[time-nuts] BBC News on Atomic Clocks for Galileo
Thomas A. Frank
ka2cdk at cox.net
Thu Apr 24 21:19:49 EDT 2008
I suggest you follow the link at the bottom, because the original
page has a nifty video embedded in it.
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Atomic rhythms give precise fix
By Jonathan Fildes
Science and technology reporter, BBC News
Launching the 'space clock' on Giove-B
In the late 18th Century, Captain Cook set out on a voyage of
discovery clutching a pocket watch to help him keep track of his
The timepiece, which he described as "our faithful guide", was
accurate to a couple of seconds per month, and helped fix the
position of his ship to a distance of two nautical miles.
Two hundred years later, the general principle of using clocks to aid
navigation still stands. But the latest generation of timepiece, to
be launched into space onboard the Giove-B satellite, is a world away
from Captain Cook's.
"Such a clock has never been flown," Pierre Waller, an engineer at
the European Space Agency (Esa), told BBC News.
The beating heart of Giove-B, the second test spacecraft for Europe's
Galileo global satellite-navigation system, is a hydrogen maser
Following its launch from the Baikonaur Cosmodrome in Kazakhstan, it
will become the most precise time piece to orbit the Earth. It will
be accurate to one billionth of a second per day, or one second in
three million years.
On board Galileo - as with GPS - we have to take into account two
different relativistic effects
By comparison, a typical wristwatch is accurate to about one second
This precision is needed, say the scientists who built the system,
because even tiny errors can cause sat-nav handsets to be way out.
A slip of just one second, for example, would produce location
inaccuracies of around 300,000 km, approaching the distance from the
Earth to the Moon.
If the technology is shown to be successful, it will be built into
all 30 of Galileo's operational satellites, eventually allowing users
to pinpoint their location with an error of just one metre, compared
to the several metres experienced with current GPS technology.
"Everything has been verified on the ground - on paper - but now we
want to verify and validate all of these assumptions on board," said
"For me, this is really the challenge of Giove-B."
The principles of satellite-navigation are well understood. Clocks
are the core of all systems and are used to generate a time code
which is continuously transmitted from the satellites.
"When you pick up that signal on the ground you can look at the time
code [which] tells you when the satellite sent it out," explained Dr
Peter Whibberley, of the National Physical Laboratory (NPL) in the UK.
"If you measure its time of arrival against the clock in your
receiver, you know how long that signal took to get to you."
This allows the distance from receiver to satellite to be calculated.
"If you have three satellites in view, you can triangulate yourself
on the surface of the Earth," explained Dr Whibberley. A fourth
satellite allows a precise fix.
"This whole process relies on satellites sending out very precisely
The more accurate the time signal, the more accurate the fix. And
currently, the most accurate timepieces are atomic clocks.
Like conventional chronometers, these use a physical constant to
measure the passing of time. But instead of the regular tick-tock of
a pendulum, they use atoms switching between different energy states.
When an atom flips between a high and low energy state, it releases
energy at a very precise frequency. Measuring this change and using
it as an input into a counter produces an accurate measure of time.
The main clock onboard Giove-B uses hydrogen as an atomic source.
This emits microwave radiation which is used as an input to
"calibrate" a quartz crystal, similar to those found in a regular
"A clock is a generator of a periodic signal," said Mr Waller. "Our
periodic signal here is generated by quartz and we are using the
[hydrogen] atoms to lock this quartz."
Although the resulting time signal is accurate to within one
nanosecond a day, the fact that the satellite is orbiting the Earth
at a height of 23,222km (14,430 miles), means the signal must be
tweaked before it is relayed.
GALILEO UNDER CONSTRUCTION
A European Commission and European Space Agency project
30 satellites to be launched in batches by end of 2013
Will work alongside US GPS and Russian Glonass systems
Promises real-time positioning down to less than a metre
Guaranteed under all but most extreme circumstances
Suitable for safety-critical roles where lives depend on service
"On board Galileo - as with GPS - we have to take into account two
different relativistic effects," said Mr Waller.
In particular, algorithms must factor aspects of Einstein's General
and Special Theories of Relativity.
For example, the so-called "relativistic Doppler effect", outlined in
the Special Theory, shows that time is perceived differently by
observers in different states of motion.
"A clock moving perpendicular to your line of sight will have a
different tick rate to one at your location," explained Mr Waller.
In addition, the Galileo system must account for what are known as
"gravitational frequency shifts", outlined in the General Theory.
"The tick rate of your clock is not the same on Earth and at
23,000km," said Mr Waller.
This aspect of Einstein's theory was confirmed on the only other
spaceflight to carry a hydrogen maser clock.
In 1976, an experiment called Gravity Probe A hurtled to a height of
10,000 km (6,200 miles) above the Earth before crashing into the
The hydrogen maser onboard confirmed the prediction that gravity
slows the flow of time.
If Galileo did not make these relativistic tweaks, it could cause
positioning errors of up to "13km over one day," according to Mr Waller.
"It is one of the few examples of where General Relativity comes into
our lives," he said.
The technology onboard Giove-B is subtly different to that which flew
on Gravity Probe A. The Galileo system uses what is known as a
passive hydrogen maser clock whilst the earlier probe used an active
"The stability of the active maser is roughly one order of magnitude
better," explained Mr Waller. "But as a result the active maser is
roughly five to 10 times heavier and bulkier."
With weight and space at a premium onboard Giove-B, active maser
technology was not an option.
In addition, the craft must pack two more atomic clocks into its
These back-up atomic chronometers use rubidium and are accurate to 10
nanoseconds per day.
One will be permanently running as a "hot" backup for the hydrogen
maser, instantly taking over should it fail. The second rubidium
clock will act as a so-called "cold" spare.
The final Galileo satellites will contain four clocks - two hydrogen
masers and two which use rubidium.
This combination should ensure that the constellation, set to be up
and running by the end of 2013, will offer uninterrupted and
unparalleled accuracy on the ground.
In addition, it should improve the precision time services that have
become so critical to economic activity, such as time-stamping of
financial transactions and co-ordinating e-mail systems.
But soon even these clocks may be consigned to history alongside
Captain Cook's pocket watch.
Scientists at NPL are currently working on next-generation optical
clocks, which use the frequency of light to help measure the passage
"The basic principle is the same as the current generation of
clocks," explained Dr Whibberley.
However, using light allows a more stable clock to be built.
"They could be placed on satellites to give much more precise time
keeping, and that promises even greater performance in positioning,"
"They could potentially be one hundred times more accurate."
Satellite-navigation systems determine a position by measuring the
distances to a number of known locations - the spacecraft
constellation in orbit
In practice, a sat-nav receiver will capture atomic-clock time
signals sent from the satellites and convert them into the respective
A sat-nav device will use the data sent from at least four satellites
to get the very best estimate of its position - whether on the ground
or in the sky
The whole system is monitored from the ground to ensure satellite
clocks do not drift and give out timings that might mislead the user
Story from BBC NEWS:
Published: 2008/04/24 08:49:18 GMT
? BBC MMVIII
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