Deep Space Atomic Clock Moves Toward Increased Spacecraft Autonomy|What Is an Atomic Clock?

Designed to improve navigation for robotic explorers and the operation of GPS satellites, the technology demonstration reports a significant milestone.

Spacecraft that venture beyond our Moon relies on communication with ground stations on Earth to figure out where they are and where they’re going. NASA’s Deep Space Atomic Clock is working toward giving those far-flung explorers more autonomy when navigating.

In a new paper published today in the journal Nature, the mission reports progress in their work to improve the ability of space-based atomic clocks to measure time consistently over long periods.

Known as stability, this feature also impacts the operation of GPS satellites that help people navigate on Earth, so this work also has the potential to increase the autonomy of next-generation GPS spacecraft.

To calculate the trajectory of a distant spacecraft, engineers send signals from the spacecraft to Earth and back. They use refrigerator-size atomic clocks on the ground to log the timing of those signals, which is essential for precisely measuring the spacecraft’s position.

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NASA’s Deep Space Atomic Clock could revolutionize deep-space navigation. One key requirement for the technology demonstration was a compact design. The complete hardware package is shown here and is only about 10 inches (25 centimeters) on each side.
Credits: NASA/JPL-Caltech

But for robots on Mars or more distant destinations, waiting for the signals to make the trip can quickly add up to tens of minutes or even hours.

If those spacecraft carried atomic clocks, they could calculate their own position and direction, but the clocks would have to be highly stable.

GPS satellites carry atomic clocks to help us get to our destinations on Earth, but those clocks require updates several times a day to maintain the necessary level of stability. Deep space missions would require more stable space-based clocks.

Managed by NASA’s Jet Propulsion Laboratory in Southern California, the Deep Space Atomic Clock has been operating aboard General Atomic’s Orbital Test Bed spacecraft since June 2019.

The new study reports that the mission team has set a new record for long-term atomic clock stability in space, reaching more than 10 times the stability of current space-based atomic clocks, including those on GPS satellites.

All atomic clocks have some degree of instability that leads to an offset in the clock’s time versus the actual time. If not corrected, the offset, while minuscule, increases rapidly, and with spacecraft navigation, even a tiny offset could have drastic effects.

One of the key goals of the Deep Space Atomic Clock mission was to measure the clock’s stability over longer and longer periods, to see how it changes with time. In the new paper, the team reports a level of stability that leads to a time deviation of fewer than four nanoseconds after more than 20 days of operation.

“As a general rule, an uncertainty of one nanosecond in time corresponds to a distance uncertainty of about one foot,” said Eric Burt, an atomic clock physicist for the mission at JPL and co-author of the new paper.

“Some GPS clocks must be updated several times a day to maintain this level of stability, and that means GPS is highly dependent on communication with the ground. The Deep Space Atomic Clock pushes this out to a week or more, thus potentially giving an application like GPS much more autonomy.”

The stability and subsequent time delay reported in the new paper are about five times better than what the team reported in the spring of 2020.

This does not represent an improvement in the clock itself but in the team’s measurement of the clock’s stability. Longer operating periods and almost a full year of additional data made it possible to improve the precision of their measurement.

The Deep Space Atomic Clock mission will conclude in August, but NASA announced that work on this technology continues: the Deep Space Atomic Clock-2, an improved version of the cutting-edge timekeeper, will fly on the VERITAS (short for Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) mission to Venus.

Like its predecessor, the new space clock is a technology demonstration, meaning its goal is to advance in-space capabilities by developing instruments, hardware, software, or the like that don’t currently exist.

Built by JPL and funded by NASA’s Space Technology Mission Directorate (STMD), the ultra-precise clock signal generated with this technology could help enable autonomous spacecraft navigation and enhance radio science observations on future missions.

“NASA’s selection of Deep Space Atomic Clock-2 on VERITAS speaks to this technology’s promise,” said Todd Ely, Deep Space Atomic Clock principal investigator and project manager at JPL. “On VERITAS, we aim to put this next generation space clock through its paces and demonstrate its potential for deep space navigation and science.” 

More About the Mission

The Deep Space Atomic Clock is hosted on a spacecraft provided by General Atomics Electromagnetic Systems of Englewood, Colorado.

It is sponsored by STMD’s Technology Demonstration Missions program located at NASA’s Marshall Space Flight Center in Huntsville, Alabama, and NASA’s Space Communications and Navigation (SCaN) program within NASA’s Human Exploration and Operations Mission Directorate. JPL manages the project.

What Is an Atomic Clock?

NASA’s Deep Space Atomic Clock could be the most stable atomic clock ever flown in space. But what does that mean, and what do clocks have to do with space navigation?

The clock is ticking: A technology demonstration that could transform the way humans explore space is nearing its target launch date of June 24, 2019. Developed by NASA’s Jet Propulsion Laboratory in Pasadena, California, the Deep Space Atomic Clock is a serious upgrade to the satellite-based atomic clocks that, for example, enable the GPS on your phone.

Ultimately, this new technology could make spacecraft navigation to distant locations like Mars more autonomous. But what is an atomic clock? How are they used in space navigation, and what makes the Deep Space Atomic Clock different? Read on to get all the answers.

Why do we use clocks to navigate in space?

To determine a spacecraft’s distance from Earth, navigators send a signal to the spacecraft, which then returns it to Earth. The time the signal requires to make that two-way journey reveals the spacecraft’s distance from Earth, because the signal travels at a known speed (the speed of light).

While it may sound complicated, most of us use this concept every day. The grocery store might be a 30-minute walk from your house. If you know you can walk about a mile in 20 minutes, then you can calculate the distance to the store.

By sending multiple signals and taking many measurements over time, navigators can calculate a spacecraft’s trajectory: where it is and where it’s headed.

Most modern clocks, from wristwatches to those used on satellites, keep time using a quartz crystal oscillator. These devices take advantage of the fact that quartz crystals vibrate at a precise frequency when voltage is applied to them. The vibrations of the crystal act like the pendulum of a grandfather clock, ticking off how much time has passed.

To know the spacecraft’s position within a meter, navigators need clocks with precision time resolution – clocks that can measure billionths of a second.

Navigators also need clocks that are extremely stable. “Stability” refers to how consistently a clock measures a unit of time; its measurement of the length of a second, for example, needs to be the same (to better than a billionth of a second) over days and weeks.

What do atoms have to do with clocks?

By space navigation standards, quartz crystal clocks aren’t very stable. After only an hour, even the best-performing quartz oscillators can be off by a nanosecond (one billionth of a second).

After six weeks, they may be off by a full millisecond (one thousandth of a second), or a distance error of 185 miles (300 kilometers). That would have a huge impact on measuring the position of a fast-moving spacecraft.

Atomic clocks combine a quartz crystal oscillator with an ensemble of atoms to achieve greater stability. NASA’s Deep Space Atomic Clock will be off by less than a nanosecond after four days and less than a microsecond (one millionth of a second) after 10 years. This is equivalent to being off by only one second every 10 million years.

Atoms are composed of a nucleus (consisting of protons and neutrons) surrounded by electrons. Each element on the periodic table represents an atom with a certain number of protons in its nucleus. The number of electrons swarming around the nucleus can vary, but they must occupy discreet energy levels, or orbits.

A jolt of energy – in the form of microwaves – can cause an electron to rise to a higher orbit around the nucleus. The electron must receive exactly the right amount of energy – meaning the microwaves must have a very specific frequency – in order to make this jump.

The energy required to make electrons change orbits is unique in each element and consistent throughout the universe for all atoms of a given element. For instance, the frequency necessary to make electrons in a carbon atom change energy levels is the same for every carbon atom in the universe.

The Deep Space Atomic Clock uses mercury atoms; a different frequency is necessary to make those electrons change levels, and that frequency will be consistent for all mercury atoms.

“The fact that the energy difference between these orbits is such a precise and stable value is really the key ingredient for atomic clocks,” said Eric Burt, an atomic clock physicist at JPL. “It’s the reason atomic clocks can reach a performance level beyond mechanical clocks.”

Being able to measure this unchangeable frequency in a particular atom offers science a universal, standardized measurement of time. (“Frequency” refers to the number of waves that pass a particular point in space in a given unit of time.

So, by counting waves, it’s possible to measure time.) In fact, the official measurement of the length of a second is determined by the frequency needed to make electrons jump between two specific energy levels in a cesium atom.

In an atomic clock, the frequency of the quartz oscillator is transformed into a frequency that is applied to a collection of atoms. If the derived frequency is correct, it will cause many electrons in the atoms to change energy levels.

If the frequency is incorrect, far fewer electrons will jump. This will determine if the quartz oscillator is off-frequency and by how much. A “correction” determined by the atoms can then be applied to the quartz oscillator to steer it back to the correct frequency.

This type of correction is calculated and applied to the quartz oscillator every few seconds in the Deep Space Atomic Clock.

What’s unique about the Deep Space Atomic Clock?

Atomic clocks are used onboard GPS satellites that orbit the Earth, but even they must be sent updates two times per day to correct the clocks’ natural drift. Those updates come from more stable atomic clocks on the ground that are large (often the size of a refrigerator) and not designed to survive the physical demands of going to space.

Up to 50 times more stable than the atomic clocks on GPS satellites, NASA’s Deep Space Atomic Clock is intended to be the most stable atomic clock ever flown in space. It achieves this stability by using mercury ions.

Ions are atoms that have a net electric charge, rather than being electrically neutral. In any atomic clock, the atoms are contained in a vacuum chamber, and in some of those clocks, atoms interact with the vacuum chamber walls.

Environmental changes such as temperature will then cause similar changes in the atoms and lead to frequency errors.

Many atomic clocks use neutral atoms, but because the mercury ions have an electric charge, they can be contained in an electromagnetic “trap” to prevent this interaction from happening, allowing the Deep Space Atomic Clock to achieve a new level of precision.

For missions going to distant destinations like Mars or other planets, such precision makes autonomous navigation possible with minimal communication to and from Earth – a huge improvement in how spacecraft are currently navigated.

The Deep Space Atomic Clock is hosted on a spacecraft provided by General Atomics Electromagnetic Systems of Englewood, Colorado.

It is sponsored by the Technology Demonstration Missions program within NASA’s Space Technology Mission Directorate and the Space Communications and Navigations program within NASA’s Human Exploration and Operations Mission Directorate. JPL manages the project.