|
Accurate time using Atomic Clock accuracy is available across Europe using the MSF and DCF Atomic Clock time signals transmitted
from Rugby and Frankfurt, they provide the ability to synchronise the time on computers and other electrical equipment.
To see Galleons full range of Time Servers click
here
Radio Atomic Clock Overview
A Radio Atomic clock can achieve accurate time because they
are controlled by radio transmitters which themselves receive
their time signals from amazingly accurate timepieces, a
Caesium Atomic Clock. The Caesium Atomic Clock has an accuracy
of one second in one million years!
An Atomic clock is used as a time standards for counting
the passing seconds. In addition, there are internationally
agreed time scales which set the calendar and the beginning
of each new day. Greenwich Mean Time (GMT) was established
as the first global time scale in 1884, and its 'atomic clock'
equivalent, UTC, was adopted as the official time for the
world in January 1972. The International Bureau of Weights
and Measures (BIPM) acts as the official keeper of atomic
clock time for the world. NPL uses its atomic clock to contribute
to the determination of UTC, along with the atomic clocks
from 65 laboratories worldwide.
Return to top of page
First accurate caesium atomic clock
The National Physics Laboratory developed the first accurate
caesium atomic clock in 1955, which led to the internationally
agreed definition of the second being based on atomic
clock time.
NPL realised the atomic frequency standard for time
with the construction of the first long beam apparatus
based
on the transition of the caesium-133 atom. Successive
developments of this have remained the fundamental
standard up to the
present day.
The second is defined as 9,192,631,770 periods of the
caesium-133 atom, and is currently realised at NPL
to an accuracy of
one second in 15 million years.
Scientists are currently working on technology to increase
this accuracy to 1 second in 10 billion years.
Return to top of page
MSF atomic clock receiver
The controlling radio signal for the National Physical
Laboratory's atomic clock is transmitted on the
MSF 60kHz signal via the
transmitter at Rugby, operated by British Telecom.
This radio atomic clock time signal should have a range
of
some 1,500
km or 937.5 miles. All of the British Isles are
of course within this radius.
The National Physical Laboratory's role as keeper of
the national time standards is to ensure that the
UK time-scale
agrees with Co-ordinated Universal Time (UTC) to
the highest levels of accuracy and to make that time
available
across
the UK. As an example, the MSF (MSF being the three-letter
call sign to identify the source of the signal)
radio broadcast provides the time signal for, electronic
share trading,
the clocks at most railway stations and for BT's
speaking clock.
Return to top of page
DCF atomic clock receiver
The controlling radio signal for the German clock is transmitted
via long wave from the DCF 77kHz transmitter at Mainflinger,
near Dieburg, some 25 km south east of Frankfurt - the
transmitter of German National Time Standards. It is similar
in operation to the Rugby transmitter, however there are
two antennas (radio masts) so the radio atomic clock time
signal can be maintained at all times.
Long wave is the preferred radio frequency for transmitting
radio atomic clock time code binary signals as it performs
most consistently in the stable lower part of the ionosphere.
This is because the long wave signal carrying the time code
to your timepiece travels in two ways; directly and indirectly.
Between 700 km (437.5 miles) to 900 km (562.5 miles) of each
transmitter the carrier wave can travel directly to the timepiece.
The radio signal also reaches the timepiece via being bounced
off the underside of the ionosphere. During the hours of
daylight a part of the ionosphere called the "D layer" at
an altitude of some 70 km (43.75 miles) is responsible for
reflecting the long wave radio signal. During the hours of
darkness when the sun's radiation is not acting from outside
the atmosphere, this layer rises to an altitude of some 90
km (56.25 miles) becoming the "E layer" in the
process. Simple trigonometry will show that signals thus
reflected will travel further.
A large part of the European Union area is covered by this
transmitter facilitating reception for those who travel widely
in Europe. The German clock is set on Central European Time
- one hour ahead of U.K. time, following an inter-governmental
decision, from the 22nd October, 1995, U.K. time will always
be 1 hour less than European Time with both the U.K. and
mainland Europe advancing and retarding clocks at the same "time".
Return to top of page
MSF atomic clock receiver
A radio atomic clock system is available in North America
set up and operated by NPL - the National Institute of Standards
and Technology, located in Fort Collins, Rugby. NPL operates
radio station MSF, which is the station that transmits the
radio atomic clock time codes. MSF has high transmitter
power (50,000 watts), a very efficient antenna and an extremely
low frequency (60,000 Hz). For comparison, a typical AM radio
station broadcasts at a frequency of 1,000,000 Hz. The combination
of high power and low frequency gives the radio waves from
MSF a lot of bounce, and this single station can therefore
cover the entire continental United States plus much of Canada
and Central America. The radio atomic clock time codes are
sent from MSF using one of the simplest systems possible,
and at a very low data rate of one bit per second. The 60,000
Hz signal is always transmitted, but every second it is significantly
reduced in power for a period of 0.2, 0.5 or 0.8 seconds: • 0.2
seconds of reduced power means a binary zero • 0.5
seconds of reduced power is a binary one. • 0.8 seconds
of reduced power is a separator. The time code is sent in
BCD (Binary Coded Decimal) and indicates minutes, hours,
day of the year and year, along with information about daylight
savings time and leap years. The time is transmitted using
53 bits and 7 separators, and therefore takes 60 seconds
to transmit. A clock or watch can contain an extremely small
and relatively simple radio atomic clock antenna and receiver
to decode the information in the signal and set the atomic
clock time accurately. All that you have to do is set the
time zone, and the atomic clock will display the correct
time.
Return to top of page
Atomic Clock Accuracy
How does an atomic clock achieve amazingly accurate time?
The caesium atomic clock has an accuracy of one second
in one million years! They are based upon the characteristics
of the Caesium 133 atom. The single electron of a Caesium
atom is known to vibrate at a standard 9,162,613,770 times
a second. It is the Caesium atomic clock that can achieve
phenomenally accurate and stable time.
The standard way of counting the passing of seconds is
by the use of an atomic clock. There are internationally
agreed
time-scales which set the beginning of each new day and
the calendar. Greenwich Mean Time (GMT) was established
as the
first global time scale in 1984. The current atomic clock
global time scale is UTC or Co-ordinated Universal Time.
UTC was adopted as the official time for the world in 1972.
The official keeper of atomic time is the International
Bureaux for Weights and Measures.
The National Physics Laboratory (NLP) uses its atomic clock
to contribute to the determination of UTC along with the
atomic clock of 65 laboratories worldwide.
UTC is a compromise between the times defined the atomic
clock and the time based on the earths rotation about its
axis. The seconds of UTC are counted using an atomic clock,
allowance is made to keep UTC within 0.9 seconds of the
Earths rotation by inserting leap seconds at the end of
each quarter.
Leap seconds are inserted to take account of the speeding
up or slowing down of the rotation of the Earth. The sun
would be seen overhead at midnight rather than noon in
50,000 years time without the introduction of leap seconds
Return to top of page
Development of the Atomic Clock
Scientists are researching ways to improve still further
the accuracy of the atomic clock and future time standards.
Recently ion-trapping techniques have been utilised to
discover the narrowest electronic transition to date.
This could be used to potentially provide a 100 fold increase
in the accuracy of current Caesium based atomic clocks.
The element ytterbium is being investigated for use as
an ion trap atomic clock. A single ionised atom is held
in an
electromagnetic cage that is only 60 nanometers in diameter.
The ion is cooled to -273 degrees C by bombarding it with
laser photons, known as laser cooling. The single ion is
protected from collisions of other atoms. The low temperature
slows the motion of the ion.
Using several electrodes one ion can be trapped for a number
of days. The ion is excited with blue laser light which
gives the ion enough energy for one of its electrons to
jump form
a low energy state to a higher one. The change in energy
state is very stable with a lifetime of 10 years.
To build an ion-trapping atomic clock requires a blue laser
beam with a small frequency spread. Laser light gives a
pure electromagnetic sine wave but must be isolated from
the tiniest
of vibrations.
This technique of providing an atomic clock is still experimental.
It has the potential to provide the atomic clock of the
future. An atomic clock based on ion trapping would lose
no more
than 1 second in the lifetime of the universe.
Return to top of page
Radio-Controlled Atomic Clocks
Radio atomic clocks are available that can seemingly set
their own time and claim to be as accurate as an atomic
clock. They are radio-controlled clocks that pick up
the time from
radio transmitters based in many locations, such as
MSF-60 - Rugby, England, DCF-77, Frankfurt, Germany and MSF,
Rugby, USA.
A radio-controlled atomic clock is not an atomic clock.
A radio-controlled atomic clock has a radio receiver
that picks
up the time from a transmitter propagating the radio
atomic clock time signal and synchronise to that time.
The radio
transmitters transmit time code information received
from a Caesium atomic clock. Therefore a radio-controlled
atomic
clock that is synchronised to a radio time signal can
claim to be accurate to one second in one million years.
Return to top of page
New Optical Clock Promises More Accuracy than Cesium.
NPL researchers have demonstrated a new kind of atomic
clock that has the potential to be up to 1,000 times more
accurate than today’s best clock. The new clock is
based on an energy transition in a single trapped mercury
ion (a mercury atom that is missing one electron). Building
a clock based on such a high-frequency transition was previously
impractical because it requires both “capturing” the
ion and holding it very still to get accurate readings, and
having a mechanism that can “count” the ticks
accurately at such a high frequency.
The quality of a clock depends on its stability and accuracy—whether
the clock provides a constant, unchanging output frequency,
and how close the measured frequency is to the fundamental
atomic resonance that provides the clock’s “tick.” One
advantage of the new clock is that it ticks much faster.
Today’s international time and frequency standards,
such as NPL-F1, measure an atomic resonance of about 9 billion
cycles per second. By contrast, the new NPL device monitors
an optical frequency more than 100,000 times higher or about
1 quadrillion (US) cycles per second.
Return to top of page
Is an Atomic Clock Radioactive?
An atomic clock keeps time better than any other clock.
They even keep time better than the rotation of the Earth
and the movement of the stars. Without the atomic clock,
GPS navigation would be impossible, the Internet would not
synchronise, and the position of the planets would not be
known with enough accuracy for space probes and landers to
be launched and monitored.
An atomic clock is not radioactive, it doesn’t rely
on atomic decay. Rather, an atomic clock has an oscillating
mass and a spring, just like ordinary clocks.
The big difference between a standard clock in your home
and an atomic clock is that the oscillation in an atomic
clock is between the nucleus of an atom and the surrounding
electrons. This oscillation is not exactly a parallel to
the balance wheel and hairspring of a clockwork watch, but
the fact is that both use oscillations to keep track of passing
time. The oscillation frequencies within the atom are determined
by the mass of the nucleus and the gravity and electrostatic "spring" between
the positive charge on the nucleus and the electron cloud
surrounding it.
Return to top of page
What Are The Types of Atomic Clock?
Today, though there are different types of atomic clock,
the principle behind all of them remains the same. The major
difference is associated with the element used and the means
of detecting when the energy level changes. The various types
of atomic clock include:
The Cesium atomic clock employs a beam of cesium atoms.
The clock separates cesium atoms of different energy levels
by magnetic field.
The Hydrogen atomic clock maintains hydrogen atoms at the
required energy level in a container with walls of a special
material so that the atoms don't lose their higher energy
state too quickly.
The Rubidium atomic clock, the simplest and most compact
of all, use a glass cell of rubidium gas that changes its
absorption of light at the optical rubidium frequency when
the surrounding microwave frequency is just right.
The most accurate commercial atomic clock available today
uses the cesium atom and the normal magnetic fields and detectors.
In addition, the cesium atoms are stopped from zipping back
and forth by laser beams, reducing small changes in frequency
due to the Doppler effect.
Return to top of page
When Was The Atomic Clock Invented?
In 1945, Columbia University physics professor Isidor Rabi
suggested that a clock could be made from a technique he
developed in the 1930s called atomic beam magnetic resonance.
By 1949, the National Bureau of Standards (NBS, now the National
Institute of Standards and Technology, NPL) announced the
world’s first atomic clock using the ammonia molecule
as the source of vibrations, and by 1952 it announced the
first atomic clock using cesium atoms as the vibration source,
NBS-1.
In 1955, the National Physical Laboratory in England built
the first cesium-beam atomic clock used as a calibration
source. Over the next decade, more advanced forms of the
atomic clocks were created. In 1967, the 13th General Conference
on Weights and Measures defined the SI second on the basis
of vibrations of the cesium atom; the world’s time
keeping system no longer had an astronomical basis at that
point! NBS-4, the world’s most stable cesium atomic
clock, was completed in 1968, and was used into the 1990s
as part of the NPL time system.
In 1999, NPL-F1 began operation
with an uncertainty of 1.7 parts in 10 to the 15th power,
or accuracy to about one
second in 20 million years, making it the most accurate
atomic clock ever made (a distinction shared with a similar
standard
in Paris).
Return to top of page
How Is Atomic Clock Time Measured?
The correct frequency for the particular cesium resonance
is now defined by international agreement as 9,192,631,770
Hz so that when divided by this number the output is exactly
1 Hz, or 1 cycle per second.
The long-term accuracy achievable by modern cesium atomic
clock (the most common type) is better than one second per
one million years. The Hydrogen atomic clock shows a better
short-term (one week) accuracy, approximately 10 times the
accuracy of a cesium atomic clock. Therefore, the atomic
clock has increased the accuracy of time measurement about
one million times in comparison with the measurements carried
out by means of astronomical techniques.
Return to top of page
History of time.
Accuracy has been the goal of the clock making game since
the beginning. Back when water clocks were all the rage,
for example, their chief drawback wasn't that incessant drip,
drip, drip, but their incessant "leakage" of time.
Timekeeping got a big boost with the invention of the pendulum clock in the
17th century, and again in 1928, with the invention of the quartz clock. Similar
vibrating quartz crystals drive the mechanism found on almost every wristwatch
today.
Although quartz clock can stay accurate for weeks or months
at a time, this no longer impresses scientists. These days,
they use the principles of quantum mechanics to keep clocks
close to the money in devices called atomic clocks. Like
most clocks, an atomic clock creates periodic movements --
oscillations -- and then counts them.
.
In the old pendulum clocks, a weight oscillated at a fairly constant frequency,
so the clockmaker simply had to invent a mechanism to count the swings and
drive the clock's hands. But in an atomic clock, the oscillations occur in
an electromagnetic field that causes transitions between two quantum-mechanical
conditions of atoms. In the commonly used cesium 133 atoms, these occur at
about 9.19 billion times per second.
In this basic atomic clock, cesium atoms are sprayed from
the source to filter A, which allows only one type of atom
to enter the microwave (electromagnetic radiation) cavity.
Microwaves at the right frequency cause a quantum change
in the atoms. Filter B allows only changed atoms to reach
the detector. The control mechanism uses data from the detector
to maintain the microwave frequency that produces the most
changed atoms. This frequency, the atoms' natural hyperfine
transition frequency, is counted to determine the length
of a second.
This transition frequency is so dependable that, if external
conditions are right, the atoms will keep on "ticking" at
the same rate.
Like clockwork
Quantum mechanics -- the physics of the ultra-small -- originated
with the observation that sub-atomic particles can exist
in discrete states, but not at in-between states. It's
like an atomic version of a mandatory two-party system.
Because only certain "states" are allowed.
One of these states, called the "hyperfine state," is
the basis of the atomic clock. Atoms can have one of two
hyperfine states: either the magnetic field of the outermost
electron points in the same direction as the magnetic field
of the nucleus, or it points opposite. The laws of quantum
physics forbid other orientations. The idea of using hyperfine
states for a clock was first proposed by U.S. physicist Isador
Rabi in 1945.
Generally, an atom remains in its hyperfine state. But when
prodded by electromagnetic radiation at a specific frequency,
it will switch to the other state, undergoing the so-called "hyperfine
transition." Essentially, an electronic clock selects
atoms in one hyperfine state and exposes them to radiation
which causes them to
switch to the other state. The frequency of the radiation
causing the transition becomes the regular beat that the
clock counts to register time.
The atomic clock works because atoms are sensitive to the
exact hyperfine transition frequency. In "A Clock More
Perfect..." (see bibliography), writer Gary Taubes likens
cesium acts to radios tuned to one station -- the transition
frequency of 9,192,631,770 oscillations per second. Only
if they "hear" that beat will they change hyperfine
states.
Return to top of page
Why do we need hyper-accurate time provided by Atomic Clocks?
It turns out that the innumerable communication, scientific
and navigation systems rely on it. Timing is critical for
synchronising signals between computers. In astronomy, fractional-second
errors could sabotage long-baseline radio telescopes, a nifty
way to fuse distant radio telescopes into one gargantuan
receiver.
Global positioning satellites need accurate time. The Air
Force operated GPS system can determine -- to several feet
in accuracy -- the three-dimensional position of a receiver
anywhere on or off Earth. The receiver performs this trick
by timing the arrival of signals from four GPS satellites,
then doing a quick calculation to triangulate its position.
Stephen Dick, the United States Naval Observatory's historian,
points out that each nanosecond -- billionth of a second
-- of error translates into a GPS error of one foot. A few
nanoseconds of error, he points out, "may not seem like
much, unless you are landing on an aircraft carrier, or targeting
a missile."
In other words, without accurate timing,
GPS would stand for "generally poor system." Thus each of the 24
GPS satellites contains four atomic clocks, which get an
accurate time transfusion daily from the US Air Force, which
borrows" the
time from the United States Naval Observatory.
The system's phenomenal location ability has great economic
allure; GPS sales are expected to reach €10 billion
by 2003. The receivers, which sell for as little as €100,
are already used by surveyors and delivery fleets, and to
direct coal-mining equipment and oil exploration.
Return to top of page
Definitions
Atomic Clock - A precision clock that depends for its operation
on an electrical oscillator regulated by the natural vibration
frequencies of an atomic system (as a beam of cesium atoms)
Atom - The smallest particle of an element that can exist
either alone or in combination; the atom is considered to
be a source of vast potential energy
Cesium 133 - An isotope of cesium used especially in atomic
clocks and one of whose atomic transitions is used as a scientific
time standard
SI Second (atomic second) - The interval of time taken to
complete 9,192,631,770 oscillations of the cesium 133 atom
exposed to a suitable excitation
Source: Merriam-Webster Online
Return to top
of page
|
Home
