RST

From HFUnderground

The RST system is a standardized way for amateur radio operators (hams) to describe the quality of a radio signal during reception.

• R (Readability): A scale from 1 to 5 that indicates how understandable the signal is, with 5 being perfectly readable and 1 being unreadable.
• S (Strength): A scale from 1 to 9 that measures the strength of the received signal, with 9 being extremely strong and 1 being very faint.
• T (Tone): Used only for CW (Morse code) or digital modes, it ranges from 1 to 9, describing the quality of the signal tone, with 9 being pure and stable.

An RST report might look like "599," which means perfectly readable, very strong signal, and pure tone. It’s a quick and efficient way to communicate signal quality during contacts.