Protecting Radio Astronomy
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.
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)
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 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.
Specific Protected radio astronomy frequencies:
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.
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”:
| Next |