The Search and Rescue Satellite Aided Tracking System (SARSAT) makes use of
several 
orbiting satellites that carry equipment capable of detecting signals from
certain types of 
emergency transmitters such as Emergency Position Indicating Buoys (EPIB),
Electronic Locator 
Transmitters (ELTs), And Emergency Position Indicating Radio Beacons (EPIRBs).

	When an emergency transmitter sends a signal, it is received by one of the
satellites which 
in turn relays the signal back to a ground terminal or Local User Terminal
(LUT) on Earth.  Through 
computer analysis of a phenomenon known as the Doppler effect, the location of
the transmitter is 
calculated and then passed on to the appropriate RCC (rescue coordination
center).

	SARSAT has proven successful at detecting signals from these emergency beacons
operating at 
121.5 / 243 MHz and ascertaining the location to within less than a 20
kilometer radius.  However, 
these types of beacons were not designed for use with satellites.  To overcome
problems with 
frequency stability and interference from voice communications, beacons using
406 MHz have been 
developed for use specifically with SARSAT.  This frequency is exclusively
assigned to satellite 
aided search and rescue and can provide a position that is accurate to within a
2 kilometer radius.  
The 406 MHz can also relay data messages such as the identity of the user.

	The time delay associated with a SAR response has little to do with the type
of emergency 
signal emitter used (other than the fact that most search and rescue units can
not DF 406), but 
rather the communications between the rescue coordination center and the search
and rescue units 
tasked to respond, the individual unit response time and the ability of the
unit to DF the signal 
where appropriate.  The ability to efficiently DF a transmitter carried by a
diver obviously is 
affected by on scene weather conditions due to the transmitter being
effectively at the waters 
surface, so it is important to ensure that the batteries are good, and to be
able to produce some 
sort of visual or audible signal as well.

-Sean T. Stevenson

 Seaman, Rescue Specialist
 Canadian Coast Guard
 Coxswain, Unit Prevention Officer, Training Officer
 Coast Guard Auxiliary Unit 5
 Coastal Search and Rescue Instructor
 Rescue Coordination Center, Victoria, BC


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