5. Near-synchronous system.
a. The Doppler shift is barely discernible because of the slow relative motion between the satellite and the
ground terminals.
b. Complex-ground-based receiving and transmitting antenna positioning systems are required to track the
drifting satellite at the relatively slow rate of about 1.3 degrees per hour.
c. Since the satellite is in sunlight most of the time, power for the satellite circuits is furnished by solar
cells.
d. Because the satellite is exposed to sunlight most of the time, temperature compensation equipment is not
required, thus saving on-board weight.
e. Two or more of the Initial Defense Communication Satellite System (IDCSS) communications satellites
are in view of any ground terminal at all times, assuring relatively uninterrupted communications. The only
time that communications may be broken is during the "handover" process. This time loss can be minimized by
using two ground terminals at each station, the antenna of one terminal pointing toward a setting satellite and
the antenna of the other terminal pointing toward a rising satellite.
f. The near-synchronous orbits greatly complicate the attempt by a potential enemy to interfere with the
satellite signal.
g. The near-synchronous orbits minimize the possibility of physical destruction of the satellite by a
potential enemy.
h. The frequency of transmission remains a relatively fixed value; the amount of frequency change that can
be accommodated by the system is limited by the ability of the satellite circuits to accommodate the change.
i. The number of communication channels in the communications system is limited by the bandwidth
capabilities of the on-board equipment in the satellite.
6. Low- and medium-altitude orbit systems.
a. A complex antenna tracking system is required because the satellite travels at relatively high speed.
b. In all active satellites, special circuits must be built into the ground receiving equipment to compensate
c. When low- and medium-altitude orbit systems are equipped with a store-and-forward repeater, they
provide a higher degree of security than other systems. The enemy stands little chance of interfering with the
signal, which is transmitted only when the satellite passes nearest the ground station.
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