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- National Time Authorities
- Institute for National Measurement Standards (INMS), Canada, NRC official time
- Physikalisch-technische Bundesanstalt, Germany, the national metrology institute
- Istituto Nazionale di Ricerca Metrologica (IMRIM), Time and Frequency Laboratory, Italy
- National Physics Laboratory, UK , Time and Frequency Home Page
- National Physics Laboratory, UK, MSF Radio Signal
- National Institute of Standards and Technology - Time and Frequency Division
- NIST's history measuring time and frequency
- US Naval Observatory
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