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Radio frequency interference (RFI) is unwanted radio waves that are disturbing what you are trying to do with radio. For radio astronomy from Earth, such RFI includes artificial sources of radio waves transmitting at the same frequency as is being observed, e.g., for communication, range-finding, etc. Some specific bands are protected for the purposes of astronomy, through regulation, but basically all frequencies are of astronomical interest so there will always be some that suffer from interference. Methods of handling RFI include filtering specific unwanted signals, or just flagging the portion of observation data that is affected by some specific RFI. If some interference's characteristics are well understood, it may be possible to apply some kind of subtraction or other very specific removal process.