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Why the Leap Second is Important to NGA

 June 29, 2015

 National Geospatial-Intelligence Agency Source Communications

No matter how you feel about the leap second, at midnight June 30, an extra second will be added to timing systems around the world.

The National Geospatial-Intelligence Agency’s Office of Geomatics has been preparing since the last leap second in 2012, testing all new software and rehearsing implementation procedures to ensure a flawless transition.

Timing is critical for the satellite navigation and Earth-observing systems essential to NGA’s mission, said Dennis Manning, a timing expert in NGA’s Global Positioning System division. To translate between a point in space and a position on the ground, one needs timing to know how much the Earth has rotated underneath your satellite constellation.

“The timing has to be precise, because the Earth’s surface at the equator is moving at over 1,000 mph,” said Manning.

A leap second is an adjustment to world time because the rotation speed of the Earth is not constant, said Cliff Minter, Ph.D., a geodetic orbit geospatial intelligence analyst in NGA’s GPS division. And, this adjustment plays into the agency’s safety of navigation mission.

“For early and modern mariners, timing is necessary for determining one’s longitude,” said Minter. “Not knowing one’s longitude is exceedingly dangerous as one risks running the ship aground.”

For more than 150 years the U.S. Naval Observatory has been responsible for calibrating the U.S. Navy’s chronometers and navigation instruments. Even today, USNO’s atomic clocks are critical for safety of navigation. USNO keeps precise time through atomic clocks, deviating about one second every 27 million years. Comparing these clocks with celestial observations reveals that the Earth’s rotation rate varies, necessitating a one-second adjustment every one to four years. In all, 25 leap seconds have been added since the first leap second in 1972.

“Without leap seconds, over the course of many years, atomic time and time perceived by Earth’s rotation would drift apart,” said Manning. “In other words, the Sun’s zenith, its highest point in the sky, would drift slowly away from the observer’s local noon.”

Why even bother with leap seconds? Why does it matter that the sun is at its highest point at noon — especially if the average observer would not notice this drift for many centuries?

According to Minter, many argue that leap seconds are too troublesome for computer systems around the world and their use should be terminated.

“In past years, the leap second has disrupted companies like Google, Yelp, Mozilla, LinkedIn, Qantas and many others,” said Minter. “On the other hand, eliminating the leap second could affect other systems too, like those that rely on celestial tracking, for example.”

While there has been much debate without consensus in the international community about whether to abolish or keep the leap second, for NGA’s mission navigation and timing are directly related, said Minter.

“With GPS, for example, time is measured in billionths of a second. Not taking into account something as long as a leap second would result in a 1/5 mile error at the equator.”

So, if you don’t notice that June 30 is one second longer, it is only because timing experts at NGA and on computer systems around the world worked diligently to ensure this one extra second would pass unnoticed.