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 34.457°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.