Spaceworthy optical clock promises centimeter-level GPS precision

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Booster stage breaking away from the sounding rocket sending the optical clock into space(Credit: Airbus Defense & Space GmbH)

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As sailors in the 17th century learned the hard way, having an accurate clock for navigation is pointless if it is too delicate to carry aboard ship. The same goes for the super accurate optical clocks being developed for future GPS satellites.

We live in a world increasingly dependent on GPS for position fixing, mobile phones, navigation, surveying, and even controlling inventory in shipyards and warehouses. By locking onto the signal of four or more satellites, it's possible in seconds to determine one's position almost anywhere on Earth within a few meters, but that level of precision just isn't good enough anymore.

The problem is that GPS depends on the atomic clocks installed in the satellites, which generate a time stamp based on the vibration of a cesium atom that oscillates in the microwave range. To achieve greater precision and accuracy, engineers want to move that oscillation into the much narrower frequencies of visible light, which are 100,000 times higher than microwaves, using what are called optical clocks.

To understand what an optical clock is, think of an atomic clock as having a pendulum like an old-fashioned wall clock. A properly made pendulum swings with great regularity and marks off time with enough accuracy to make pendulum clocks the dominant timekeepers until the invention of the chronometer in the late 18th century.

Now imagine that instead of beating once or twice a second, the pendulum beats 300 million times a second – you now have the accuracy of an atomic clock, which uses cesium atoms to beat at the frequency of microwaves. Multiply those beats by 100,000 and you are in the realm of the optical clock – so called because it's operating in the frequency of visible light.

The use of optical clocks in GPS satellites would result in accuracies 100 to 1,000 times greater than conventional atomic clocks. But to read these frequencies it will be necessary to reduce the output of these optical clocks, so the beats can counted and linked to a microwave-based reference atomic clock for verification.

This is where a frequency comb comes in. A frequency comb in an optical clock acts like the reduction gears in a mechanical clock to shift the output down from the visible to the microwave spectrum. These devices use lasers fired at intervals of a tenth of a trillionth of a second that are uniformly spaced in time, so across the visible frequency they form millions of teeth like a comb. By linking these teeth to the beat of an atomic clock, the result is the much more accurate and readable optical clock.

Getting optical clocks off the ground

The problem is that until now frequency combs (and therefore optical clocks) have been heavy, bulky, delicate benchtop monsters. This leads us to the work of Matthias Lezius of Menlo Systems and his team, who have built a compact optical clock that is capable of operating in a zero-gravity environment and has the potential to one day produce centimeter-level GPS location fixing.

The team's advanced version of the frequency comb uses optical fibers and is not only fully automated, but weighs only 22 kg (44 lb) and measures only 22 x 14.2 cm (8.7 x 5.6 in). It also uses only 70 W of power and is rugged enough to be launched in a rocket, which is exactly what the team did in April 2015 after combining it with an atomic cesium clock and a rubidium optical clock.

For this test of its ability to operate in space, the completed clock was installed in a payload module built by Airbus Defense & Space, then fired into space on a suborbital trajectory atop a sounding rocket from the Esrange Space Center, which is located above the Arctic Circle in Sweden. During its six-minute flight, the optical clock's performance was monitored by means of a low-frequency radio telemetry link.

Booster stage breaking away from the sounding rocket sending the optical clock into space(Credit: Airbus Defense & Space GmbH)

Though the clock worked, it proved to have only a tenth of the accuracy of a GPS satellites, but the team is working on the next generation, which they say will be several orders of magnitude more accurate. This new optical clock, scheduled to fly in 2017, will be vacuum hardened, so it won't need to kept in a pressurized container, and will be more resistant to cosmic rays, so it should last several years in space. The hope is one day to have an optical clock weighing only a few kilograms, taking up a volume of 3 liters (183 in3) and using a mere 10 W of power.

The team sees the new technology as having applications not only in GPS satellites, but also in greenhouse gas sensing satellites, precision meteorology, and Earth observation.

"Our device represents a cornerstone in the development of future space-based precision clocks and meteorology," says Lezius. "The optical clock performed the same in space as it had on the ground, showing that our system engineering worked very well."

The research is published in Optica.

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