Q: As we all know, atomic clocks are being used to measure time and the GPS system. But I was wondering based on what was the first atomic clock calibrated and how accurate this calibration was based on our standards nowadays?

A: More specifically, caesium atomic clocks realize the second (see this Q&A for the meaning of realization) or, said another way, they are a primary frequency standard. Generally, when a new primary standard is being developed—for whatever quantity, not only time—and has not yet become, by international agreement, a primary standard, it should be calibrated against the primary standards of the time.

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