High precision timing system based on GPSD

　　Accurate time is necessary for astronomical observation. The accurate pointing of astronomical telescope in a specific time, the control of CCD exposure time and the high precision synchronization ratio of observation data in different wavebands require the system to have at least sub millisecond time accuracy. However, at present, the accuracy of crystal oscillators used in general computers and embedded devices is several or tens of ppm (one millionth of a second), and will be affected by temperature drift, so that the daily error can reach the second level. If factors such as aging of components or external interference are considered, the error may exceed 10 seconds. If not corrected in time, the error accumulation will not be ignored.

　　Network Time Protocol (NTP) was proposed by Professor MILLS David L. of Delaware University in 1982. It is designed to transfer uniform and standard time using Internet resources. At present, there is a lot of research work on the use of GPS signals to achieve time correction. Most of them only read the serial data decoded by the GPS module and extract the time information to correct the system clock. This process does not involve the use of NTP, and the accuracy is low, usually tens to hundreds of milliseconds. In this regard, this paper makes full use of the support of NTP server software for GPS clock source, and uses the combination of serial data and second pulse to calibrate time, which greatly improves the accuracy of time calibration.

　　Accurate time is necessary for astronomical observation. The accurate pointing of astronomical telescope in a specific time, the control of CCD exposure time and the high precision synchronization ratio of observation data in different wavebands require the system to have at least sub millisecond time accuracy. However, at present, the accuracy of crystal oscillators used in general computers and embedded devices is several or tens of ppm (one millionth of a second), and will be affected by temperature drift, so that the daily error can reach the second level. If factors such as aging of components or external interference are considered, the error may exceed 10 seconds. If not corrected in time, the error accumulation will not be ignored.

　　Network Time Protocol (NTP) was proposed by Professor MILLS David L. of Delaware University in 1982. It is designed to transfer uniform and standard time using Internet resources. At present, there is a lot of research work on the use of GPS signals to achieve time correction. Most of them only read the serial data decoded by the GPS module and extract the time information to correct the system clock. This process does not involve the use of NTP, and the accuracy is low, usually tens to hundreds of milliseconds. In this regard, this paper makes full use of the support of NTP server software for GPS clock source, and uses the combination of serial data and second pulse to calibrate time, which greatly improves the accuracy of time calibration.

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