Page 464 - Satellite Communications, Fourth Edition

444 Chapter Fourteen

Interburst interference may be a problem with the tuned-circuit
method because of the energy stored in the tuned circuit for any given
burst. Avoidance of interburst interference requires careful design of the
tuned circuit (Miya, 1981) and possibly the use of a postamble, as men-
tioned in the Sec. 14.7.2.
Other methods of carrier recovery are discussed in Gagliardi (1991).

14.7.4 Network synchronization
Network synchronization is required to ensure that all bursts arrive at
the satellite in their correct time slots. As mentioned previously, timing
markers are provided by the reference bursts, which are tied to a highly
stable clock at the reference station and transmitted through the satel-
lite link to the traffic stations. At any given traffic station, detection of
the unique word (or burst code word) in the reference burst signals the
start of receiving frame (SORF), the marker coinciding with the last bit
in the unique word.
It would be desirable to have the highly stable clock located aboard
the satellite because this would eliminate the variations in propagation
delay arising from the uplink for the reference station, but this is not
practical because of weight and space limitations. However, the refer-
ence bursts retransmitted from the satellite can be treated, for timing
purposes, as if they originated from the satellite (Spilker, 1977).
The network operates what is termed a burst time plan,a copy of
which is stored at each earth station. The burst time plan shows each
earth station where the receive bursts intended for it are relative to
the SORF marker. This is illustrated in Fig. 14.16. At earth station
A the SORF marker is received after some propagation delay t , and
A

Figure 14.16 Start of receive frame (SORF) marker in a time burst plan.