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 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.
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.
"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."
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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