Sources of Interference
Radio frequency “noise” complicates the task of the radio astronomer, at times making it difficult to distinguish emissions from an object under study from extraneous emissions produced by other nearby sources. Interference comes from both natural and artificial sources, the latter ones becoming a bigger problem every day. By international agreement (the World Administrative Radio Conference), certain frequencies have been allocated strictly for radio astronomy. However, there is disagreement about how far beyond the restricted limits is acceptable spillover” (for example, radio broadcasters may think 10mm over their wavelength limit is acceptable, while radio astronomers may think .001 mm is too much). In some countries, the restrictions are not enforced, so may as well not exist.
Natural sources of interference include:
 • Radio emissions from the Sun
•  Lightning
•  Emissions from charged particles (ions) in the upper atmosphere
Among the growing list of human-made sources of interference are:
 • Power-generating and transforming facilities
•  Airborne radar
•  Ground-based radio and television transmitters (which are getting more powerful
all the time)
• Earth-orbiting satellite transmitters and transponders, including Global Positioning
Satellites (GPS)
•  Cellular phones
Human-generated interference that originates on the ground (such as radio and television transmissions) travels along the ground and over the horizon. It used to be that such interference tended to be weak at ground level, increasing in strength with height above ground. For this reason, most radio telescopes have been situated in valleys or other low places, unlike optical telescopes which are often built on mountain tops. However, more and more, interference at ground level is becoming a problem even for low-lying radio telescopes.