From the calendar and timekeeping to navigation to satellite communications
systems to advanced medical imaging technology,
astronomy has brought
innumerable benefits to civilization. Today, the universe is a laboratory
holding undiscovered knowledge that may spawn unimagined new benefits and
entirely new industries.
Astronomy is our tool for unlocking that knowledge. To preserve our ability
to discover this new knowledge, we must prevent interference that blocks
the universe from our view. For optical astronomers, that means reducing
light pollution of the night sky. For radio astronomers, it means preventing
interference from "dirty" radio transmitters that spill over into our
sensitive receiving systems.
Using well-known and readily available
engineering techniques, operators of communication and satellite systems
can avoid interfering with radio astronomy. Just as we insist that
industrial firms use good engineering to avoid polluting the air, water
and soil, we must insist that firms operating radio transmitters use good
engineering to preserve humanity's precious window on the universe.
What is radio astronomy?
Radio astronomy is the study of distant objects in the universe by collecting and
analyzing the radio waves emitted by those objects. Just as optical astronomers
make images using the light emitted by celestial objects such as stars and
galaxies, radio astronomers can make images using the radio waves emitted
by such objects, as well as by gas, dust and very energetic particles in the
space between the stars. Radio astronomy has been a major factor in revolutionizing
our concepts of the universe and how it works. Radio observations have provided a
whole new outlook on objects we already knew, such as galaxies, while revealing
exciting objects such as pulsars and quasars that had been completely unexpected.
From revealing the remnant of the Big Bang to showing the afterglows of the
superenergetic Gamma Ray Bursters, radio observers have provided science with
insights unobtainable with other types of telescopes. Of the ten astronomers
who have won the Nobel Prize in Physics, six of them used radio telescopes
for the work that won them the Nobel.
Radio telescopes today are among the most powerful tools available for astronomers
studying nearly every type of object known in the universe.
What kinds of signals interfere with radio astronomy?
By international agreement, radio frequencies are divided up into
blocks, or bands, designated for different types of uses. For example,
you know that AM radio stations all are within a certain range of
frequencies that is different from the band of frequencies in which
you find FM stations. Similarly, TV stations use different frequencies
than, say, police two-way radios. These international frequency designations
are designed to prevent one type of station from interfering with stations
of another type.
A number of frequency bands are allocated to radio astronomy. Because
radio astronomers do their work with extremely sensitive receiving
equipment, transmitting is generally prohibited in the radio
astronomy bands. However, transmitters using frequencies near those
assigned to radio astronomy can cause interference to radio telescopes.
This occurs when the transmitter's output is unduly "broad," spilling
over into the radio astronomy frequencies, or when the transmitter
emits frequencies outside its intended range. Other interference arises
because radio transmitters often unintentionally emit signals at multiples
of their intended frequency.
As use of radio for devices such as cellular telephones, wireless
computer networks, garage door openers, and a whole host of other uses
continues to increase, the threats to radio astronomy from inadequately
engineered transmitters increases. A prime threat comes from transmitters
in orbiting Earth satellites, since those transmitters are located
overhead, precisely where radio astronomers must aim their telescopes
to study the universe. In addition, many types of equipment not normally
considered to be radio transmitters, particularly computers or systems
incorporating microprocessors, emit undesirable radio signals.
Graphic: 'Spillover'
into a Radio
Astronomy Band
Graphic: Effect of Radio
Interference
on Astronomical Observations
Good engineering can prevent or minimize interference to radio astronomy.
Spillover from overly-broad transmitters and other unintended signals do
nothing to improve the performance of a communication system. Technology
readily available to radio engineers can eliminate or drastically reduce
these unwanted signals that threaten radio astronomy. It is especially
important that such interference-reducing technology be included in
orbiting satellites.
Radio astronomers do much on their own to minimize the effect of
interfering signals, from locating radio telescopes far from urban centers
whenever possible to designing their antennas and electronic equipment
with features that reduce interference. Still, they need the help and
cooperation of those who design and operate radio transmitting equipment
to preserve our ability to gain new knowledge from the universe.
Communication between radio astronomers and other users of the radio
spectrum is vital. Engineers at radio telescope facilities often can help with
suggestions for ways to minimize interference. There are numerous examples
of situations in which a radio observatory and a transmitting facility have
cooperated to implement a technical solution allowing both to achieve their
objectives. For example, in 1958, the National Radio Quiet Zone
was established to minimize possible harmful interference to the NRAO in
Green Bank.
Preserving the ability to gain the scientific knowledge that can only
be gathered by radio telescopes requires effort and resources. Given
astronomy's long history of contributing to human progress and the
tremendous promise of future advances, this is an investment that will
repay handsomely for all humanity.
Further reading:
"Interference and Radioastronomy",
A. Richard Thompson, Tomas E. Gergely and Paul A. Vanden Bout,
Physics Today, November 1991, pp. 41-49.
"Light Pollution, Radio Interference and Space Debris",
conf. ser. 17, D.L. Crawford, ed., Astronomical Society of
the Pacific, San Francisco, 1991.
"Preserving the Astronomical Sky", IAU Symposium No. 196,
eds. R.J. Cohen and W.T. Sullivan III, Astronomical Society of the
Pacific, San Francisco, 2001.
National Radio
