BPC (time signal)

BPC is the callsign of a time signal broadcasting from the BPC Shangqiu Low-Frequency Time-Code Radio Station, cooperatively constructed by the National Time Service Center of the Chinese Academy of Sciences and Xi'an Gaohua Technology Co., Ltd., beginning April 25, 2002.

BPC transmits a time signal on 68.5 kHz, which can be used for synchronizing radio controlled clocks. The transmission site is situated near Shangqiu, Henan Province ^[1] at 34°27′25″N115°50′13″E / 34.457°N 115.837°E / 34.457; 115.837 . ^[2]

BPC broadcasts at 90 kW for 20 hours per day, with a 4-hour break from 05:00–09:00 China Standard Time daily (21:00–01:00 UTC). ^[3] BPC includes both a conventional amplitude-modulated time code transmitted during the first 400  of each second, and an additional phase-modulated spread-spectrum time code transmitted during the last 600 ms of each second, about which little is known. ^[4]

Time code

BPC transmits the time every 20 seconds, using an amplitude-modulated binary code sent at 2 bits per second. Each 20-second block encodes the China Standard Time of the beginning of that block. ^[5]

To encode each pair of bits, the transmitter is reduced by 10 dB (to 10% of normal power) at the beginning of each second, and restored to full power after a multiple of 0.1 seconds. The duration of the reduction encodes the bits, as follows:

BPC bit coding
Signal reduced at the beginning of each second

Duration	MSbit	LSbit
0 ms	Start of time code
100 ms	0	0
200 ms	0	1
300 ms	1	0
400 ms	1	1

If there is no signal reduction at all, that is a special marker which marks the beginning of the time code.

BPC time code, ^[6] sent every 20 seconds on the minute

Second	MSbit LSbit	Meaning		Second	MSbit LSbit	Meaning
00	—	Start of time code No signal gap		10	12 P1	Hour (0=AM 1=PM) Even parity over 01–09
01	40 20	Second (00, 20 or 40)		11	0	Unused
					16	Day of month (01–31)
02	0 0	Unused		12	8 4
03	8 4	Hour (00–11)		13	2 1
04	2 1			14	8 4	Month (01–12)
05	32 16	Minute (00–59)		15	2 1
06	8 4			16	32 16	Year (00–99)
07	2 1			17	8 4
08	0	Unused		18	2 1
	4	Day of week 1=Monday 7=Sunday
09	2 1			19	64
					P2	Even parity over 11–18

Note that the bits sent in the same second as the parity bits are not parity-checked.

References

1. "Low-frequency radio time signals". Cl.cam.ac.uk. Retrieved 2011-10-20.
2. "::时间服务::". Archived from the original on 2012-06-17. Retrieved 2013-03-16.
3. 西安高华科技有限公司 (2020-01-10). "河南商丘BPC电波塔发播时间公布" (in Chinese). Archived from the original on 2021-07-29. Retrieved 2021-07-29.
4. Feng, Ping; Wu, Guichen; Bai, Yan; Ding, Xiaofeng (August 2010), "Additional spread spectrum modulation timing method in BPC" (PDF), in Tan, Jiubin; Wen, Xianfang (eds.), Sixth International Symposium on Precision Engineering Measurements and Instrumentation, Proceedings of SPIE, vol. 7544, pp. 75441X–75441X–6, Bibcode:2010SPIE.7544E..1XF, doi:10.1117/12.885845, S2CID   110865336 The paper appears to be describing a proposed addition to the time signal, but the tenses are confusing and sometimes implies that the addition has been implemented.
5. Nikitin, Vladimir (27 Dec 2014). 68.5 time signal – via YouTube. The audio quality is good enough that the AM modulation can easily be seen in an audio editor. The sample is 65 seconds long, from 23:33:06 to 23:34:11 displayed time, so includes two complete and two partial time frames. In the last 11 seconds of the video, starting at 23:34:00 on a Saturday, the encoded time of day is — 00 00 01 11 10 00 10 01 11 00, which decodes to 07:34:00 on a Sunday. This makes sense if the displayed time UTC, as CST is UTC+8.
6. Description (in Chinese) Includes sample of time code captured around 2019-12-01 15:52. The time code diagram starts with the seconds field numbered "0" and ends with the marker numbered "19" but note in the text below that it corresponds to second 1, 21 or 41. The screenshot of a waterfall plot with the BPC signal shows that there is no signal reduction at 19:37:40.

External links

• 低频时码授时, from the National Time Service Center, Chinese Academy of Sciences