2. Orbits. Some satellite systems are designed for medium altitude (about 5,000 miles). The satellite tracking
problem is very similar to that of passive satellites. The tracking antenna at each station must track each
satellite as it passes overhead in its orbit. A second antenna system must pick up each of the various satellites
as it comes over the horizon and track it to maintain continuity of communications. It is evident that to
maintain worldwide satellite communications, a number of identical satellites in different orbits will be
necessary, as shown in Figure 1-8.
Figure 1-8. Active-repeater satellite communications systems, medium altitude.
3. Doppler shift. When radio signals are sent to or received from an orbiting communications satellite, a
change in frequency occurs due to a phenomenon known as Doppler shift. Doppler shift is defined as the
change in frequency with which energy reaches a receiver when the source of radiation and the receiver are in
motion relative to each other. As the satellite approaches the transmitting station, the signal frequency being
received in the satellite will increase, and as the satellite moves away from the transmitting station, the signal
frequency will decrease. Likewise, as the satellite approaches the receiving station, the frequency of the
received signal will increase, and as it moves away from this station, the frequency will decrease.
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