From the Mono Sheriff's Website:
http://www.monosheriff.org/emergencyservices.html
Personal Locator Beacon Information
PLB's are part of the NOAA (National Oceanic and Atmospheric Administration) SARSAT (Search And Rescue Satellite-Aided Tracking) system. When an individual (or group) is in distress, activating a PLB results in a 406MHz signal being transmitted. The distress signal is received by the SARSAT system which uses NOAA satellites in low-earth and geostationary orbits to detect and locate the source of the signal.
Initially, the geostationary satellites detect the signal and relay it to a network of ground stations and then ultimately to the U.S. Mission Control Center (USMCC) in Suitland, Maryland. The actual location of the transmitting PLB is then determined using doppler technology from the low-earth satellites which can take up to 45 minutes depending on the location of the PLB transmitter compared with the closest low-earth satellite.
Encoded in the transmitted signal is a serial number which is used to determine the registered owner of the PLB device. Some models of PLB’s include and/or allow a GPS (Global Positioning System) module. The GPS module is able to determine the PLB location by acquiring information from GPS satellites. Once the position is acquired using GPS satellites, the position coordinates are transmitted as part of the PLB signal thereby providing position information within minutes instead of the potential delay of 45 minutes. With the GPS information included in the transmission, the beacons location can be determined within feet instead of miles.
The USMCC provides the location information to the local Search and Rescue authorities. In addition to the 406MHz transmitted signal used by the SARSAT satellites, the PLB devices also transmit a 121.5MHz homing signal that is used by Search and Rescue teams to locate the person in distress once they get close to the location provided by the SARSAT system.
Carrying a PLB while on backcountry trips can provide an extra sense of security; however there are a few things to think about prior to relying on a PLB for rescue in the event of an emergency.
The actual process of rescuing someone in a remote location takes considerable time and effort to complete, regardless of activation of a PLB. The PLB merely notifies others of where you are and of your emergency. Factors such as remoteness, weather, altitude and available rescuers all affect the time it takes to complete a search and rescue mission. Below is an example of the flow of a PLB rescue mission, the time involved can vary and usually takes a number of hours or even days to complete.
First, the PLB must be maintained in proper condition. The user should know how to activate it, keep the batteries charged, keep the PLB with them and should use common sense as to when to use it. No one should depend solely on a mechanical device as it in not a substitute for good judgment.
To broadcast a distress call, the PLB should be in a good position-visible to satellites. Various PLB devices have different electronic configurations and performance can vary. Does your PLB work in water? Does the antenna need a ground plane? Has it been exposed to sub-zero temperatures?
Upon activation, a COSPAS/SARSAT system satellite should pickup the signal and transmits it to the US Mission Control Center (USMCC). The control center will then contact the Office of Emergency Services (OES) warning center of the state where the PLB is activated. For California, the OES warning center will determine what county and/or jurisdiction to contact to initiate a search effort.
Upon contact, Mono County Sheriff’s Department will begin a search effort and a rescue mission. Depending on the factors noted above, a helicopter and/or ground searchers may be utilized to locate the PLB user. The location will be continually tracked and refined to assist aircraft and searchers.
Once the PLB user is found, the situation will be assessed and the rescue mission completed including transport to medical treatment if required. The PLB is not an insurance policy and the technology takes time to notify potential rescuers of an emergency.
DO NOT rely solely on a PLB to save your life..!!