Undecided on Leap Seconds

article reprinted from http://spectrum.ieee.org

Leap Second Heads into Fierce Debate

When Earth’s rotation gets far enough out of sync with the drumbeat of atomic time, a leap second is added to Coordinated Universal Time (UTC) and the world’s clocks count off 59, then 60, then 00 seconds.

The fix is intended to pair two very different ways of keeping time, one grounded in the unchanging world of atomic physics and the other pinned to Earth’s spin, which is slowing due to tidal friction with the Moon.

Some say the leap second is a good compromise. It’s a way to link atomic clocks to the position of the sun in the sky. Others argue it’s an inconvenience and potential danger to modern systems. The leap second has been called “Y2K’s distant cousin” and “a crude hack added…to paper over the fact that planets make lousy clocks compared with quantum mechanical phenomena.”

Whatever the leap second is, it will not be ignored. Next week, its fate will come up for debate before the International Telecommunication Union’s World Radiocommunication Conference (WRC), which will run nearly an entire month, from 2 to 27 November in Geneva.

Many countries are strongly split over what to do: some favor keeping the leap second while others want it dropped from the definition of UTC. “I’m expecting difficult discussion,” says Vincent Meens of France’s National Center for Space Studies, who chairs the study group in the telecommunications union that’s responsible for the topic.

In the first week of the conference, a group will be spun off to focus on the leap second, says Brian Patten of the U.S. National Telecommunications and Information Administration. He anticipates the question won’t be resolved quickly: “I am predicting this will go all the way through the conference. There won’t be a conclusion until the last week.”

Patten will represent a regional group of countries in the Americas that includes Canada, Mexico, and the United States. Together they’re advocating the elimination of the leap second, with a grace period to allow time for legacy hardware and software to be updated. “The most fundamental thing that we’re proposing is to stop using the leap second in UTC, as the most economically viable and simplest method to implement,” he says.

“The world has changed a lot since 1972,” when the leap second was first introduced, Patten says. At the time, the addition helped celestial navigation. Now, satellite navigation systems offer far better accuracy, Patten says. At the same time, new vulnerabilities have emerged: “There is a huge underlying infrastructure of computer networks and telecommunication systems and all these other machines all talking to each other all over the world all the time.”

Since the rotation rate of the Earth doesn’t slow at a steady, entirely predictable rate, leap seconds aren’t scheduled at regular intervals. Each time a leap second is announced, system administrators must plan ahead to ensure there is no problem. Sometimes there is: past leap seconds have caused hiccups in web services and an outage in 2012 of an airline reservation system used by Quantas. “There hasn’t been a huge disaster but there could be,” Patten says, “and we’re being proactive in trying to prevent a future problem.”

The Americas aren’t going it alone. Other regional groups have weighed in. The Asia-Pacific Telecommunity, which includes Australia, China, and Japan, advocates dropping the leap second. Two others regions, one that includes Russia and a number of other formerly Soviet countries and the Arab Spectrum Management Group advocate preserving the leap second.

Notably missing from the regional proposals is Europe, which could not get the needed support for a proposal to drop the leap second. “We had intense debate,” says Alexander Kühn, who chaired the conference preparatory group for the region. At the last meeting in Norway in September, 20 countries voted in favor of dropping the leap second, he says, but the U.K., Russia, and six other countries opposed the proposal, which was enough to quash it.

A key concern for Russia seems to be the impact to the country’s GLONASS satellites. Kühn says he’s consulted with an engineer who says there is logic to Russia’s argument, but that the problem could be overcome by a software fix.

The U.K., home of the place where the sun is overhead at noon UTC, strongly advocates keeping the leap second. “It is our view that the technical problems associated with the insertion of leap seconds have been overstated and do not justify this radical change to the world’s time-scale,” the U.K. and several other countries state in their break-out proposal to the WRC.

Decoupling civic time from the Earth’s rotation might eventually mean—absent other changes—that the sun will reach its noon-time peak at 8 p.m. But this isn’t something that will happen any time soon: over the next hundred years, the drift between UTC and Earth-tracking time UT1 expected to be on the order of a minute.

Still, the U.K. and its proposal co-signers advocate an approach that would keep UTC the same but make it clear that International Atomic Time, the leap-second-less time that UTC is based on, could be used when someone needs a continuous time scale.

That wouldn’t make implementing leap seconds in UTC any easier of course, and it could complicate matters. “UTC was formulated to be the real-time and distributable reference clock for the world,” Patten says, while TAI is synthesized from many atomic clocks around the world, and “is not readily available for distribution like UTC.” “It would be another information distribution problem which turns out to be very difficult in the timekeeping area,” says Ron Beard, who chairs Working Party 7A, a group that has studied the technical issues associated with eliminating the leap second.

This isn’t the first time the question of the leap second has come before the WRC. In 2012, a decision on the leap second was postponed to allow for more study.

It’s an open question whether an agreement will be reached this time around. “Theoretically you need a majority,” Kühn, of the European group, says. “In practice it’s a common habit of the WRC that they try to reach a concensus where everyone is so-called ‘equally unhappy’.”

If those in favor of eliminating the leap second are successful, it could be the last time questions about UTC come before the diplomats of the International Telecommunication Union (ITU).

The ITU has been responsible for UTC because, early on, the time signals were primarily transmitted by radio. As metrologist Terry Quinn has noted, this is no longer the case: “These days, time is disseminated by many other means, notably by satellite navigation systems and the internet but also by optical fibres, coaxial cable as well as by many systems related to satellite communications.”

In the future, control over defining UTC could go to the International Bureau of Weights and Measures just outside Paris, which is already responsible for maintaining UTC and International Atomic Time.


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