The effects of the January 2016 UTC offset anomaly on GPS-controlled clocks monitored at NIST

Errors in the Coordinated Universal Time (UTC) offset parameters broadcast by Global Positioning System (GPS) satellites caused many thousands of GPS-controlled clocks to be in error by approximately -13 μs on January 25-26, 2016. The erroneous UTC offset information was broadcast by 15 GPS satellites, or half of the available constellation, during the anomaly. This paper discusses the technical reasons for the UTC offset anomaly, its effect on clocks controlled by GPS satellites (hereafter referred to as GPS clocks), and how it was detected at the National Institute of Standards and Technology (NIST). It then discusses the impact of the UTC offset anomaly on GPS clocks maintained and monitored by NIST, including clocks located in Boulder, Colorado and those at remote locations. Some analysis is presented of the effects of the UTC offset anomaly on GPS clocks located at 19 sites in North, Central, and South America.