LORAN-C is a radionavigation system operated by the United States Coast Guard. LORAN-C signals originate from 100 kHz transmitters that are referenced to cesium oscillators. Because the ground wave signal is stable and easy to receive, LORAN-C receivers are often used as a reference for frequency calibrations.

NIST continously monitors the signals from three different LORAN-C transmitters from Boulder, Colorado and compares the phase of the received signal to the NIST frequency standard. Values are published in the archive that represent the daily accumulated phase shift for each LORAN-C transmitter relative to UTC(NIST). The archived data can be used to support claims of frequency traceability to NIST through the use of LORAN-C.

To view archived LORAN-C data select the month and year and then click the "Get data" button. Since new data are added to the archive monthly, last month is selected by default.