Figure 1-11. Communications and beacon section of a synchronous
communications satellite, block diagram.
LEARNING EVENT 19: NEAR-SYNCHRONOUS COMMUNICATIONS SATELLITE
1. Near-synchronous satellite. The block diagram of a near-synchronous satellite is similar to that of a
synchronous satellite. The primary difference between the two diagrams is the absence of a command channel
in the near-synchronous type. Inclusion of a command channel raises the possibility that interference might
change the orbit of the satellite. Deleting the command capability removes this possibility, thus improving the
security against tampering with orbital position. A beacon feature is included so that ground terminals may use
automatic tracking. A telemetry subsystem is also included to inform ground stations of equipment status as
well as environmental conditions.
2. Random access. Random orbits facilitate multiple access; several ground stations may use a satellite as a
common facility. Simultaneous communications by several ground terminals through the same satellite may
cause interchannel interference. One possible solution to the problem is to include an on-board signal source
that sounds a busy signal when the satellite is queried. The busy signal could be included as part of the
telemetry signal sent out by the satellite. A second possible solution to the problem might be a coded signal that
opens an electronic gate to the channel. Use of the satellite would thereby be limited to the ground terminals in
3. Transmission bandwidth. Several information (communications) channels must be available as choices for
the uplinks and downlinks. Frequency conversion is performed to generate a minimum of four information
channels for each two-way communications systems between ground terminals. The transmission channel
bandwidth must be large enough to simultaneously pass all the information channel frequencies. In this way,
link frequencies may be added or removed as traffic loads change; also it permits shifting the frequencies of the
information channels for flexibility and for security operations. Moreover, the bandwidth must be broad
enough to accommodate the signals used in broadbanding techniques.