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GPS satellites carry atomic clocks that provide extremely accurate time. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast.
Please Share! How contact lenses work to correct vision is the same way eyeglasses do: they alter the direction of light rays to focus light properly onto the retina. If you are nearsighted, light rays focus too early within your eye  they form a focus point in front of the retina instead of directly on it.