How Does It Work - mcmurdo Smartfind G8 AIS User Manual

11

HOW DOES IT WORK?

When you activate the EPIRB, its on board transmitters start broadcasting
distress signals at several frequencies:

A 406 MHz alert signal can be received by the Cospas-Sarsat satellite
system (see page 39). Ground stations track these satellites and
process the distress signals to obtain a location using Doppler location
techniques (see
articles/230-doppler-processing

A 121.5 MHz transmission can be detected by overflying aircraft and
Search & Rescue (SAR) homing receivers.

For EPIRB models equipped with AIS (SmartFind G8 AIS), an
additional transmission operates at 162 MHz. The AIS message
includes the unit ID, GPS position and a simple text message and is
repeated 8 times per minute using an 'Impolite AIS Protocol' giving it
priority over other AIS signals in the vicinity. This signal will typically be
received by vessels with VHF AIS antennae out to a four-mile range
(typical).

With EPIRB models equipped with a GNSS (Global Navigation Satellite
System) receiver (SmartFind G8 AIS / SmartFind G8), the current
position is transmitted as part of the distress signal. GNSS systems
require a clear view of the sky in order to work effectively, because the
receiver needs signals from several satellites to calculate its position. If
the GNSS receiver within the EPIRB cannot determine its position, it
"fails safe" - it does not transmit an inaccurate position - but instead
sends information to indicate that no position is available. Position
information is then dependent on the orbiting satellite system.
The transmissions will continue for a minimum of 48 hours and continue until
the battery is exhausted.
https://www.cospas-sarsat.int/en/21-embedded-
).
Page 38