Traceability of network time service to UTC(NIM): online calibration

Network time service provided by Network Time Protocol (NTP) servers is widely used to synchronize the clocks of computers and other networked equipment. With the rapid development of smart devices and their applications, more NTP servers as time sources need to be calibrated to a legal time reference. This work proposes using a low-cost and convenient calibration method to investigate the time service capability of ten NTP servers located at different cities in China. The uncertainties (k = 2) of the time offsets between Coordinated Universal Time (National Institute of Metrology) and network time from NTP servers for a single measurement are generally less than 150 ms, and the uncertainties (k = 2) of the mean time offsets for 24 h are less than 85 ms. Calibration results were found at the same level when applying the two time references, which were maintained a time offset less than 10 ns and located more than 1000 km apart.