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 at.

BPC broadcasts at 90 kW for 20 hours per day, with a 4-hour break from 05:00–09:00 China Standard Time daily (21:00–1:00 UTC). BPC includes both conventional amplitude modulated time code and additional spread-spectrum time code, about which little is known.

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.

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:

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

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