Protecting Radio Astronomy

By Tim Hunter
 

Introduction – The Radio Spectrum and Its Regulation

The average citizen is familiar with the radio spectrum through enjoyment of AM and FM radio, television, short wave radio, amateur radio, and the use of ancillary items, such as garage door openers, cordless phones, microwave ovens, and wireless computer networks. Radio frequency use is ubiquitous and taken for granted. However, it is not an unlimited or harmless resource that can be used at will.

The radio spectrum encompasses a very wide range of frequencies and wavelengths. For the purposes of this essay, all electromagnetic emissions beyond the infrared portion of the spectrum with wavelengths of 1000 microns (1mm) or longer will be considered part of the radio spectrum. The main portions of the radio spectrum are:

From http://www.jneuhaus.com/fccindex/spectrum.html by John Neuhaus, WA2JXE

Most radio waves travel through the atmosphere with little attenuation and can be detected at large distances. Transmissions from one source may greatly interfere with signals from another source. Some interference is natural, such as radio static produced by a thunderstorm, but much of it is manmade. In most countries, no one is allowed to transmit any radio signal of appreciable strength without being licensed to do so or without using equipment that has been certified to be appropriate for its intended use.

The International Telecommunications Union (ITU)

The International Telecommunications Union (ITU) located in Geneva, Switzerland, is the world’s oldest international organization, established in 1865 to manage the first international telegraph networks. It is a specialized agency of the United Nations, and its membership includes almost all the world’s countries and over 500 private members (ITU, 2004). ITU’s purpose is to develop and manage the world’s telecommunications resources.

Under the auspices of the ITU, World Radiocommunication Conferences (WRC) are held every two to three years to review, and, if necessary, revise the Radio Regulations, which is the international treaty governing the use of the radio frequency spectrum, and satellite orbits and communications (ITU, 2004). The Radio Regulations are designed to provide an orderly operation of the radio frequencies worldwide.

The United States Communications Act of 1934 (as revised) authorizes the United States Commerce Department’s National Telecommunications and Information Administration (NTIA) and the Federal Communications Commission (FCC) to manage the use within the United States of the radio frequency spectrum. Some portions of the spectrum are managed jointly by NTIA and the FCC, and some are managed exclusively by either one or the other (Schroeder, 2004). Below is a reduced image showing a chart of the United States frequency allocations:

From http://www.ntia.doc.gov/osmhome/allochrt.html

Most countries of the world comply with spectrum allocations specified in the ITU Radio Regulations’ Article 5 (International Table of Frequency Allocations). In the US, thirty different radio services are allocated portions of the radio spectrum over 450 separate frequency bands.

Radio Astronomy Interference

Radio astronomy’s reach extends from the Solar System to distant galaxies and the cosmic microwave background radiation left over from the Big Bang. “Radio telescopes today are among the most powerful tools available for astronomers studying nearly every type of object known in the universe” (NRAO, 2004).

Radio Astronomy Frequencies

The portion of the radio spectrum typically used in radio astronomy is from approximately 13 MHz to 10 GHz with the 1-10 GHz range being especially important. Frequencies above I GHz (1000 MHz) are referred to as microwave frequencies.

Some significant radio astronomy frequencies:

13.36 - 13.41 MHz - Solar and planetary observations

25.55 - 25.67 MHz - Solar and planetary observations

73.00 - 74.60 MHz – Pulsar detection

150.05 - 153.00 MHz – Pulsar detection

406.10 - 410.00 MHz – Pulsar detection

1400.0 - 1427.0 MHz – Hydrogen line measurements

1600-1700 MHz – Hydroxyl line measurements

from: SETI League, 2003. See also Appendix A, NTIA.

Specific Protected radio astronomy frequencies:

• 13.36-13.41 MHz
• 25.55-25.67 MHz
• 38.0-38.25 MHz
• 73-74.6 MHz
• 406.1-410 MHz
• 1400-1427 MHz [Hydrogen Line at 1420.406 MHz]
• Radio Astronomy on a secondary basis 1610-1626.5 MHz
• 1660.5-1668.4 MHz [Hydroxyl Spectral Line 1665 and 1667 MHz]
• Radio Astronomy on a secondary basis 1718.8-1722.2 MHz
• 2690-2700 MHz
• 4825-4865, 4950-4990 MHz
• Formaldehyde Line 14,470-14,500 MHz
• 31.2-31.3, 36.43-36.5, 42.5-43.5, 48.94-49.04 GHz
• 97.88-98.08, 140.69-140.98, 142-149 GHz
• 262.24-262.76, 265-275 GHz

Adapted from: http://www.jneuhaus.com/fccindex/10_khz.html#10_KHz ; http://www.jneuhaus.com/fccindex/index.html#Freq_chart; http://www.ntia.doc.gov/

Frequencies have been allocated to radio astronomy to keep these bands clear of radio transmission, allowing radio astronomers to detect faint signals from cosmic sources. Radio frequency interference (RFI) is any radio signal that is not of cosmic origin that interferes with radio astronomical observations (ATNF, 2004). It is very important to note that in some instances what radio astronomers consider RFI is a signal of considerable value for other users, and radio astronomers are very careful to coexist with such uses (NRAO, 2004).

The radio signals from astronomical objects are extremely weak, millions or billions of times weaker than signals used by ordinary communications systems. Detectable astronomical radio sources range in strength from around 10^-3 to 10^-6 Jansky. Translating this into watts per square meter per Hertz of frequency means the very brightest radio sources produce a signal of only 10^-20 W m-2 Hz-1 (Kitchen, 2003). A cellular phone used on the Moon produces a strong signal compared to most radio astronomical signals (NRAO, 2004). Not only can manmade interference overwhelm astronomical radio signals, but weak interfering signals can contaminate legitimate data and lead to erroneous interpretations.

Just because a frequency is allocated to radio astronomy, does not mean that there isn’t considerable interference. Some of the allocated frequency bands are close to legal high power transmission users. Even very well designed transmitters may have spurious signals that send a small fraction of their power into bands away from their allocated frequencies. Though such signals are very small, they are often many times larger than signals used by radio astronomers. For this reason, there is great effort to protect important radio astronomy frequencies, and most radio telescopes are located as far away from civilization as possible.

Radio astronomers sometimes use bands that are not officially protected. Jupiter has interesting radiation in the 15-30 MHz range, but most of this is not protected (Ukaranet, 2004). The FM broadcast band at 88-108 MHz is another region where radio astronomers sometimes wish to work.

Radio astronomy is usually passive (receive only) (ATNF, 2004). Sometimes, large radio telescopes transmit radar signals to study Solar System objects. Threats to radio astronomy come from mostly mundane sources, cellular telephones, wireless computer networks, garage door openers, and, especially from Earth orbiting satellites (NRAO, 2004). Satellite transmitters are often overhead when a radio telescope is poised for observation.

The Water Hole

Between 1 GHz to 10 GHz there is a relatively quiet region in the radio spectrum with little natural Galactic, extragalactic, or atmospheric radio noise (Exploratorium, 2004). Neutral hydrogen gas (HI) emits radio signals at 1.42 GHz. The hydroxyl ion OH- emits at 1.64 GHz. The frequency range between 1.42 and 1.64 GHz is sometimes called the “water hole,” and it is a critical band for research and for the Search for Extraterrestrial Intelligence (SETI). The diagrams below show Galactic and atmospheric noise and the “water hole”:

from http://www.exploratorium.edu/learning_studio/news/october97/mainstory5_oct97.html.

from http://www.setileague.org/general/waterhol.htm.

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