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Communications satellites are in orbit around the Earth. The properties of a satellite’s orbit control its visibility from Earth and the details of how we link to it. We need, therefore, to have a basic understanding of the properties and relative advantages and disadvantages of different kinds of orbits. The force of gravity is constantly acting on the satellite. A satellite stays in orbit because the downward force of gravity pulling the satellite toward Earth counterbalances the momentum of the satellite imparted to it by the launch vehicle and which continues even after the rocket has stopped thrusting. Without this gravitational pull, the satellite would fly off into space. Without friction or drag from air resistance, an object will theoretically remain in orbit forever. Recall that the moon has been orbiting the earth for billions of years without anyone pushing it around! In addition to the pull of the Earth (and we will see later that even that is not uniform), the Sun and Moon pull the satellite in various directions. This causes what are called perturbations, which slightly pull the satellite out of its intended orbit and orbital slot. This drifting must be corrected for the satellite to operate properly, and so an important subsystem of most satellites keeps the satellite in place. This need to control the satellite adds to its mass, complexity, and cost, as we will see later in this chapter. About four centuries ago, an astronomer in Prague named Johannes Kepler used accurate years-long records of planet observations to arrive at an empirical understanding of how planets - and everything else - orbit. About 70 years later, Isaac Newton used newly formulated principles of physics and mathematics to show not only that Kepler was right, but to demonstrate the physical basis of satellites’ behavior.
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