Time synchronization over radio

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Time synchronization over radio is the procedure used for time transfer performed by humans over two-way radio circuits, including voice, telegraph, and teletype. [1]

Time transfer is a scheme where multiple sites share a precise reference time. Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomena with each other, as well as cell phone towers coordinating handoffs as a phone moves from one cell to another.

Two-way radio A radio that can do both transmit and receive a signal

A two-way radio is a radio that can both transmit and receive a signal, unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirectional person-to-person voice communication with other users with similar radios. Two-way radios are available in stationary, mobile, and hand-held portable models. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds.


Radiotelegraph procedure

ACP 124(A) is the earliest Allied military document of the Cold War to define a time transfer method. It consists of:

Cold War Geopolitical tension after World War II between the Eastern and Western Bloc

The Cold War was a period of geopolitical tension between the Soviet Union with its satellite states, and the United States with its allies after World War II. The historiography of the conflict began between 1946 and 1947. The Cold War began to de-escalate after the Revolutions of 1989. The collapse of the USSR in 1991 was the end of the Cold War. The term "cold" is used because there was no large-scale fighting directly between the two sides, but they each supported major regional conflicts known as proxy wars. The conflict split the temporary wartime alliance against Nazi Germany and its allies, leaving the USSR and the US as two superpowers with profound economic and political differences.

RequestINT ZUA"Request a time signal now (or at ______)"
ReplyZUA 1500"Time signal will be transmitting at 1500." The numerals indicating the time will be followed by a 5-second dash, terminating exactly at the time indicated.

Note that ZUA is a military-only operating signal. Civilians would need to use the appropriate Q code operating signal, which also uses IMI following the signal, instead of INT preceding the signal:

Operating signals are one type of brevity code wherein the signals are designed and used primarily to support the communication of the communications operators among themselves with respect to communications operations, instead of communicating abbreviated messages about non-communications related activities.

The Q-code is a standardized collection of three-letter codes all of which start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially amateur radio. To distinguish the use of a Q-code transmitted as a question from the same Q-code transmitted as a statement, operators either prefixed it with the military network question marker "INT" or suffixed it with the standard Morse question mark UD.

RequestQTR IMI"Request a time signal now (or at ______)"
ReplyQTR 1500"Time signal will be transmitting at 1500." The numerals indicating the time will be followed by a 5-second dash, terminating exactly at the time indicated.

Telephone and radiotelephone procedure

ACP 125(A) describes the format:

Request"Request a time signal now" or "Request time signal at _____."
Reply"When I say 'TIME' it will be exactly 1500. 15 seconds, 10 seconds, 5-4-3-2-1 TIME 1500."

ACP 125(G) updates the above procedure as follows: [2]

Stations without the ability to acquire a time signal accurate to at least one second should request a time check at the start of every shift, or once a day minimum. Stations may ask the NCS for a time check by waiting for an appropriate pause, keying up and stating your call sign, and then using the prowords "REQUEST TIME CHECK, OVER" when the NCS calls on you. Otherwise, you may ask any station that has access to any of the above time signals for a time check.

Once requested, the sending station will state the current UTC time plus one minute, followed by a countdown as follows:


The receiving station will then use the proword "TIME" as the synch mark, indicating zero seconds. If the local time is desired instead of UTC, substitute the time zone code "JULIETT" for "ZULU".

Instead of providing time checks on an individual basis, the NCS should give advance notice of a time check by stating, for example, "TIME CHECK AT 0900 JULIETT", giving all stations sufficient time to prepare their clocks and watches for adjustment. A period of at least five minutes is suggested.

Teletypewriter procedure

ACP 126(C): CommunicationCP 168s Instructions—Teletypewriter (Teleprinter) Procedures [3] no longer describes the format

ACP 126(A) and possibly (B) described the format:

RequestINT ZUA"Request a time signal now (or at ______)"
ReplyZUA 1500, 5-4-3-2-1-0.

Due to the fact that different types of machines are used by the Member Nations, the time will be indicated by the printing:

Visual procedure

ACP 129: Communication Instructions—Visual Signaling Procedures originally documented the visual signalling procedure for time transfer. Subsequently, ACP 129 was combined with ACP 168: Pyrotechnic Signals to become ACP 130(A):Communications Instructions—Signalling Procedures in the Visual Medium. [4]

Time by signal lamps

Example: D46 conveys intention to D06 to make a timing signal.
D46 makesD06 makes
D06 (until answered)K
0845 (time zone)Flash
D46 makes a timing signal
D06 (until answered)K
0845 (time zone)Flash
5 second flash5 second flash

ZUJ means "Stand by."

ZUA means "Timing signal will be transmitted now (or at...)"

Time by flag Hoists

Flag TANGO, followed by two or four numerals, also signifies a time check.

D46 makesD06 makes
D06 (until answered)K
Flag TFlash
1645 (time zone)Flash
(to execute)
D06 (until answered)K
5 second flash5 second flash

Time by time bells

The time signal indicated is at the instant of execution or commencement of hauling down of the shape.

See also

Related Research Articles


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A radiogram is a formal written message transmitted by radio. Also known as a radio telegram or radio telegraphic message, radiograms use a standardized message format, form and radiotelephone and/or radiotelegraph transmission procedures. These procedures typically provide a means of transmitting the content of the messages without including the names of the various headers and message sections, so as to minimize the time needed to transmit messages over limited and/or congested radio channels. Various formats have been used historically by maritime radio services, military organizations, and Amateur Radio organizations.

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ACP 125 is the short name for Allied Communications Publication 125: Communications Instructions—Radiotelephone Procedures, developed and published by the Combined Communications Electronics Board, for use by the Five Eyes nations and the rest of NATO. According to the latest version,

"The aim of ACP 125 is to prescribe the voice procedure for use by the armed forces of Allied nations on secure and non-secure tactical voice nets. Its purpose is to provide a standardized way of passing speech and data traffic as securely as possible consistent with accuracy, speed and the needs of command and control."

Allied Communication Procedures is the set of manuals and supplements published by the Combined Communications Electronics Board that prescribe the methods and standards to be used while conducting visual, audible, radiotelegraph, and radiotelephone communications within NATO member nations. These procedures relate to procedure words, radiotelephony procedure, Allied Military phonetic spelling alphabets, plain language radio checks, the 16-line message format (radiogram), and others.

Allied Communications Publications are documents developed by the Combined Communications-Electronics Board and NATO, which define the procedures for communicating in computer messaging, radiotelephony, radiotelegraph, radioteletype (RATT), air-to-ground signalling, and other forms of communications used by the armed forces of the five CCEB member countries and/or NATO.


  1. "SGM-136-52: Agreement on Methods of Synchronising Time" (PDF).
  2. Mills, C.J., ed. (2016). ACP 125(G): Communications Instructions – Radiotelephone Procedures (PDF) (Unclassified, public military procedures document.) ((G) ed.). Combined Communications-Electronics Board (published 2016-11-28). Retrieved 2017-12-14.
  3. "ACP 126(C): Communications Instructions—Teletypewriter (Teleprinter) Procedures" (PDF).
  4. "ACP 130(A):Communications Instructions—Signalling Procedures in the Visual Medium" (PDF).