GPS has become ubiquitous technology that provides real-time positioning, navigation and timing (PNT) data in cars, boats, planes, trains, smartphones and wristwatches, and has enabled advances as wide-ranging as driverless cars, precision munitions, and automated supply chain management. Phones and other GPS-enabled devices pinpoint your location on Earth by contacting at least four satellites bearing atomic clocks. Each of these satellites provides a time stamp, and the system calculates your location based on the relative differences among those times.
The atomic clocks used on today’s satellites are based on natural oscillation of the cesium atom — a frequency in the microwave region of the electromagnetic spectrum. Atomic clocks are extremely accurate because they are based on natural and universal atom vibrations. However, even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month. The accuracy of GPS Navigation is primary degraded due to Earth’s ionosphere, which interferes with the timing signals as they commute from a satellite to your GPS receiver. But the second biggest contribution of error comes from the stability of the clocks onboard the GPS satellites.
Now, scientists in China have successfully demonstrated a cold atom clock in space, an achievement that could lead to more accurate terrestrial timekeeping and better tests of fundamental physics. The device, called Cacs, or Cold Atomic Clock in Space, was launched in Sep 2017 along with other instruments of the Tiangong-2, China’s second orbital lab. According to the South China Morning Post, it will slow down by only one second in a billion years. In comparison, the NIST-F2 atomic clock, which serves as the United States’ primary time and frequency standard, loses a second every 300 million years.
“It is the world’s first cold atomic clock to operate in space … it will have military and civilian applications,” said Professor Xu Zhen, a scientist involved with the Cacs project.
China’s Beidou satellite navigation network currently provides less precise guidance than the US GPS system, but Xu said that using Cacs as a time reference in space would give a “significant boost” to Beidou’s performance.
China’s atomic clock passes space test
An atomic clock uses vibrations of atoms to measure time, which are very consistent as long as the atoms are held at constant temperature. In fact, since 1967 the definition of second has been “9,192,631,770 vibrations of a cesium-133 atom.”
In a cold atomic clock, the atoms are cooled down with a laser to decrease the effect of atom movement on the measurements. Cacs goes even further and eliminates the pull of Earth’s gravity by being based in orbit. Chinese engineers had to miniaturize their device so that it could be sent into space. It can fit in a car trunk, while the NIST-F2, along with all its support equipment, is about the size of a room.
The Cold Atom Clock Experiment in Space (CACES) involves trapping, cooling, and probing rubidium atoms within a box that could fit in the trunk of a car. In orbit at an altitude of 400 kilometers, the experiment was launched on board China’s Tiangong-2 space laboratory last September. Now, a year later, it is performing just as expected, according to a paper posted to the arXiv server by scientists at the Chinese Academy of Sciences’s Shanghai Institute of Optics and Fine Mechanics.
Cacs was launched before the European Space Agency could place their atomic clock, the Pharao, in orbit, which is scheduled to be launched next year. The US abandoned a similar project due to budgetary cuts. Unlike Pharao, which uses the traditional design with cesium atoms, the Chinese clock uses rubidium atoms. Developers say the element offers better performance in terms of accuracy and reliability.
Cacs is not the most accurate timepiece in the world. German researchers at Physikalisch-Technische Bundesanstalt have built an experimental atomic clock, which uses ytterbium ions and is two orders of magnitude more accurate than regular cesium clocks. The device, however, is used only for demonstration purposes, while Cacs is meant for practical applications.
The Chinese plan to improve their BeiDou Navigation Satellite System with synchronization signals from the new orbital atomic clock.
Next, Liu and colleagues plan to install a more stable clock with links to the ground on board China’s space station, which is due to start taking shape in 2020. Then both they and a group of European researchers led by Schiller aim to test orbital “optical clocks,” which would use a laser beam to probe atoms with higher frequency emissions in order to generate even more precise ticks than a microwave device. “There is an exciting future for high-precision clocks in space,” Schiller says.
ACES project; Time to go atomic on space station
THE International Space Station will soon host the most accurate clock ever sent into space. It will allow for better synchronisation of clocks on Earth and also probe exotic physics.
The experiment, called Atomic Clock Ensemble in Space (ACES), will be built by EADS Astrium and was scheduled to fly to the space station in 2014, however got delayed according to the European Space Agency. It will keep track of time by measuring the frequency of microwaves absorbed by cooled caesium atoms.
On Earth, the accuracy of caesium clocks is limited by gravity. The atoms are cooled by using lasers to slow them down, then tossed upwards into a cavity where measurements are made to determine the precise frequency of microwave radiation that they absorb and emit. In microgravity, the atoms linger in the cavity, allowing for longer and more accurate measurements, explains John Prestage of NASA’s Jet Propulsion Laboratory in Pasadena, California, who is not involved in the project.
ACES should be at least 100 times as accurate as the clocks on GPS satellites, adds Prestage.
Using the space-station clock as a common point of reference, ground-based atomic clocks could be more accurately compared with one another. What’s more, variations between atomic clocks could reveal if a physical constant called alpha – which governs the electromagnetic force – is not constant after all.
The Chinese clock, however, is less advanced than its European counterpart, which will use cold cesium atoms. For one thing, its estimated stability—three parts in 1013—is a third of ACES design value, meaning it would take nine times as long to reach a given accuracy. In addition, CACES doesn’t transmit its ticking to Earth, so its accuracy can’t be regularly monitored. (It has a data link but not one that can send stable time and frequency information.) ACES, on the other hand, will communicate its timekeeping to Earth through a microwave link, allowing atomic clocks on the ground to be calibrated and also enabling tests of the theory of general relativity involving the effect of altitude on a clock’s ticking rate.
ACES Principal Investigator Christophe Salomon, an atomic physicist at the Ecole Normale Supérieure in Paris, says that although Liu and colleagues have demonstrated the basics of a cold atom clock in orbit, they haven’t fully exploited that capability—the longest recorded time measurement was actually shorter than the 0.5 seconds typically reached on Earth. “They have made a nice technology demonstration,” he says, “but it is disappointing that they haven’t taken advantage of the microgravity.”