites. You pay a monthly fee for these, but they do not use a MMSI number. These only report your information to people you give access to. Hopefully they will be watching when you need them to.
I’ve left out a lot of details. These are just the basics of some different options. The only one I currently own is DSC. If I plan to go outside of VHF range of land, I'll consider getting something else.
Edit:
Bottom line is this - if you want the mother ship to find you, then you want AIS or DSC. If you want a worldwide rescue agency to be notified so that they can send the local authorities after you, then you want a PLB. If you want your friends to be able to track your position, then you want a Spot Tracker, In Reach, or similar item.
