Page 420 - Satellite Communications, Fourth Edition

400 Chapter Thirteen

Figure 13.1 Possible interference modes between satellite circuits and a terres-
trial station. (Courtesy of CCIR Radio Regulations.)

A , A , C , and C are possible modes of interference between space
2
1
2
1
and terrestrial services. B and B are possible modes of interference
1
2
between stations of different space systems using separate uplink and
downlink frequency bands, and E and F are extensions to B and B 2
1
where bidirectional frequency bands are used.
The Radio Regulations (ITU, 1986) specify maximum limits on radi-
ated powers (more strictly, on the distribution of energy spectral density)
in an attempt to reduce the potential interference to acceptable levels in
most situations. However, interference may still occur in certain cases,
and what is termed coordination between the telecommunications admin-
istrations that are affected is then required. Coordination may require
both administrations to change or adjust some of the technical parame-
ters of their systems.
For geostationary satellites, interference modes B and B set a
1
2
lower limit to the orbital spacing between satellites. To increase the
capacity of the geostationary orbit, the Federal Communications
Commission (FCC) in the United States has in recent years authorized
a reduction in orbital spacing from 4° to 2° in the 6/4-GHz band. Some