pool cn.pool.ntp.org            iburst maxsources 4
pool ntp.nict.jp                iburst maxsources 4

server ntp.tuna.tsinghua.edu.cn iburst

refclock PPS /dev/pps0 lock NMEA refid GPS
refclock SHM 0 offset 0.2 delay 0.1 refid NMEA noselect