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Location Glossary Science of Maps Weekend Reads Maps

Precision timing: the atomic clock that could power sub millimeter GPS

When it comes to navigation, clocks are incredibly important. As we’ve seen previously, GPS relies on atomic clocks to locate the receivers’ position on earth. Atomic clocks are used because they’re the most precise timekeepers we have.

Not only that, but atomic clocks are becoming more precise all the time. In fact, the latest generation of atomic clocks is so precise that in 5 billion years (roughly the age of the earth) it would not lose or gain even one second.

The only problem is that this atomic clock, created at the Joint Institute for Laboratory Astrophysics, Colorado USA, and its accompanying equipment is massive, taking up two large laboratories.

Paul-Griffin

Dr Paul Griffin, a lecturer and Chancellor’s Fellow at the University of Strathclyde, who has a PhD in atomic physics, is currently working on ways to create smaller atomic clocks - ones that you could actually fit in a car for example, for GPS. In the next 5 years they hope to create a small clock that would only lose one second in 16 million years.

A brief history of time (keeping)

In understanding why atomic clocks are so precise, Dr. Griffin takes us on a brief journey through time, explaining what methods man has used to track time. It appears that what all methods of time keeping have in common is the counting of oscillations.

Griffin says, “Around 4,000 years ago, at the start of civilization, we figured out that when the sun was at its highest point it was noon. We can measure that quite accurately by using a sundial, for example, which would show noon when the shadow was at its shortest. So, essentially, that was the old definition of time, this oscillation of the sun around us.”

However, when it comes to precision, the sun is not the most reliable of timekeepers. Firstly, the length of the day varies throughout the year due to Earth’s orbit around the Sun being an ellipse rather than a circle - over a year this means the day varies by up to a minute. Secondly, the length of the day is gradually increasing due to the tidal drag of the moon - about 2 milliseconds per hundred years.

PaulGriffinworkngonAtomicClock

The next big revolution in time keeping came in the form of the pendulum. The pendulum clock, which, for many years, was as precise an instrument anyone needed, increasing accuracy from 15 minutes per day to 15 seconds a day.

All at sea

However, pendulum clocks weren’t very good at sea. The rocking of the boat upset the pendulums, making them useless for navigation. This led to the marine chronometer created by John Harrison, using a balance wheel and spiral spring in place of the pendulum.

The next revolution came with the use of quartz crystal and electric oscillation in the 20th century. Griffin says, “By putting a voltage across quartz it vibrates and creates an extremely accurate oscillator.”

However, even though it was theoretically very accurate, fluctuations in temperature can disturb the frequency of oscillation. And this is when we arrive at atomic clocks.

Pendulums and springs are all very well, but each of them is slightly different. The advantage of atoms is that they are identical. “If I make a clock that works on the oscillations of an atom, it doesn’t matter who builds it, if it is built in France or Germany, anywhere, it should come out exactly the same and it’s that accuracy and repeatability that is a very important factor.”

sr_lattice_clock_artwork

The creation of atomic clocks has led to an incredibly precise definition of a second, namely the duration of 9,192,631,770 oscillations in a caesium atom.

Griffin explains, “When you shine light at an atom it will absorb certain frequencies of light…a second is how long it takes for 9.2 billion oscillations to take place inside a caesium atom and that means that the light has a frequency of 9.2 gigahertz.”

People placement

While the theory is fairly mind blowing, people may be asking why we need such precise clocks. Well, when it comes to GPS, increased precision could prove incredibly useful.

Griffin says, “The speed of light is 186,000 miles per second and the clocks on the 24 satellites orbiting the earth are set at exactly the same time. So when the receiver in my car receives the signals from the different satellites at slightly different times, because the speed of light is fixed, we can use basic trigonometry to work out where you are.”

However, the more accurate the clocks, the more precisely it is possible to locate a GPS receiver.

 

Griffin: "This is a photograph of approximately one billion rubidium atoms cooled down to a temperature of 100 microKelvin, or one-tenth of a millionth of a degree above absolute zero. The atoms are cooled using lasers and are held in a vacuum chamber at a pressure less than one trillionth of atmospheric pressure. These cold atoms are the “oscillators" in our atomic clock." Griffin: "This is a photograph of approximately one billion rubidium atoms cooled down to a temperature of 100 microKelvin, or one-tenth of a millionth of a degree above absolute zero. The atoms are cooled using lasers and are held in a vacuum chamber at a pressure less than one trillionth of atmospheric pressure. These cold atoms are the “oscillators" in our atomic clock."

 

Griffin says, “We have got to a stage now that we can pinpoint a receiver’s location to a resolution of 5 meters… But with even more accurate atomic clocks we aim to create GPS sensors that can be pinpointed to less than a millimetre.”

With this form of precision GPS could graduate from a navigation tool in cars to controlling them in the move towards autonomous cars.

Other applications range from checking the slow movement of building constructions, the precise timing of financial transactions to measuring the effects of general relativity, including the testing of the theory that increased gravitational fields slow down time.

Apparently then, absurdly accurate timekeeping could help to advance civilisation. On the flip side it will mean turning up to a meeting 0.0000001 seconds late will become totally unacceptable.

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