>> dr. matsakas is talking about his job at the u.s. naval observatories time service department. >> we provide the official time for gps and via gps, it goes to much of the world. >> so it's not an exaggeration to say, you are the time keeper. >> yes, we are the time keeper. >> and keeping time, precise time, is important. for the financial system, for the internet, for the pentagon. the department has more than 100 atomic clocks, and there are three different types. one measures the oscillation of atoms. how often does the atom oscillate in one second? >> that's a definition of a second. >> the doctor took us to see another kind of clock, called an atomic fountain. >> we use it to freeze atoms to