The time-space continuum
By comparing two times—the time a signal was transmitted by one of those satellites in space with the time it was received by your GPS receiver on Earth—your phone can calculate how far away each satellite is. By doing this with multiple satellites, your GPS is able to triangulate its location on Earth and to compute the time. The more satellites in view—the minimum is four at any given time—the more your GPS receiver can average out errors like random measurement noise and atmospheric delays.Getting precise location depends upon getting precise time. If one GPS satellite is off by a billionth of a second, your GPS receiver will be a foot off. If the satellite's clock were off by one full second, your location on Google Maps would appear to be about two-thirds of the way to the Moon. If you've ever been told by your phone that you're in the middle of a river when you're standing at a subway station, you may have been the victim of these errors.There are many other reasons for accurate time, but we civilians can't know them all. The Master Clock also keeps time for a host of other military operations: flying drones, aiming missiles, establishing secure communications, and other secret things. That is to say, not all time is created equal: the more critical you are to America's national security (say, you're a Navy SEAL, or a drone missile), the more accurately you might need to know the time or your location.IF ONE GPS SATELLITE WERE OFF BY ONE FULL SECOND YOUR LOCATION ON GOOGLE MAPS WOULD APPEAR TO BE ABOUT 2/3 OF THE WAY TO THE MOON
Conversely, in the event that the Master Clock (and an Alternate Master Clock, which is located in Boulder, Colorado), were destroyed in, say, a terrorist attack, GPS could serve as a backup system. "Especially in the context of nuclear war: if a whole bunch of cities get destroyed, at least GPS would work for awhile."Still, without the Master Clock to calibrate them, the clocks on the GPS satellites would gradually drift, per the law of special relativity. "If the Master Clock broke for 24 hours, the world would probably be okay. But I wouldn't do that for a week."There have been some [attacks]… we think about it all the time.
A Brief History of (Disseminating) Time
Between the Master Clock's contribution to Universal time—it makes up about 1/3 of the average—and the dominance of GPS, the U.S. has become the world's dominant timekeeper
Time wars: Greenwich drift
Cesiums. Strontiums. Rubidiums. Ytterbiums.
In February, the Naval Observatory in Colorado upgraded the Master Clock with several brand new fountain clocks that use the element rubidium. (Counting the oscillations of rubidium could boost the timing reference for GPS by 10-fold, from 1 to 2 nanoseconds down to 300 picoseconds.) The clock is so delicate that installing it required an "airsled hover lifter"—essentially, a hovercraft—in order to ensure that it didn't come into contact with the floor or the walls.The Master Clock now relies on up to four rubidium fountains, which, Matsakis said makes it, for now at least, "the most precise continuously-operating system ever constructed to measure anything."Increasing precision is increasingly costly. A good cesium clock can now be purchased for as little as $75,000, a maser can cost around $250,000, and a rubidium fountain simply cannot be bought. Its parts can cost as much as $600,000, but that doesn't include the salaries of the half-dozen PhDs who've worked for a decade developing these fountains. In total, the Observatory runs on less than $20 million a year, a modest budget for the Department of Defense. (The Observatory is also allocated funds by NASA, the Air Force, and other agencies for special projects that are performed at cost.)For now at least, the Master Clock is the most precise continuously-operating system ever constructed to measure anything.
Wait a second: what is time?
A Short History of Long-Term Thinking, for Our Fifty Thousand Year Time Capsule
If We Want Anyone to Remember Humanity, We Need to Talk About Time Capsules
"Where Time Comes From": The Atlantic Video