The future of the world’s satellite infrastructure depends on a device as old as time: the humble clock.
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A mainstay on larger, legacy satellites such as the Global Positioning System (GPS), high-precision atomic clocks have enabled essential navigation and telecommunication functions while providing a timekeeping standard for financial markets, power grids and the internet.
Fundamentally, all clocks measure the passage of time by counting oscillations of a resonator. Grandfather clocks count the oscillations of a pendulum (i.e., the number of times the pendulum arm passes through the vertical), while most modern timepieces count the vibrations of electrified quartz.
But differences in manufacturing, as well as the effects of heat and pressure, can cause resonators likes quartz and pendulums to vary in their oscillation period over time.
Because satellites need to send and measure signals traveling at lightspeed, even the smallest variations in frequency can cause substantial errors; a deviation of one one-thousandth of a second could throw off a GPS route – or a missile’s target – by 300 km.
Precision timekeeping could enable swarms of small satellites to communicate with each other more effectively and help spacecraft navigate in deep space. NASA's Jet Propulsion Laboratory is currently testing their Deep Space Atomic Clock, which they hope will assist spacecraft to reach destinations as far as Mars and beyond.
For applications that require this level of precision, clocks aboard satellites must not only be synchronized, but also syntonized, or kept at the same frequency, said James Camparo, an Aerospace Fellow and atomic clock physicist.
To maintain syntonization, most atomic clocks employed in space use the same quartz found in other timepieces, but tie the quartz’ oscillation to a fundamental atomic constant of nature, which in principle will not change: the energy-level spacing on an atom’s energy-ladder diagram. Camparo describes the resonant frequency associated with this energy-level spacing as the backbone of ultra-precise timekeeping.
In one type of atomic clock, cesium atoms are subjected to microwave radiation at exactly 9,192,631,770 Hz – a frequency generated by a quartz oscillator. If the quartz oscillator produces this exact frequency, the cesium atoms will change from one rung on their energy-ladder to another.
After the initial cesium-atom/microwave interaction, a detector in the clock measures how many atoms changed rungs. If most of the atoms have jumped from one rung of the energy-ladder to another, the frequency of the microwaves – and by extension, the quartz oscillator – is correct.
If the atoms have not jumped a rung on their energy-ladder, the frequency has drifted – factors such as temperature or pressure may have altered the quartz, just as they do in everyday timepieces. When this happens, an electric circuit in the clock will correct the oscillator frequency back to 9,192,631,770 Hz.
While most atomic clocks follow this scheme, some use different elements, such as rubidium, and different forms of radiation. The result is the same: precision timekeeping that can self-correct.
As the scientific community works toward smaller, higher precision devices, Camparo and his team conduct critical research on alternative clock designs and study the long-term impact of environmental factors on timekeeping through simulation and testing.
Working in parallel with a commercial manufacturer developing a new laser-based clock, the team is currently evaluating the efficacy of a simplified version with a lower-powered laser – a design that could significantly extend the life of the device.
“If we succeed, we can pass that information through the open literature to the manufacturer. If we fail, we haven’t interrupted the contractor’s progress and in fact, we can alert the timekeeping community not to go down that path,” Camparo said. “This is how Aerospace’s research – success or failure – can be of benefit to America’s place in space.”
Ultimately, the lab’s strength in foundational research, combined with Aerospace’s public interest mission, enable it to bridge the divide within the atomic clock community.
“We take on problems that academia might not be interested in and manufacturers don’t have the time and resources to look at,” he said. “If you don’t know the basic atomic physics, you may find an engineering solution, but it may not be the optimal solution.”
Atomic clocks are designed to measure the precise length of a second, the base unit of modern timekeeping. The International System of Units (SI) defines the second as the time it takes a caesium-133 atom in a precisely defined state to oscillate exactly:
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9 billion, 192 million, 631 thousand, 770 times.
The official definition provides more detail: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 Kelvin.”
Planet Earth is an exceptionally accurate timekeeper
What is time?
In an atomic clock, the natural oscillations of atoms act like the pendulum in a grandfather clock. However, atomic clocks are far more precise than conventional clocks because atomic oscillations have a much higher frequency and are much more stable.
There are many different types of atomic clocks, but they generally share the same basic working principle, which is described below:
First, the atoms are heated in an oven and bundled into a beam. Each atom has one of two possible energy states. They are referred to as hyperfine levels, but let's call them state A and state B.
A magnetic field then removes all atoms in state B from the beam, so only atoms in state A remain.
The state-A atoms are sent through a resonator where they are subjected to microwave radiation, which triggers some of the atoms to change to state B. Behind the resonator, atoms that are still in state A are removed by a second magnetic field. A detector then counts all atoms that have changed to state B.
The percentage of atoms that change their state while passing through the resonator depends on the frequency of the microwave radiation. The more it is in sync with the inherent oscillation frequency of the atoms, the more atoms change their state.
The goal is to perfectly tune the microwave frequency to the oscillation of the atoms, and then measure it. After exactly 9,192,631,770 oscillations, a second has passed.
Why Telling Time Is ComplicatedThe accuracy of atomic clocks varies and is constantly improving. With an expected error of only 1 second in about 100 million years, the NIST-F1 in Boulder, Colorado, is one of the world's most precise clocks.
It is called a caesium fountain clock where lasers concentrate the atoms into a cloud, cool them down, and then toss them upwards. This method slows the atoms down, allowing for a longer measurement period and a more precise approximation of the natural frequency of the atoms.
Scientists are currently developing a device that is even more accurate than the current atomic clocks. The optical atomic clock uses light in the visible spectrum to measure atomic oscillations. The resonance frequency of the light rays is about 50,000 times higher than that of microwave radiation, allowing for a more precise measurement. The expected deviation of the new optical clock is 1 second in 15 billion years.
Some 450 atomic clocks around the world contribute to the calculation of International Atomic Time (TAI), one of the time standards used to determine Coordinated Universal Time (UTC) and local times around the world.
Satellite navigation systems like GPS, GLONASS, and Galileo also rely on precise time measurements to calculate positions accurately.