[TenTec] Off Topic - Time

[Cliff]

DOES ANYBODY REALLY KNOW WHAT TIME IT IS? 

Since the establishment of Coordinated Universal Time (UTC) in 1972, 22
leap seconds have been added to keep atomic time in synch with the rotation
of the Earth. When the Global Positioning System (GPS) started in 1980, it
was synchronized with UTC. However, GPS time does not add leap seconds, and
as a result GPS time is now 13 seconds ahead of UTC. So, if you are getting
your time from a GPS satellite, is it correct? 

Before we try to clarify, let's confuse you even more. The frequency or
rate of UTC is computed by the International Bureau of Weights and Measures
(BIPM) located near Paris, France. The BIPM uses a weighted average from
about 250 atomic clocks located in about 50 national laboratories to
construct a time scale called International Atomic Time (TAI). Now TAI time
is a uniform and stable scale which does not keep in step with the slightly
irregular rotation of the Earth. Therefore TAI time is many seconds off
from UTC time (32 seconds ahead, to be precise, and that's because of an
additional 10 second drift that occurred from when UTC was introduced in
the late 1950s and the introduction of leap seconds). 

But don't despair. It is all about to get clearer. 

For public and practical purposes it is necessary to have a scale that, in
the long term, does keep in step with the rotation of the Earth. Such a
scale is UTC, which is identical with TAI except that it is adjusted to add
the leap seconds to ensure that, when averaged over a year, the Sun crosses
the Greenwich meridian at noon UTC to within 0.9 seconds. But this still
does not help us with the time differential between GPS and UTC. 

According to the National Institute of Standards and Technology (NIST)
"Precise time synchronization has many uses in everyday life.
Synchronization between two or more locations is necessary for high speed
communication systems, synchronizing television feeds, calculating bank
transfers, and transmitting everything from email to sonar signals in a
submarine. Power companies use precise time to regulate power system grids
and reduce power losses. Radio and television stations require both precise
time-of-day and frequency in order to broadcast programs. Precise time
measurements are also essential for accurate navigation and the support of
communications on earth and in space. Scientific organizations such as NASA
depend on reliable and consistent time measurement for projects such as
interplanetary space travel. Fractional disparities in times between a
space probe and tracking stations on Earth can dramatically affect the
positions of spacecraft. Precise time measurements are also essential to
radio navigation systems like the Global Positioning System (GPS). By
synchronizing the satellite clocks within nanoseconds of each other, it
makes it possible for a receiver to know its position on earth within a few
meters."