Fibre-optic satellite synchronisation helps pinpoint distant space objects
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To determine the position of distant objects in space, multiple radio telescopes are pointed at the same objects and the differences between the precise time at which a signal is detected are recorded and compared.
This technique allows researchers to combine all the observations and pinpoint the object’s location and other characteristics.
Linking radio telescopes in an array requires that each telescope has access to an atomic clock to record the precise time at which a signal is detected, a feat that adds significant cost and complexity to the operations.
For the first time, researchers have demonstrated that a stable frequency reference can be reliably transmitted more than 300km over a standard fibre-optic telecommunications network and used to synchronise two radio telescopes. This eliminates the need to install an atomic clock at each location.
Stable frequency references, which are used to calibrate clocks and instruments that make ultra-precise measurements, are usually only accessible at facilities that generate them. The new technology could allow scientists anywhere to access the frequency standard simply by tapping into the telecommunications network.
The ability to send stable frequency references over the telecommunications network could be particularly useful for radio telescope arrays such as the Square Kilometer Array (SKA), an international effort to build the world’s largest radio telescope using arrays in Australia and South Africa.
When complete, SKA will detect faint radio waves from deep space with a sensitivity about 50 times greater than that of the Hubble telescope. Individual radio telescopes will be linked to create a total collecting area of approximately one million square metres.
The results of the test showed that the technique is capable of compensating for signal fluctuations in the fibre-optic network introduced by environmental factors such as temperature changes or vibrations. The demonstration was even performed over a network that was transmitting live telecommunications traffic at the same time.
“By running the experiment on optical fibres also carrying normal traffic, we showed that transmitting the stable frequency standard doesn’t affect the data or telephone calls on the other channels,” said Kenneth Baldwin, a member of the research team from the Australian National University. “This is necessary to gain the cooperation of the telecommunications companies that own these fibre networks.”
Importantly, the new technique doesn’t require any substantial changes to the rest of the fibre-optic network and is easy to implement.
To keep the frequency stable during transmission, the researchers send the signal through the network to a destination and then reflect it back. The returning signal is used to determine if any changes occurred. After each round trip, any transmitted frequency shift is passively subtracted to exactly compensate for the measured changes.
For every 100km of fibre, the round trip takes about one millisecond. Even though the compensation process happens very quickly, the time on the receiving end can drift during the round trips. To solve this problem, a quartz oscillator at the remote location keeps the time steady between round trips.
“The frequency of the quartz oscillator will also eventually drift, so our unique process combines local stabilisation with the quartz oscillator for short time lengths, with the longer - greater than round-trip time - stabilisation provided by the transmitted stable frequency reference technique,” said Baldwin. “This highly stable method for transmitting the frequency reference allows an atomic clock, which cost around two hundred thousand dollars, to be replaced with a system that only costs tens of thousand dollars.”
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