ACR Electronics PLB-350B - Manual (Page 7)

Y1-03-0241A 6
The 406 MHz signal is detected by multiple satellites and from that
information the location of the beacon can be calculated. This data
alone is sufficient for SAR to find persons or ships in distress in a
reasonable timeframe. However, as a further enhancement, some
beacons have a GPS engine onboard. This feature allows the beacon to
acquire current location coordinates from an internal GPS receiver. The
purpose of this feature is to send an even more precise location of the
beacon to the satellites, i.e., latitude and longitude data. This helps SAR
to reach the location even faster.
2. Internal GPS position system
The PLB-350 is fitted with an internal GPS receiver that will determine
the coordinates (latitude and longitude) of the beacon’s position on the
globe, to be transmitted to the Cospas-Sarsat emergency system.
When the beacon is activated, the internal GPS immediately attempts to
acquire positional coordinates. GPS coordinates can be acquired any
time that the GPS is on, but only valid data is saved. Once the beacon
acquires valid coordinates, the data is included as part of the next
transmitted 406 MHz digital message.
The internal GPS operates on a schedule during a beacon activation.
The GPS is on for a time, actively acquiring coordinates, and off for a
time, in a standby state. The schedule is designed to conserve battery
but, at the same time, assure that navigational coordinates are regularly
updated.