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Summary: because they have atomic clocks. GPS reciever clocks are not as accurate, but by using a fourth satellite distance measurement, they can compensate, and simulate atomic clock accuracy. Each GPS satellite carries 4 atomic clocks on board. Atomic clocks are the most accurate time keeping devices ever invented by man. Atomic clocks usually range in price from $50K to $100K.
Image Dimensions: 115 x 76
Image originally found here.