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. Lasers May Shine Spacecraft Data To Earth, Researchers Say

a telescope with laser transmitter in Mars orbit, comparable in size to the Hubble Space Telescope, acts as a communications relay for Mars probes to send their data back to earth at high data rates in a beam of light. A receiving station might be on the Earth's surface as shown here, or in Earth orbit.
Canberra - Jul 29, 2002
Lasers and telescopes could replace radio transmitters on satellites in ten years, suggests a paper in last week's issue of 'Science' [July 26, 2002].

They'd fix a worsening bottleneck in getting data back from satellites and spacecraft, say authors Joss Bland-Hawthorn (Anglo-Australian Observatory), Alex Harwit (Transparent Networks), and Martin Harwit (Cornell University).

The problem is bandwidth -- the size of the data 'pipe'. Currently data is sent to Earth as radio waves. The higher the carrier frequency, the greater the bandwidth, and the faster the data can be sent.

But the sky's the limit -- literally. Gases in Earth's atmosphere absorb radio frequencies above 300 GHz.

Today's spacecraft do 'onboard processing', choosing some data to send and discarding the rest. But in the long term a different solution is needed.

"Detector chips for astronomy now have millions of pixels," said Dr Bland-Hawthorn. "Hubble's successor, the Next Generation Space Telescope, will carry a set of these. Future astronomy missions will have detectors with a thousand times more pixels than today's."

To download all their data these satellites would have to transmit at 100 gigabits a second. But current systems are hundreds of times too slow.

Earth's atmosphere lets through near-infrared radiation. A thousand times higher in frequency than radio waves, it can carry data at a far higher rate. So why not use infrared lasers to shine down the data? ask the Science paper's authors.

A wavelength around 1 micrometre would be used. "Most of the necessary technologies have already been developed for optical fibre communications," said Dr Alex Harwit.

Ten-metre optical telescopes on mountaintops would receive the signals.

Cloud blocks near-infrared radiation. But some mountaintop sites have up to 350 cloudless days a year. "If you have a number of receiving stations, there's almost no chance they'd all be clouded out," said Dr Martin Harwit.

Because the narrow laser beam would target the signal better than a radio beam, less energy would go astray, and onboard power consumption would be slashed. A laser would need only a fraction of the power of today's transmitters.

Last year the European Space Agency tested a laser link between two satellites, SPOT-4 and Artemis. NASA too is exploring the option of laser communications.

The technique would work well for spacecraft orbiting Mars, 400 million kilometres away, according to Dr Bland-Hawthorn.

The Science paper calls for "energetic action" from the scientific community to head off the looming crisis in data transmission.

"Space missions flying ten years from now will collect huge amounts of data," said Dr Alex Harwit. "We'd like to see work start now on a laser telemetry system that can handle it."

"Astronomers, weather forecasters, resource managers -- anybody who uses a satellite or spacecraft would benefit from this new technology," said Dr Andrew McGrath of the Anglo-Australian Observatory.

Related Links
Lasers at Anglo-Australian Observatory
Transparent Networks
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Europe Puts A Laser Through Artemis To SPOT The Data
Paris (ESA) Nov 27, 2001
Last week, for the first time, a data link between satellites was established using a laser beam as signal carrier. On board ESA's Artemis satellite - launched last July by an Ariane 5 - is the SILEX system.
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