500 kHz

To facilitate communication between operators speaking different languages, standardized abbreviations were used, including a set of "Q codes" specified by the 1912 Service Regulations.

For example, Regulation 44, from the 27 July 1914, edition of Radio Communication Laws of the United States, stated: "The international standard wave length is 600 meters, and the operators of all coast stations are required, during the hours the station is in operation, to 'listen in' at intervals of not more than 15 minutes and for a period not less than 2 minutes, with the receiving apparatus tuned to receive this wave length, for the purpose of determining if any distress signals or messages are being sent and to determine if the transmitting operations of the 'listening station' are causing interference with other radio communication."

As a visual memory aid, a typical clock in a ship's radio room would have the silence periods marked by shading the sectors between h+15ᵐ to h+18ᵐ and h+45ᵐ to h+48ᵐ in RED.

Similar sectors between h+00ᵐ to h+03ᵐ and h+30ᵐ to h+33ᵐ are marked in GREEN which is the corresponding silence period for the 2182 kHz voice communications distress signals.

In addition, during this silent period all coastal and ship stations were required to monitor the frequency, listening for any distress signals.

[3][full citation needed] All large ships at sea had to monitor 500 kHz at all times, either with a licensed radio operator or with equipment (called an auto-alarm) that detected an automatically sent distress signal consisting of long dashes.

Terman's Radio Engineering Handbook (1948) shows the maximum distance for 1 kW over salt water to be 1,500 miles, and this distance was routinely covered by ships at sea, where signals from ships and nearby coastal stations would cause congestion, covering up distant and weaker signals.

However, NAVDAT uses QAM modulation (in comparison to the SITOR used by NAVTEX) and is therefore capable of much higher data throughput.