Knowledge of signal communications can be very valuable if radio equipment is inoperative, or there is no common frequency between you and the station you wish to communicate with, or where there is a language difference. Vessels within sight or hearing of each other may communicate using code flags, flashing lights, or sound signals.
The International Code of Signals
The International Code of Signals is published by the National Imagery and Mapping Agency as Publication No. 102. Pub. 102 contains signals that can be sent by flag hoists, flashing light and sound, and other means. The International Morse code is used for flashing light and sound signals, as well as radiotelegraphy. When you know these signals, try your hand at translations using semaphore, Morse code, or international code flags.
Signals basically consist of single letters and two-letter combinations; three-letter combinations, all starting with "M," are used solely for medical messages. Many signals can be expanded or made more specific with complements. These may express a variation in the meaning of the basic signal; questions and answers related to the meaning of the basic signal; or supplementary, specific, or more detailed meanings of the basic signal. Most complements consist of a single numeral, but there may be two, three, four, five, or six numerals when signaling such values as latitude and longitude, time, or date.
Signals consisting solely of one letter are used for the most basic messages. Typical ones are listed above. There is also another set of single-letter signals that is used only with numerical complements to communicate azimuth or bearing, course, speed, latitude and longitude, or time and date. Yet another set of single-letter signals, usually made by sound or radiotelephony, is used between icebreakers and assisted vessels.
The General Signal Code consisting of signals of two letters, many with a single numerical complement, is used for many types of messages. Typical signals are:
Flag Hoist Signaling
The most common use of the International Code of Signals is visual signaling using code flags. A set of flags consists of 26 flags for the letters of the alphabet, ten number pennants, three substitute pennants, and the answering pennant. (The U.S. Navy uses a fourth substitute and calls them all "repeaters.") The substitutes are necessary because a set contains only one flag for each letter and some flag hoists require repetitions of one or more letters.
Five standard colors are used — red, white, blue, yellow, and black. Most of the flags are of two colors, selected and arranged for maximum contrast. Two flags are of a single color only, three use three colors, and one uses four colors.
Flag hoists are read from the top flag or pennant downward; if there is more than one hoist from a single spreader or yardarm, each hoist is read in turn from outermost inward. Additional details on flag hoist signaling can be found in the International Code of Signals Pub. 102.