especially in reception reports written by shortwave listeners. Each letter of the code stands for a specific factor of the signal, and each item is graded on a 1 to 5 scale (where 1 stands for nearly undetectable/severe/unusable and 5 for excellent/nil/extremely strong).
The code originated with the CCIR (a predecessor to the ITU-R) in 1951, and was widely used by BBC shortwave listeners to submit signal reports, with many going so far as to mail audio recordings to the BBC's offices.
The use of the SINPO code can be subjective and may vary from person to person. Not all shortwave listeners are conversant with the SINPO code and prefer using plain language instead.
Each category is rated from 1 to 5 with 1 being 'unusable' or 'severe' and 5 being 'perfect' or 'nil'. MANY raters misunderstand the code and will rate everything either 55555 or 11111 when in reality both extremes are unusual in the extreme. '55555' essentially means 'perfect reception akin to a local station' while that is occasionally possible, when talking about long-distance short-wave reception, it is almost never the case.