The Global Positioning System has become one of the fundamental pillars of 21st-century living. Most people in the developed world own a GPS receiver and numerous industries rely on them, sometimes for life-critical tasks.
This system is so important that governments, industry leaders, and the military have long studied its shortcomings and worried over how easy it would be to bring it down. The results of their hand-wringing are by no means reassuring.
One nightmare scenario is the Kessler syndrome, the worrying possibility that a collision in Earth orbit could cause a cascade of further collisions that dramatically increase the density of space debris. This could destroy the entire GPS satellite constellation in just a few hours.
So many groups have begun to think about how the GPS system could be restored without relying on satellites. And today, the US government’s National Institute of Standards and Technology publishes the results of one program that could make this possible.
First some background. GPS satellites are essentially orbiting clocks that broadcast precise, synchronized time signals. A receiver on the ground can triangulate its position by comparing the arrival times of signals from three or more satellites.
The orbiting timepieces are atomic clocks based on cesium atoms. When electrons orbiting the atoms jump from one state to another, they produce radiation with a frequency of exactly 9,192,631,770 hertz—the so-called cesium standard. This is used to keep time.
These atomic clocks are accurate to within 10-6 seconds and are regularly synchronized with ground-based systems. This synchrony is what determines the system’s positioning accuracy.
Ground-based clocks can be synchronized with much greater accuracy, however. Indeed, a technique known as optical two-way time-frequency transfer has synchronized clocks to within 10-19 seconds.
But applying this to moving clocks has not been possible. The synchronization procedure assumes that the time it takes light to travel from one clock to the other is the same in both directions. But this is not the case if either clock is moving. So this can’t be used for GPS-type systems
Today, that looks set to change, thanks to the work of Hugo Bergeron and colleagues at the NIST facility in Boulder, Colorado. These guys have developed a technique for synchronizing a pair of moving clocks with extraordinary accuracy.
The secret behind the new approach is relatively straightforward. The goal is to find a way to take into account the movement of the clocks. So Bergeron and co simply measure the relative movement.
“The speed is found from the rate-of-change of the measured time-of-flight over three roughly continuous measurements requiring ~1.5 ms across a turbulent link,” they say.
They put this new technique to the test using time signals relayed by retroreflectors fitted to a pair of quadcopters that can move at relative speeds of up to 24 meters per second.
And the results are impressive. “The synchronized clocks agree to ~10−18 in frequency,” they say.
That’s obviously much more accurate than current GPS signals and opens up a whole raft of new applications. Not least of these is more accurate navigation, but there is also the potential for distributed mobile scientific experiments looking for everything from gravitational waves to dark matter.
And it also raises the possibility of replacing the GPS satellite constellation quickly should disaster strike, using balloons, drones, or other flying vehicles.
Ref: arxiv.org/abs/1808.07870 : Femtosecond Synchronization of Optical Clocks Off of a Flying Quadcopter
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