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Written by David H. Johnston
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 Global Positioning System satellites transmit
signals to equipment on the ground. GPS receivers passively receive satellite signals;
they do not transmit. GPS receivers require an unobstructed view of the sky, so they are
used only outdoors and they often do not perform well within forested areas or near tall
buildings. GPS operations depend on a very accurate time reference, which is provided by
atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on
board.
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Dear Weather Guy,
