a. Altitude of orbit. The orbital altitude of a communications satellite greatly affects the extent of Doppler
shift. Low-altitude orbits have the greatest Doppler shift because the relative motion between the ground
terminals and the satellite is great. Medium-altitude orbits have less Doppler shift because of the reduced
relative motion between the ground terminal and the satellite. Synchronous-altitude (or near-synchronous)
orbits have the least Doppler shift because the relative motion between the ground terminals and the satellite is
minimal.
b. Compensation.
Compensation for frequency variation caused by Doppler shift is normally
accomplished in the radio receiver within the ground terminal. This placement minimizes circuit complexity
within the satellite. The basic component of the compensation circuit is a discriminator. The discriminator
produces an output voltage that is proportional to the frequency difference between the incoming signal and the
normal signal. This output voltage is applied to a voltage-controlled oscillator to make it track the incoming RF
signal, and thus produce the correct IF used in the receiver.
LEARNING EVENT 15: ACTIVE REPEATER SYNCHRONOUS SATELLITE COMMUNICATIONS
A synchronous satellite does not change its position to any marked degree with respect to the Earth. It travels in
a circular orbit but, because of its great distance from the Earth (22,300 miles), its orbit speed keeps it
suspended above a selected spot on the equator. By placing three synchronous satellites equidistantly around
the Earth, it is theoretically possible to cover most of the Earth's surface with the radiation pattern from these
satellites. Any radio station in the radiation field pattern (Earth area) covered by one satellite can relay
messages to any other radio station in the field pattern of that satellite on a real-time basis. A problem exists in
coordinating the stations desiring to use the services of the satellite because of the limited number of
communications channels available. The problem is solved by the DCS taking control of the satellite
communications channels after the satellite has been placed in orbit. High-priority messages concerning the
safety and security of the country have first call on the available channel.
LEARNING EVENT 16: SYSTEM CHARACTERISTICS
Each type of satellite communications system was designed to fulfill a specific need or to satisfy a particular
experiment. The characteristics of each design type are summarized in the following paragraphs:
1. Passive satellite system.
a. It is universally adaptable; that is, anyone having the necessary facilities can make use of it as long as
the satellite is in view. There is no limit to the number of users, so long as each user uses a different frequency.
SS0031
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