Page 39 - Satellite Communications, Fourth Edition

Overview of Satellite Systems 19

1.7 Cospas-Sarsat*
COSPAS is an acronym from the Russian Cosmicheskaya Sistyema Poiska
Avariynich Sudov, meaning space system for the search of vessels in dis-
tress and SARSAT stands for Search and Rescue Satellite-Aided Tracking
(see http://www.equipped.com/cospas-sarsat_overview.htm). The initial
Memorandum of Understanding that led to the development of the
system was signed in 1979 by agencies from Canada, France, the USA,
and the former USSR. There are (as of November 2004) 37 countries and
organizations associated with the program. Canada, France, Russia
and the USA provide and operate the satellites and ground-segment
equipment, and other countries provide ground-segment support. A full
list of participating countries will be found in Cospas-Sarsat (2004).
The system has now been developed to the stage where both low earth
orbiting (LEO) satellites and geostationary earth orbiting (GEO) satel-
lites are used, as shown in Fig. 1.8.
The basic system requires users to carry distress radio beacons, which
transmit a carrier signal when activated. A number of different beacons
are available: emergency locator transmitter (ELT) for aviation use; emer-
gency position indicating radio beacon (EPIRB) for maritime use; and per-
sonal locator beacon (PLB) for personal use. The beacons can be activated
manually or automatically (e.g., by a crash sensor). The transmitted
signal is picked up by a LEO satellite, and because this satellite is moving
relative to the radio beacon, a Doppler shift in frequency is observed. In
effect, if the line of sight distance between transmitter and satellite is
shortened as a result of the relative motion, the wavelength of the emit-
ted signal is also shortened. This in turn means the received frequency
is increased. If the line of sight distance is lengthened as a result of the
∗
http://www.cospas-sarsat.org/

LEOSAR satellites

GEOSAR satellites
Figure 1.8 Geostationary orbit search and rescue (GEOSAR) and low earth orbit search
and rescue (LEOSAR) satellites. (Courtesy of Cospas-Sarsat Secretariat.)