Lasers and telescopes could replace radio transmitters on satellites in ten
years, suggests a paper in this 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.

###

PAPER

Bland-Hawthorn, J., Harwit, A., and Harwit, M. “Laser Telemetry from Space.”
Science, v.297 p.253

IMAGE

Telescope with laser transmitter in Mars orbit
www.aao.gov.au/lasers/ (400 x 300 pixels, with caption)

http://www.aao.gov.au/lasers/concept2.jpg (1280 x 960 pixels)