Page 41 - Satellite Communications, Fourth Edition

Overview of Satellite Systems 21

Figure 1.9 Showing the Doppler shift in received frequency on successive passes of the
satellite. ELT—emergency locator transmitter.

downlink frequency of 1544.5 MHz to one of several local user terminals
(LUTs) established at various locations throughout the world.
In the original Cospas-Sarsat system, the signal from the emergency
radio beacons was at a frequency of 121.5 MHz. It was found that over
98 percent of the alerts at this frequency were false, often being caused
by interfering signals from other services and by inappropriate handling
of the equipment. The 121.5-MHz system relies entirely on the Doppler
shift, and the carrier does not carry any identification information. The
power is low, typically a few tenths of a watt, which limits locational accu-
racy to about 10 to 20 km. There are no signal storage facilities aboard the
satellites for the 121.5-MHz signals, which therefore requires that the
distress site (the distress beacon) and the LUT must be visible simulta-
neously from the satellite. Because of these limitations, the 121.5-MHz
beacons are being phased out, and the 121.5-MHz service will terminate
on February 1, 2009. Cospas-13, planned for launch in 2006, and Sarsat-
14, planned for launch from 2009, will not carry 121.5-MHz beacons.
However, all Cospas-Sarsat satellites launched prior to these will carry
the 121.5-MHz processors. (Recall that Sarsat-7 is NOAA-15, Sarsat-8
is NOAA-L, Sarsat-9 is NOAA-M, and Sarsat-10 is NOAA-N.)