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Old 27-06-2021, 07:19 PM
gary
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The Australian ultra precise Sapphire Clock helps Jindalee Radar see even clearer

In a 24 June 2021 article at Institute of Electrical and Electronics Engineers
(IEEE) web site, Ewen Levick reports on the development of the
Sapphire Clock, the most precise clock yet created, by Andre Luiten
at the University of Western Australia.

Quote:
Originally Posted by Ewen Levick, IEEE
Luiten calls it the Cryogenic Sapphire Oscillator, and it could bolster technologies as varied as military radar and quantum computing. He and his colleagues are working on these applications at the University of Adelaide, also in Australia, where he now serves as director of the Institute for Photonics and Advanced Sensings.

The new clock—also known as the Sapphire Clock—isn’t better than an atomic clock; it’s different. That’s because accuracy and precision are different things: Accuracy is how well a clock can measure a true second, now defined as the time it takes cesium atoms under controlled conditions to oscillate between two energy states exactly 9,192,631,770 times. Since 2013, even more accurate types of atomic clocks have been built, but over 400 atomic clocks based on cesium-133 atoms are still used to create civil time across the globe. If you’re reading this article on a smartphone or a laptop, the time displayed on the edge of your screen is derived from one of those atomic clocks.

For many applications, such as satellite-based global positioning systems, accuracy is paramount. And make no mistake, even cesium atomic clocks are stunningly accurate. The NIST-F2 cesium clock operated by the U.S. National Institutes of Standards and Technology in Boulder, Colo., is so accurate that it would have to run for 300 million years to gain or lose a second.

But for some applications, accuracy is less important than precision. Precision has to do not with delineating the perfect second but rather with creating extremely regular ticks, or oscillations. Imagine a game of darts. Atomic clocks are able to land all their darts, or oscillations, broadly around the bull’s-eye so that the average position is right on target, even though any given dart might be a centimeter or two away from dead center. Luiten’s device doesn’t aim for the bull’s-eye: instead, it is able to land all its darts at exactly the same point on the dartboard. In other words, each tick is really, really, really just like another.
One application is the Jindalee Operational Radar Network (JORN).

Quote:
Originally Posted by Ewen Levick, IEEE
The receiver stations consist of 480 antenna pairs arranged in two parallel lines along the red desert sand, each 3 km long. They rely on the Doppler effect, in which objects moving toward the radar return higher frequency echoes than objects moving away—that is, the signal undergoes a phase shift.

“We propagate signals out, and if the target is moving toward or away from us, then we see a Doppler shift. Over time, we are able to develop target direction and velocity to develop target tracks,” Wynd says.

The signals’ refraction off the ionosphere allows the radar to see over the horizon, but the movement of the ionosphere introduces variations in the signal, as do reflections from the Earth’s surface. The radar cross section of the Earth’s surface can be large, on the order of a million times as great as the cross section of targets. That immensity can make targets hard to identify.

“One of the challenges we have is resolving targets from the background clutter,” Wynd says. “If the clutter is too high, then the signal disappears.”

This is where precise timing really matters. The frequency of the outgoing signal is controlled using the ticks of a reference clock, currently a quartz-based oscillator. If those ticks aren’t very precise, then the outgoing signal becomes irregular, and it’s harder to measure changes in the returning echoes. In addition, if the ticks of the clocks at the transmission and receiver stations get out of sync, then the whole system inaccurately measures the distance to the target.
Full story here :-
https://spectrum.ieee.org/computing/...-atomic-clocks

Sapphire Clock group web site including videos :-
https://www.adelaide.edu.au/ipas/our...sapphire-clock

Jindalee Operational Radar Network :-
https://en.wikipedia.org/wiki/Jindal..._Radar_Network
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