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On this page, we have collected technical details on the reception of the JJY signal. If you have further questions, please check the Frequently Asked Questions!

Time code of the JJY signal

The carrier signal of the Standard Time and Frequency transmission at 40kHz or 60kHz directly provides an accurate frequency reference, but additional information is needed to obtain the actual time and date. This information is included in a time code that is transmitted by pulses of different length.

Recorded as audio, these pulses sound like this (original format file):

At the beginning of each second, the amplitude is increased from 10% to 100% to start a new pulse. The moment when it crosses the midpoint of 55% amplitude is synchronized with the second of Japan Standard Time. After a certain pulse duration, the amplitude then returns to 10% until the next second. Three different pulse durations encode different information:
0.8s ± 5ms represents "0" binary value.
0.5s ± 5ms represents "1" binary value.
0.2s ± 5ms is a marker ("M" or "P") for the current position in the time code.

The time information is repeated every 60s. The following example shows a signal that was transmitted on Friday, June 10 2016 at 5:15 pm.

The initial marker (M) indicates second 0 of a new minute. During the first 20 seconds, the numerical values describing this minute and the current hour are transmitted.

The minute (0 to 59) is transmitted as a 7 bit binary-coded-decimal (BCD) number. It can be found by summing the assigned values for each bit that is a binary "1". The respective bit values are 40, 20, 10, 8, 4, 2 and 1, as shown above.

The hour (0 to 23) is transmitted as a 6 bit BCD number, with individual bit values of 20, 10, 8, 4, 2 and 1.

 

The second block of 20s contains the date given as the current day of the year. Two parity bits help detect reception errors.

The day of the year is counted from "1" for January 1. It is transmitted as a 10 bit BCD number as shown above.

Each of the two parity bits is a 1-bit even parity:
The expected value of PA1 depends on the 6 bits of the hour data. Each bit that is a binary "1" adds 1 to the total. The state of PA1 is the remainder in a dividing this total by two. Here, 4 bits of the hour data were set. As 4 is divisible by 2 with no remainder, the PA1 has an expected value of "0".
The expected value of PA2 depends on the 7 bits of the minute data in the same way. Here, 3 bits are set, and the PA2 has an expected value of "1" because the dividing 3 by 2 leaves a remainder of 1.

The bits SU1 and SU2 (in the next block) are reserved. They would identify the period of daylight saving time if such a system were to be introduced in Japan.
SU1 would be set to "1" 6 days before a change to or from daylight saving time.
SU2 would be "1" during the period that daylight saving time is active.
Currently both bits are always "0", indicating normal time and no expected change.

 

The final block usually contains the last two digits of the current year, the day of the week, and an indication in the case that a leap second will be inserted or removed on the first day of the following month. Additional position markers repeat throughout the entire time code signal at each second that ends in the digit 9. Receiving two marker pulses in succession identifies the second pulse as the start of a new minute, and the time code evaluation begins from the start.

The reserved bit SU2 is described in the previous section.

The year is transmitted as a 8 bit BCD number representing the final two digits (0 to 99) as shown above. For a radio-controlled clock, these two digits are usually sufficient. There is one exception: While the year 2000 was a leap year since it is divisible by 400, the year 2100 is not. For this reason, clocks made after 2000 should consider the year "00" to be 2100 in their programming. In 2100, clocks that are over 100 years old may show an incorrect date after February 28!

The day of the week is transmitted as a 3 bit value, counting from 0 for Sunday to 7 for Saturday.

UTC allows leap seconds to be inserted or removed just before UTC 0:00 on the first day of a month, which is just before 9:00 in Japan Standard Time.
LS1 indicates that a leap second adjustment will occur within this month. In this case, it is set to "1" at 9:00 on the second day of the month, and returned to "0" after the leap second has been inserted or removed.
LS2 is set to "1" if a leap second will be inserted. It is set to "0" if a leap second will be removed (or if there is no adjustment).

The additional position markers at 9s, 19s, 29s, 39s and 49s are labeled P1 to P5. The final position marker P0 always directly precedes the start marker M at 0s. P0 occurs at 59s, unless there is a leap second adjustment. If a leap second is inserted, an additional "0" is inserted, and P0 moves to 60s. If a leap second is removed, the preceding "0" is omitted and P0 occurs at 58s.

When the minute is 15 or 45, a different final block is used. In this block, morse code replaces the regular second pulses to indicate the station call sign JJY. The final block also contains a number of bits to indicate time and duration of any scheduled interruptions of service. When no interruption is planned, all these bits are zero.

The bits ST1 to ST6 warn of an imminent service interruption of the Standard Time and Frequency signal due to maintenance or other reasons. During normal operation, all of them are "0". Otherwise, they follow these logic tables:

ST1 ST2 ST3 meaning
0 0 0 no interruption scheduled
0 0 1 service will stop within 7 days
0 1 0 service will stop within 3 to 6 days
0 1 1 service will stop within 2 days
1 0 0 service will stop within 24 hours
1 0 1 service will stop within 12 hours
1 1 1 service will stop within 2 hours

ST3 ST4 ST5 meaning
0 interruption during entire day (or no interruption)
1 service interruption only during the daytime
0 0 no scheduled interruption
0 1 unknown duration of interruption, or 7 or more days
1 0 interruption will last 2 to 6 days
1 1 interruption will last less then 2 days

 

Propagation of the JJY signal

Low-frequency signals can be received over large distances, far beyond the visibility of the transmitting antenna. Due to their large wavelength, which amounts to several kilometers for the JJY signals, interaction with the Earth's surface results in a diffraction effect that causes the emitted wave to follow the surface curvature.

A second, weaker component arrives at the same destination by reflecting off the ionosphere. The combination of the two creates a ring-shaped interference pattern around the transmitter when considering the field strength of the received signal.

Map of available 60kHz signal strength during day-time.
Expected field strength of the 60kHz JJY signal transmitted by Hagane-yama station during day-time. In the white regions (0), the amplitude of the received signal is less than 1% of the dark red region (3).
Map of available 60kHz signal strength during night-time.
Map of available 40kHz signal strength during day-time.
Map of available 40kHz signal strength during night-time.
Additional maps of expected field strength of the JJY signals. (Click to enlarge.)

These values were calculated for typical winter conditions (February 2004). The "day-time" data shows conditions that are typical between 10:00 and 14:00, while the "night-time" data represents 22:00 to 2:00. The values were set to match actual measurements taken approximately 100km from each antenna. The effective transmitted power is calculated as 12.5kW at 40kHz and 25kW at 60kHz.

The images show regions with a very weak signal, but as conditions change throughout the day, reception of the JJY signal is possible throughout Japan, unless prevented by the surrounding environment, such as a reinforced building.

Field strength predictions with distance and time

The following plots show the electric field strength prediction for the current month as a function of distance and time. Click to enlarge!

Ohtakadoya-yama Standard Radio Transmission Station

transmission frequency: 40kHz, antenna power: 50kW, transmitted power: 12.5kW

3d plot of signal strength
animated density plot
Hagane-yama Standard Radio Transmission Station

transmission frequency: 60kHz, antenna power: 50kW, transmitted power: 25kW

3d plot of signal strength
animated density plot

Predicted electric field strength by prefecture

We provide a calculated value for the expected field strength of the standard radio signal depending on the month and the location where the signal is received. The data represents the values for each hour of the day. Please note that the actual received field strength may differ.

After selecting time and location, click "Show data" and a pop-up window will open to display the data. If no window appears, you may need to set your browser to allow pop-ups for this page. If you select "archive of all prefectures", then the data for both signals will be provided as a ZIP or LZH file. If the result indicates "File not found", please try different parameters, the data might not yet be available.