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SIO is an abbreviation for "Strength, Interference, Overall". These three categories are used to describe how well a radio signal was received. Signal strength, signal quality as affected by interference, and overall quality of reception, are each given a value from 1 to 5, with higher numbers meaning better reception. So, a perfect signal, very strong, would rate an SIO of 555. A strong signal with a lot of interference might rate SIO of 523, and a barely audible signal wither no interference may get a rating of SIO 151. See also SINPO.