American Radio Relay League (ARRL) employs postcard based recipient verification for amateur radio operators by mailing to their registered license address with

American Radio Relay League (ARRL) employs postcard based recipient verification for amateur radio operators by mailing to their registered license address with FCC, a public record. They use this to sign a user certificate (X.509) which presently can be used to access Logbook of the World (LoTW), a web based logbook for radio operators to record contact with other operators.