April 25, 2008
New ‘Space Clock’ to Assist in Satellite Navigation
For centuries, timepieces have been used to aid navigation. However, the latest hydrogen maser atomic clock is a world away from any used ever before. The new "Ëspace clock' will be launched onboard the Giove-B satellite when it launches from the Baikonaur Cosmodrome in Kazakhstan.
"Such a clock has never been flown," Pierre Waller, an engineer at the European Space Agency (Esa), told BBC News.
The clock will serve as the beating heart of Giove-B, the second test spacecraft for Europe's Galileo global satellite-navigation system. It will be accurate to one billionth of a second per day, or one second in three million years, making it the most precise time piece to ever orbit the earth.
For perspective, a typical wristwatch is accurate to about one second per day.
The scientists who constructed the navigation system said the high level of precision is necessary because even the tiniest of errors can cause significant errors in satellite navigation handsets.
A one second inaccuracy, for example, would throw off a location approximately 186,000 miles.
If the technology proves successful, it will be incorporated into all 30 of Galileo's operational satellites, and would allow users to identify their location to within one meter, a significant improvement over current GPS technology that pinpoints location to within several meters.
"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."
Clocks are the core of all satellite navigation systems, and are used to generate a time code that the satellites continuously transmit.
"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," Dr Peter Whibberley, of the National Physical Laboratory (NPL) in the UK, explained during a BBC News interview.
"If you measure its time of arrival against the clock in your receiver, you know how long that signal took to get to you."
From this, the distance from receiver to satellite is calculated.
"If you have three satellites in view, you can triangulate yourself on the surface of the Earth," explained Dr Whibberley, adding that a fourth satellite would allow a precise fix.
"This whole process relies on satellites sending out very precisely timed signals."
Atomic clocks are currently the most accurate timepieces. The accuracy of the time signal correlates with the accuracy of the fix.
Like conventional chronometers, atomic clocks implement a physical constant to measure the passing of time. With atomic clocks, time is measured by atoms switching between different energy states, instead of the regular "Ëtick-tock' of a mechanical pendulum.
These atoms release energy at very precise frequencies when switching between high and low energy states. Measuring these changes and using them as a counter input produces highly accurate measures of time.
Giove-B's primary onboard clock uses hydrogen as an atomic source, which emits microwave radiation that 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," explained Mr. Waller. "Our periodic signal here is generated by quartz and we are using the [hydrogen] atoms to lock this quartz."
Although the atomic clock time signal is accurate to within one nanosecond per day, the signal must be nevertheless be adjusted before it is relayed since the satellite is orbiting the Earth at an altitude of 14,430 miles (23,222km).
"On board Galileo - as with GPS - we have to take into account two different relativistic effects," Mr. Waller said.
The algorithms have to factor in certain aspects of Einstein's General and Special Theories of Relativity. For example, the "relativistic Doppler effect", outlined in the Special Theory, demonstrates 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.
The Galileo system must further account for what are known as "gravitational frequency shifts", also 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 portion of Einstein's theory was confirmed on the only other vehicle, the Gravity Probe A, to carry an onboard hydrogen maser clock. The experimental craft was launched in 1976 and achieved an altitude of 6,200 miles (10,000km) before it crashed in the Atlantic Ocean.
The vehicle's onboard hydrogen maser clock confirmed that gravity does indeed slow the flow of time.
Mr. Waller explained that if Galileo did not make these relativistic tweaks, positioning errors of up to "13km over one day" would result.
"It is one of the few examples of where General Relativity comes into our lives," he added.
Giove-B's onboard technology is slightly different to that which flew on Gravity Probe A in that it uses a passive hydrogen maser clock. The earlier probe had used an active maser instead.
"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."
Active maser technology was not an option onboard Giove-B due to weight and space considerations. Furthermore, the vehicle must pack two additional atomic clocks in its chassis to serve as back-up chronometers. These clocks use rubidium and are accurate to 10 nanoseconds per day. One will run permanently as a "hot" backup for the hydrogen maser, instantly taking over should it fail, while the other rubidium clock will serve as a "cold" spare.
In total, the Galileo satellites will then carry four clocks, two hydrogen masers and two with rubidium. This should guarantee the constellation, which should be up and running by the end of 2013, would offer uninterrupted and with unprecedented accuracy.
It should also improve critical precision time services, such as time-stamping of financial transactions and co-ordination of e-mail systems.
As impressive as Giove-B's systems are, scientists at NPL are already working on next-generation optical clocks, which measure time using light frequency.
"The basic principle is the same as the current generation of clocks," Dr Whibberley explained
But with light, an even more stable clock can be built.
"They could potentially be one hundred times more accurate."
"They could be placed on satellites to give much more precise time keeping, and that promises even greater performance in positioning," he said
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