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Mainly due to the slowing down of earth rotation, it is about time to add another (the 35th) leap second to UTC in order to keep its time of day close to the mean solar time. NASA features an explanation of why the leap seconds have to be inserted, how earth rotation can be measured precisely and why it is impossible to give precise predictions on when the next one will happen.
The length of day is influenced by many factors, mainly the atmosphere over periods less than a year. Our seasonal and daily weather variations can affect the length of day by a few milliseconds over a year. Other contributors to this variation include dynamics of the Earth's inner core (over long time periods), variations in the atmosphere and oceans, groundwater, and ice storage (over time periods of months to decades), and oceanic and atmospheric tides. Atmospheric variations due to El Niño can cause Earth's rotation to slow down, increasing the length of day by as much as 1 millisecond, or a thousandth of a second.
VLBI [Very Long Baseline Interferometry] tracks these short- and long-term variations by using global networks of stations to observe astronomical objects called quasars. The quasars serve as reference points that are essentially motionless because they are located billions of light years from Earth. Because the observing stations are spread out across the globe, the signal from a quasar will take longer to reach some stations than others. Scientists can use the small differences in arrival time to determine detailed information about the exact positions of the observing stations, Earth's rotation rate, and our planet's orientation in space.
Original Submission
(Score: 2) by Non Sequor on Tuesday June 30 2015, @01:52AM
The argument that breaking UTC from UT1 would require changing laws, regulations, and technical specs seems like self inflicted brain damage. All of these things are already overruled by enforcement of a de facto standard through time synchronization mechanisms and they all have to tolerate some drift due to variations in synchronization quality among various parties (or else they must already have some mechanism for policing drift).
Doesn't the GPS network effectively define its own context specific time scale which should be sufficient relative to the accuracy of its distance coordinates? Skew in time signal between satellites should be independent of the relationship between the satellite timescale and terrestrial timescales, so long as the satellites stay in synch in terms of the time they let treat as elapsed.
Write your congressman. Tell him he sucks.
(Score: 0) by Anonymous Coward on Tuesday June 30 2015, @06:51PM
You are correct that GPS keeps its own time. It is a continuous counting clock that started in 1980 [nps.edu], and since it uses atomic clocks, it is a pretty nice clock indeed. Because of the ubiquity of GPS, it gives us all a very nice clock in our watches, phones, cars, etc. Anything with a GPS receiver in it automatically becomes a nice clock. However, to be useful, one needs to convert GPS time to UTC, which is where the leap-seconds come in to play.
Now let's suppose you have a few billion burning a hole in your pocket and you want to go into the space imaging business. You put up some cubesats. They take pictures of the Earth, and time-stamp their images with GPS time, which makes the most sense. Now you need to know where on the Earth you're pointing. You get that from your orbit, which is specified as a Two Line Element (TLE), which is defined at a specific point in time, in GMT. That inexpensive GPS receiver you used on the cubesat had the firmware to convert GPS time properly, but it can't be updated and it will be off by a second the next time a leap second gets added, so you'll need to handle that in ground processing. Or, you do away with the leap seconds so your cubesat now always keeps continuous time, but now you need to work your orbital calculation side. Leap seconds are not really an issue when you're talking about your computer, or phone, or if your car GPS gives you time that is off by a second or two. However, when it comes to things that depend upon precision synchronization, like orbits, it does matter.