Research On NTP Network Clock Synchronization System Based On Satellite Time Service

With the increasing popularity of network technology and the increasing demand of distributed system for high precision and high stability time benchmark,how to meet the demand of time benchmark for each terminal equipment in the system is particularly important.Using network clock synchronization technology to synchronize the clock of terminal equipment is one of the mainstream research methods.This technology uses standard clock reference source to correct the time of terminal equipment in the system to meet the clock requirements of each unit.NTP(Network Time Protocol)is a protocol specially used for time synchronization of network system.Network clock synchronization technology uses NTP protocol to synchronize the time of servers and client devices at all levels in the system with universal time coordinated(UTC).With the completion of BDS-3 system,the network clock synchronization technology based on satellite time service has a very broad application scenario,which is worthy of research and improvement.NTP provides time synchronization service,the key is to have a standard clock reference source,atomic clock,crystal oscillator and other clock sources can be used as NTP external clock reference source.By calibrating the crystal oscillator with good stability by satellite navigation system,the standard frequency source synchronized with the satellite time reference can be obtained.Using the clock source as the external clock reference source of NTP time service unit,the clock synchronization of other devices in the system can be realized through NTP protocol.In this dissertation,the satellite time service technology and NTP network time service technology are combined to synchronize the clock of the distributed network equipment in the system.Firstly,the satellite navigation receiver obtains the standard second pulse signal from the satellite navigation message,and then uses the standard second pulse signal to lock and adjust the frequency output of the local crystal oscillator,so as to obtain the frequency standard source synchronized with the satellite time reference.Then the standard frequency source is used as the reference clock of NTP time service unit,and the appropriate working mode is selected to process the synchronization request of terminal equipment,so as to keep the clock in the network synchronized with UTC time.In this dissertation,the principle and method of feedback control system based on FPGA,VCOCXO and DAC are introduced in detail.The digital phase detector unit is designed with tapped delay line in FPGA,and the clock deviation between the fractional second pulse of crystal oscillator and 1PPS second pulse output from satellite navigation receiver is measured.Then the interference in the time interval is eliminated by Kalman filter,and the control quantity is quantized by PI controller.Then,the output frequency of the thermostatic crystal oscillator is adjusted by DAC conversion to control voltage,thus the standard time source with high precision and stability is obtained.After that,the standard time source from satellite time service is transmitted to NTP time service unit through serial port,and the client / server working mode is selected to synchronize the clock of terminal equipment and correct the time of terminal equipment.