KRISS (Korea Research Institute of Standards and Science) has been operating the standard time and frequency broadcasting station (call sign: HLA) since 1984. The station HLA broadcasts continuous time and frequency signals on 5 MHz carrier frequency and 2 kW transmit power. But the poor reception due to radio interference is decreasing users. Therefore to increase the radio reception, a digital modulation method and an increase of bit rate are introduced additionally to the present amplitude-modulated radio signal. We describe a digital modulation method for improving radio reception of the standard time and frequency by shortwave.
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