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Why must satellite radio signals have frequencies above the ionosphere’s highest plasma frequency?

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Satellite radio signals must have frequencies higher than the ionosphere’s highest plasma frequency to ensure they can propagate through the ionosphere and reach their destination in space.1

When radio waves encounter a plasma, their ability to pass through depends on their frequency relative to the plasma's characteristic oscillation frequency. If a signal's frequency is near or below this plasma frequency, the ionosphere acts as a mirror, causing the waves to be reflected or absorbed rather than allowing them to escape into space. By broadcasting at frequencies higher than the ionosphere's maximum plasma frequency, satellite systems ensure that the signals are not reflected away from the Earth, enabling reliable communication between the ground and satellites.1