[time-nuts] BBC News on Atomic Clocks for Galileo

[Thomas A. Frank]

I suggest you follow the link at the bottom, because the original  
page has a nifty video embedded in it.

Enjoy!

Tom Frank
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Atomic rhythms give precise fix


By Jonathan Fildes
Science and technology reporter, BBC News



Please turn on JavaScript. Media requires JavaScript to play.

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  
location.

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  
atomic clock.

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
Pierre Waller
By comparison, a typical wristwatch is accurate to about one second  
per day.

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  
Mr Waller.

"For me, this is really the challenge of Giove-B."

Precise fix

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  
timed signals."





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  
wristwatch.

"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."

Relative times

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  
Atlantic Ocean.

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.

Light fantastic

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  
maser.

"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  
chassis.

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  
of time.

"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,"  
he said

"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  
distances
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:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/7360762.stm

Published: 2008/04/24 08:49:18 GMT

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