If you recorded an electromagnetic transient at a known time, using your watch or an NTP-synchronized device, you might notice that when you open the time series in EMpower, the transient appears several seconds off from what you would expect.


This is normal. The difference comes from how different systems keep time:

  • Your watch or NTP device uses UTC time (Coordinated Universal Time).

  • Phoenix MTU-5C systems use GPS time.


As of 2025, GPS time is 18 seconds ahead of UTC.


Why is there a difference?


GPS time runs on an atomic clock system that counts time steadily and continuously. UTC clocks, on the other hand, are adjusted occasionally to match the Earth's rotation.


Because Earth’s rotation is slowly getting longer (the earth is slowing down), UTC sometimes adds a “leap second”— in other words, a civilian watch sometimes stops counting time for a second. This is done to keep civilian clocks synchronized with solar events that we can observe from Earth, such as sunrise or solar zenith.


Since January 6, 1980, when UTC and GPS time were the same, 18 leap seconds have been added. In October 2025 (the date when this article was written), GPS is 18 seconds ahead of UTC.


So, if you recorded a transient synchronized with a civilian watch (a UTC clock) in October 2025, the MTU-5C data in EMpower will show the transient 18 seconds away as compared to the time that was shown in your watch when the transient happened.


Keep in mind that in the future this difference might be other than 18 seconds, as more leap seconds are added to UTC clocks.


Why does the MTU-5C use GPS time?


Using GPS time provides indirect key benefits for our users. The most significant are:

  1. Consistent remote referencing and recording synchronization: Since GPS time keeps a continuous steady count of time, data from different systems can be remote referenced and synchronized based on their time stamps, regardless of if the recordings started before or after a leap second was introduced to UTC clocks.

  2. Faster synchronization: The MTU-5C can sync more quickly by using a GPS-based clock rather than with UTC-based sources.