UFFC Society – Frequency Control
http://www.ieee-uffc.org/frequency_control/index.asp
NIST Physical Measurement Laboratory – Time and Frequency Division
http://www.nist.gov/pml/div688/
BIPM
http://www.bipm.org/en/home/
National Time Service Center – Chinese Academy of Sciences
http://english.ntsc.cas.cn/