A Little More About Time
OK, pop quiz for the day … what do James Gandolfini (Tony Soprano), Bob Feldman, Richard Gottenhrer, and Jerry Goldstein all have in common?
Give up? Tony Soprano sang and the other gentlemen wrote a song (released by the “Jive Five”) entitled, “What Time Is It?”.
One of man’s older questions and yet one that gets harder and harder to answer as time and technology march on. Like most other “magic” (Arthur C. Clarke’s Third Law: “Any sufficiently advanced technology is indistinguishable from magic”.), GPS makes answering the question a lot easier or a lot harder. You pick.
When thinking about the magic of precise location and navigation, many forget that the basis of all GPS’s magic is, indeed, TIME. To make GPS work, the developers had to devise much more accurate standards and methods of monitoring and transferring time.
Once upon a time, the accuracy of a handmade clockwork or an hourglass was sufficient for all man had to do. These devices usually varied a few seconds per day but when real accuracy was required, the local time could be updated each noontime as the sun passed overhead the local observer. This served well until the introduction of the train, when it became possible to travel fast enough to require almost constant resetting of clocks.
Standard time, where all clocks in a region use the same time, was invented to solve this problem. Standard time divides the world into a number of “time zones“, each one covering, in theory at least, 15 degrees. All clocks within each of these zones would be set to the same time as the others, but so as to differ by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time in order to set local clocks.
Today, the common name for Greenwich time is Greenwich Mean Time, often abbreviated GMT and referred to by the military and some other government organizations as Z or Zulu time, since the letter “Z” designates the time zone that covers the location of Greenwich, England.
A nice list of the time zones and their letters are here, few readers, for example might know that in Colorado Springs where I am writing this, the clocks are currently set to Sierra time, moving to Tango time when daylight savings goes back to standard.
Well, the sun overhead is a nice measurement, been used since Roman times with sundials, but it has one fatal flaw. The earth is slowing down. Thus, the sun is ‘late’ for noon a fraction of a second every day. It’s about 33 seconds slower than its time in 1972, as a matter of fact, so a more accurate standard was sought. This lead to the most accurate measurement yet devised (I’m sure there will something even more accurate in the future, just don’t know what it is yet), measuring the decay of Cesium atoms … the so-called Atomic Clock.
There’s some good information of atomic timekeeping on Wikipedia.org here:
“Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. It is a weighted average of the time kept by about 300 atomic clocks (including a large number of cesium atomic clocks) in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1, 1972, as decided by the 14th General Conference on Weights and Measures (CGPM). The International Bureau of Weights and Measures is in charge of the realization of TAI..”
These 300 or so atomic time standards include several at the US Naval Observatory, off Massachusetts Avenue at the end of Observatory Circle, NW, Washington, DC. In addition to many other “timely duties” the USNO supervises, the operation of a number of atomic standards at Schriever AFB near Colorado Springs, which is the Master Control Station (MCS) for the GPS.
See…you knew I’d get you to the GPS eventually, did you not? Oh ye of little faith…
The MCS uses their on-station time standards to control the clocks (typically 4 ‘Atomic’ standards, either Cesium or Rubidium) aboard each operational GPS satellite.
The object of this discourse and travelogue? When your GPS receiver locks onto GPS satellites, the time available from that receiver is within 340 nanoseconds of true TAI - about the most accurate standard ever available to the “man on the street” or the “man on the seas”.
If you only want precise time signals in a fixed location, here’s a sample of the kind of stationary GPS standards available. A unit like this costs in the area of $300 and gives time performance equal to mid-level laboratory clocks that cost $3,000 to $30,000. Just another way your tax dollar was really put to good use with the GPS. And if you have a need to give accurate time to many folks all at the same time, you’ll like this project.
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