refinements for the use of 500 kHz were specified in later agreements, including the 1932 Madrid Radio Conference. In later years, except for distress traffic, stations shifted to nearby "working frequencies" to exchange messages once contact was established: 425, 454, 468, 480, and 512 kHz were used by ships while the coast stations had their own individual working frequencies. Twice each hour, all stations operating on 500 kHz were required to maintain a strictly enforced three-minute silent period, starting at 15 and 45 minutes past the hour. 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. 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. Shore stations throughout the world operated on this frequency to exchange messages with ships and to issue warning about weather and other navigational warnings. At night, transmission ranges of 3,000–4,000 miles (4,500–6,500 kilometers) were typical. Daytime ranges were much shorter, on the order of 300–1,500 miles (500–2,500 kilometers). 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. During the silence, a distress signal could more easily be heard at great distances. NAVDAT Following the adoption of GMDSS in 1999 and the subsequent obsolescence of 500 kHz as a Morse distress frequency, the 2019 World Radiocommunication Conference (WRC-19) allocated to the maritime NAVDAT service. NAVDAT is intended for the broadcast of data from shore-to-ship and may thus be compared to NAVTEX. However, NAVDAT uses QAM modulation (in comparison to the SITOR used by NAVTEX) and is therefore capable of much higher data throughput. This allows NAVDAT broadcasts to carry images and other data as well as plain text,