What's Up With "10-4" And Other Radio Lingo?

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Reader and frequent question-asker Nate J. wrote in wondering why we say things like "10-4" and "Roger" on walkie-talkies and other two-way radios.

The Ten-Codes

The ten-codes or ten-signals are code words used as stand-ins for common phrases in radio communication. Charles Hopper, a communications director with the Illinois State Police, developed them in 1937 to combat the problem of the first syllables or words of a transmission being cut off or misunderstood. Preceding every code with "ten" gave the sometimes slow equipment time to warm up and improved the likelihood that a listener would understand the important part of a message. The codes also allowed for brevity and standardization in radio message traffic.

The codes were expanded by the Association of Public-Safety Communications Officials-International (APCO) in 1974 and were used by both law enforcement agencies and civilian CB radio users. Over time, differing meanings for the codes came about in different agencies and jurisdictions, undoing the codes' usefulness as a concise and standardized system. The problem came to a head in 2005 during rescue operations after Hurricane Katrina. After several instances of inter-agency communication problems, the United States Federal Emergency Management Agency (FEMA) discouraged the use of ten-codes and today the federal government recommends they be replaced with plain, everyday language. Here's the official APCO list.

Roger That

In the days of the telegraph, the Morse code letter R (dot-dash-dot) was sometimes used to indicate "received" or "message received/understood." When radio voice communication began to replace telegraphs, Roger, the code word assigned to the letter R in the Joint Army/Navy Phonetic Alphabet (the radio alphabet used by all branches of the United States military from 1941 to 1956), took on the same role.

Contrary to what Hollywood would have you think, Roger only means "last transmission received/understood" and does not also mean or imply "I will comply." Wilco (Will Comply) is the code used if the speaker intends to convey "message received and will comply." Given that, the phrase Roger Wilco, which you so often hear in the movies, is redundant and not really used since Wilco alone covers all the bases and acknowledges receipt of message and states intent to comply.


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Mayday is an international code word used to signal life-threatening emergencies. It was originated in 1923 by Frederick Stanley Mockford, a senior radio officer at London's Croydon Airport. He was given the task of coming up with a unique and easily understandable emergency code word. Most of the air traffic at Croydon was either coming from or going to Le Bourget Airport in Paris, so Mockford chose mayday because of its similarity to the French m'aider ("come help me").

Because it is an emergency signal, there are plenty of rules governing the use and format of a mayday call. A mayday call can only be made when life or craft is in imminent danger of death or destruction (and, as with fake 9-1-1 calls, fake mayday calls are considered serious crimes. In the U.S., making a fake distress call is a federal crime that can carry large fines and jail time), and once one is made, no other messages can be transmitted except to assist in the emergency.

Correct format for a Mayday call is:

- "Mayday, Mayday, Mayday." The call is always given three times in a row to keep it from being mistaken for a similar-sounding phrase, or to distinguish an actual distress signal from a message about a mayday call.

- "This is ____." The vessel name is repeated three times, followed by call sign if available.

- "Mayday. [vessel name]."

- "My position is ____." Position is given in latitude-longitude coordinates, or bearing and distance from a fixed point.

- "I am _____." The type of emergency, e.g. fire or sinking.

- "I require immediate assistance."

- "I have _____." The number of people on board and their condition, as well as any other pertinent information, e.g. abandoning to life rafts.

- "Over."

Some radio instructors suggest the mnemonic MIPDANIO for learning mayday signal format: Mayday, Identify, Position, Distress, Assistance, Number of crew, Information, Over.

Phonetic or Spelling Alphabets

The problem: a lot of letters sound alike. If two people are talking to each other on the telephone or a two-way radio, an "N" might be misheard as an "M," or a "B" might be heard as a "D," "T," or "C." If one person tells the other that the secret code that must be entered to save the world from imminent disaster is "M-A-T-T", there's a chance that we might all die because a little bit of static could cause the code to be heard as "N-8-B-D."

The solution: a phonetic alphabet or spelling alphabet, where code words are assigned to the letters of the alphabet acrophonically (that is, the code word assigned to a given letter starts with that letter, like Alpha for "A"). Using the Joint Army/Navy Phonetic Alphabet mentioned earlier, the above code becomes "Mike-Able-Tare-Tare" which is a little harder to mishear.

Today, the most commonly used spelling alphabet is the International Radiotelephony Spelling Alphabet. The alphabet was developed by the International Air Transport Association (IATA) and International Civil Aviation Organization (ICAO) after World War II and later adopted by organizations like International Maritime Organization (IMO), the Federal Aviation Administration (FAA), the American Radio Relay League (ARRL), and the North Atlantic Treaty Organization (NATO) (whose use of the alphabet led to its global spread and use of the common name, NATO phonetic alphabet).

In addition to these (and other) organizations, the alphabet is used by the military and in civilian industries (retail, IT and airlines for instance). The alphabet in its current form is:

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October 23, 2012 - 5:30am
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