Where does the expression "Roger that" come from ?

"Roger" was "phonetic" for "R" (received and understood". In radio communication, a "spelling alphabet" (often mistakenly called a "phonetic alphabet) is used to avoid confusion between similarly sounding letters. In the previously used US spelling alphabet, R was Roger, which in radio voice procedure means "Received". While in the current spelling alphabet (NATO), R is now Romeo, Roger has remained the response meaning "received" in radio voice procedure. In the US military, it is common to reply to another's assertion with "Roger that", meaning: "I agree".