By the diffraction of long wave, the cover range of a regular 50kW time signal station (ERP = 23kW) can achieve to 1,500km, and such coverage undoubtly meet the requirement for a country that has a big territory, like Japan and Germany. Figure 2a shows the coverage of JJY, built by Japan.
However, we should concern that some countries has a smaller territory, like Taiwan, it is only 400 km in length. For such country, it is not necessary to spent 17 million US dollars for buying a 50kW site. From the field strength plot in ITU-R P.368-9, a 1kW time signal station (ERP = 460W) can meet the goal to cover the country with 50dBuV/m at least. Figure 2b and 2c show the field strength estimation and the coverage in geography.
Before I joined the research group (Telecom Lab 113, Chunghwa Telecom), the research group had already defined the time code format/symbol for Taiwan, see figure 2d. A valid time code "packet" is 120 bits in length, being transmitted in 1 minutes; namely, the bitrate is 2 bps (no typo, the popular spec is 1bps.). As the figure 2d shows, 18 seconds are used to send the time information, telling the receiver about the minutes, hours, day, weekday, month, year, leap shift, and daylight saving time. Meanwhile, there are 38 seconds used to transmit the defined function, like the weather, disaster, and some other public information, and the rest of 4 seconds are used to do synchronization.
After the infomation is encoded, these bits applies the Amplitude-Shift-Keying, with variant reduction period to represent for different symbols (kinda feeling of Pulse Width Modulation, PWM). There are 5 symbols defined at the moment, 200ms for symbol 0(00), 400ms for symbol 1(01), 600ms for symbol 3(11), 800ms for symbol 2(10), and 0ms for symbol SYNC. These manners can be observed from figure 2e.
When the baseband keying is done, the ASK signal will be sent to the RF modulator/Amplifier, and the radio applies conventional AM scheme with the 77.5kHz carrier. Due to the long wavelength, it is impossible to build a quarter-wavelength antenna (nearly 970 meters). Therefore, a shorter antenna (200 meters) makes the power efficiency of the transmitter seriously decay, and that is the reason for us to discuss the specs of RF transmitter in kW range.