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Radionavigation Satellite Service (RNSS) systems, such as the
U.S. Global Positioning System (GPS) and the Russian Global
Navigation Satellite System (GLONASS), are very important for
worldwide navigation. Their transmission is mainly from the
satellites to the Earth. These systems have widespread
application for public safety, but they unfortunately have
various spectrum allocations from approximately 900 MHz to 1700
MHz putting the radio astronomy water hole at risk.
The GLONASS transmissions, for example, are so powerful they are
received in the side lobes and back lobes of radio telescopes no
matter where in the sky the telescope is pointing as long as one
of the satellites is above the horizon. Such interference may
mean almost half of a radio telescope observations have to be
discarded (Galt, 1991). The figure below demonstrates the
potential deleterious effects of satellite transmissions:
From: http://www.nrao.edu/whatisra/images/goodbad2.jpg
Protecting Radio Astronomy
National Radio Quiet Zone
In 1958, a 13,000 square mile National Radio
Quiet Zone (NRQZ) was established in the United States near the
state border between Virginia and West Virginia. It is designed
to protect the National Radio Astronomy Observatory (NRAO) in
Green Bank, WV, and the radio receiving facilities for the
United States Navy in Sugar Grove, WV (NRAO, 2004). Similar
radio quiet zones exist or have been proposed for other large
radio telescopes.
The IAU and WRC-00
The International Astronomical Union (IAU), the
main professional organization of the world’s astronomers, has
long recognized the importance of preserving astronomical
observing sites from degradation, including light pollution and
radio frequency interference. Commission 40 (Radio Astronomy)
and Commission 50 (Protection of Existing and Potential
Observatory Sites) of the IAU have been especially active in
this regard (IAU, 2004). In North America and in Europe there
are respective coordinating groups (CORF, the Committee on Radio
Frequencies, and CRAF, the Committee for Radio Astronomy
Frequencies) to integrate the interests of radio astronomers
with those of others using the radio spectrum.
WRO-00, which met in Istanbul, Turkey, in 2000 set aside for
protection all the frequencies between 71 and 275 GHz currently
being used in radio astronomy, and added more than 90 GHz of
spectrum to the 44 GHz of the spectrum already set aside in this
frequency range (IDA, 2004). The frequencies previously
allocated to satellite downlinks in the 71-275 GHz range were
reset at frequencies not presently being used for scientific
purposes. No satellites currently operate at these high
frequencies, and protecting the 71-275 GHz range means no
present commercial equipment needs to be changed.
Such protection is welcome for existing millimeter wave
telescopes and for large projects coming on line, such as the
Square Kilometer Array (SKA) and the Atacama Large Millimeter
Array (ALMA).
Conclusions
Protecting radio astronomy is a difficult task. Legal protection
of those frequencies of importance to radio astronomy is a
requisite first step. Transmissions near important radio bands
should be limited, particularly satellite transmission. This
limitation can range from frequency restrictions to power
restrictions and times of operation. Where possible, such
transmissions should cease whenever the satellite is within the
viewing area of a major radio telescope.
Radio telescopes situated in remote locations should receive
radio quiet zone protection. Radio astronomers have become adept
at shielding their equipment and developing algorithms for
finding and eliminating RFI from their data. Radio astronomy is
on the frontier of astronomical research. For it to continue,
the astronomical community and the public must vigorously
protect it.
References:
Appendix A. World radio astronomy observatories. NTIA web site
at: http://www.ntia.doc.gov/osmhome/reports/pub9835/Raspapnd.htm.
Australian Telescope National Facility (ATNF). International
Interference Mitigation (for Radio Astronomy) at: http://www.atnf.csiro.au/SKA/intmit/
Exploratorium at: http://www.exploratorium.edu/learning_studio/news/october97/mainstory5_oct97.html.
Galt, J. In: Crawford DL. Editor. Light Pollution, Radio
Interference and Space Debris. Conference Series 17,
Astronomical Society of the Pacific, San Francisco, 1991,
213-221.
International Dark-Sky Association (IDA) at: http://www.darksky.org
IAU Commissions at: http://www.iau.org/Organization/divcom/commissions.html.
International Telecommunications Union at: http://www.itu.int/aboutitu/overview/foreward.html
Kitchin CR. Astrophysical Techniques. 4th Edition. Institute of
Physics Publishing, Bristol and Philadelphia, 2003, 103.
National Radio Astronomy Observatory (NRAO) at: http://www.nrao.edu/whatisra/rfi.shtml
Schroeder N, Murray M. Radiofrequency spectrum allocations in
the United States. Background paper. January 2004. At: http://www.ntia.doc.gov/osmhome/chart_03.htm.
SETI League. Significant radio astronomy frequencies; IAU
protected segments at: http://www.setileague.org/articles/protectd.htm ; General
information at: http://www.setileague.org/general/waterhol.htm.
Ukaranet at: http://www.ukaranet.org.uk/basics/frequency_allocation.htm.
First posted 11 February 2005
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