IMPROVING ATOMIC CLOCKS

FROM:  NATIONAL SCIENCE FOUNDATION
The challenge of building a better atomic clock and why it matters

Prior to the mid-18th century, it was tough to be a sailor. If your voyage required east-west travel, you couldn't set out to a specific destination and have any real hope of finding it efficiently.

At the time sailors had no reliable method for measuring longitude, the coordinates that measure a point's east-west position on the globe. To find longitude, you need to know the time in two places--the ship you're on, and the port you departed from. By calculating the difference between those times, sailors got a rough estimate of their position. The problem: The clocks back then just couldn't keep time that well. They lost their home port's time almost immediately after departing.

Today, time is just as important to navigation, only instead of calculating positioning with margins of errors measured in miles and leagues, we have GPS systems that are accurate within meters. And instead of springs and gears, our best timepieces rely on cesium atoms and lasers.

But given the history, it's fitting that scientists like Clayton Simien, a National Science Foundation (NSF)-funded physicist at the University of Alabama at Birmingham who works on atomic clocks, was inspired by the story of John Harrison, an English watchmaker who toiled in the 1700s to come up with the first compact marine chronometer. This device marked the beginning of the end for the "longitude problem" that had plagued sailors for centuries.

"If you want to measure distances well, you really need an accurate clock," Simien said.

Despite the massive leaps navigation technology has made since Harrison's time, scientists--many NSF-funded--are looking for new ways to make clocks more accurate, diminishing any variables that might distort precise timekeeping. Some, for example, are looking for ways to better synchronize atomic clocks on earth with GPS satellites in orbit, where atmospheric distortion can limit signal accuracy to degrees that seem minute, but are profound for the precise computer systems that govern modern navigation.

The National Institute of Standards and Technology, Department of Defense, join NSF in the search for even better atomic clocks. But today's research isn't just about building a more accurate timepiece. It's about foundational science that has other ramifications.

'One Mississippi,' or ~9 billion atom oscillations

Atomic clocks precisely measure the ticks of atoms, essentially tossing cesium atoms upward, much like a fountain. Laser-beam photons "cool down" the atoms to very low temperatures, so the atoms can transfer back and forth between a ground state and an excited state.

The trick to this process is finding just the right frequency to move directly between the two states and overcome Doppler shifts that distort rhythm. (Doppler shifts are increases or decreases in wave frequency as the waves move closer or further away -- much like the way a siren's sound changes depending on its distance.)

Laser improvements have helped scientists control atoms better and address the Doppler issue. In fact, lasers helped to facilitate something known as an optical lattice, which can layer atoms into "egg cartons" to immobilize them, helping to eliminate Doppler shifts altogether.

That shift between ground state and excited state (better known as the atomic transition frequency) yields something equivalent to the official definition of a second: 9,192,631,770 cycles of the radiation that gets a cesium atom to vibrate between those two energy states. Today's atomic clocks mostly still use cesium.

NSF-funded physicist Kurt Gibble, of Pennsylvania State University, has an international reputation for assessing accuracy and improving atomic clocks, including some of the most accurate ones in the world: the cesium clocks at the United Kingdom's National Physical Laboratory and the Observatory of Paris in France.

But accurate as those are, Gibble says the biggest advance in atomic clocks will be a move from current-generation microwave frequency clocks -- the only kind currently in operation -- to optical frequency clocks.

The difference between the two types of clocks lies in the frequencies they use to measure the signals their atoms' electrons emit when they change energy levels. The microwave technology keeps reliable time, but optical clocks offer significant improvements. According to Gibble, they're so accurate they would lose less than a second over the lifetime of the universe, or 13.8 billion years.

Despite that promise of more accurate performance, the optical frequency clocks don't currently keep time.

"So far, optical standards don't run for long enough to keep time," Gibble said. "But they will soon."

Optical frequency clocks operate on a significantly higher frequency than the microwave ones, which is why many researchers are exploring their potential with new alkaline rare earth elements, such as ytterbium, strontium and gadolinium.

"The higher frequency makes it a lot easier to be more accurate," Gibble said.

Gibble is starting work on another promising elemental candidate: cadmium. Simien, whose research employs gadolinium, has focused on minimizing--or eliminating if possible--key issues that limit accuracy.

"Nowadays, the biggest obstacle, in my opinion is the black body radiation shift," Simien said. "The black body radiation shift is a symptomatic effect. We live in a thermal environment, meaning its temperature fluctuates. Even back in the day, a mechanical clock had pieces that would heat up and expand or cool down and contract.

"A clock's accuracy varied with its environment. Today's system is no longer mechanical and has better technology, but it is still susceptible to a thermal environment's effects. Gadolinium is predicted to have a significantly reduced black body relationship compared to other elements implemented and being proposed as new frequency standards."

While Simien and Gibble agree that optical frequency research represents the next generation of atomic clocks, they recognize that most people don't really care if the Big Bang happened 13 billion years ago or 13 billion years ago plus one second.

"It's important to understand that one more digit of accuracy is not always just fine tuning something that is probably already good enough," said John Gillaspy, an NSF program director who reviews funding for atomic clock research for the agency's physics division. "Extremely high accuracy can sometimes mean a qualitative breakthrough which provides the first insight into an entirely new realm of understanding--a revolution in science."

Gillaspy cited the example of American physicist Willis Lamb, who in the middle of the last century measured a tiny frequency shift that led theorists to reformulate physics as we know it, and earned him a Nobel Prize. While research to improve atomic clocks is sometimes dismissed as trying to make ultra-precise clocks even more precise, the scientists working in the field know their work could potentially change the world in profound, unexpected ways.

"Who knows when the next breakthrough will come, and whether it will be in the first digit or the 10th?" Gillaspy continued. "Unfortunately, most people cannot appreciate why more accuracy matters."

From Wall Street to 'Interstellar'

Atomic clock researchers point to GPS as the most visible application of the basic science they study, but it's only one of this foundational work's potential benefits.

Many physicists expect it to provide insight that will illuminate our understanding of fundamental physics and general relativity. They say new discoveries will also advance quantum computing, sensor development and other sensitive instrumentation that requires clever design to resist natural forces like gravity, magnetic and electrical fields, temperature and motion.

The research also has implications beyond the scientific world. Financial analysts worry that worldwide markets could lose millions due to ill-synchronized clocks.

On June 30 th at 7:59:59 p.m. EDT, the world adds what is known as a "leap second" to keep solar time within 1 second of atomic time. History has shown, however, that this adjustment to clocks around the world is often done incorrectly. Many major financial markets are taking steps ranging from advising firms on how to deal with the adjustment to curtailing after-hours trading that would occur when the change takes place.

Gibble says the goal of moving to ever more accurate clocks isn't to more precisely measure time over a long period.

"It's the importance of being able to measure small time differences."

GPS technology, for example, looks at the difference of the propagation of light from multiple satellites. To provide location information, several GPS satellites send out signals at the speed of light--or one foot per nanosecond--saying where they are and what time they made their transmissions.

"Your GPS receiver gets the signals and looks at the time differences of the signals--when they arrive compared to when they said they left," Gibble said. "If you want to know where you are to a couple of feet, you need to have timing to a nanosecond--a billionth of a second."

In fact, he said, if you want that system to continue to accurately operate for a day, or for weeks, you need timing significantly better than that. Getting a GPS to guide us in deserts, tropical forests, oceans and other areas where roads aren't around to help as markers along the way--one needs clocks with nanosecond precision in GPS satellites to keep us from getting lost.

And if you're not traveling to those locales, then there's still the future to think about.

"Remember the movie, 'Interstellar,'" Simien said. "There is someone on a spaceship far away, and Matthew McConaughey is on a planet in a strong gravitational field. He experiences reality in terms of hours, but the other individual back on the space craft experiences years. That's general relativity. Atomic clocks can test this kind of fundamental theory and its various applications that make for fascinating science, and as you can see, they also expand our lives."

-- Ivy F. Kupec,
Investigators
Kurt Gibble
Clayton Simien
Related Institutions/Organizations
University of Alabama at Birmingham
Pennsylvania State Univ University Park

STRATEGIC COMMAND AND WARFIGHTERS

Warfighters might not realize it, but U.S. Strategic Command provides many of the capabilities they rely on in combat operations. Here, Army 1st Lt. Michael Kim, platoon leader of 2nd Platoon, Apache Company, 1st Battalion, 23rd Infantry Regiment, checks his location with a handheld GPS -- a capability provided through Stratcom -- while on patrol in southern Afghanistan, July 30, 2012. U.S. Army photo by Sgt. Matt Young.

FROM: U.S. DEPARTMENT OF DEFENSE
Strategic Command Provides Vital Warfighter, Operational Support
By Donna Miles
American Forces Press Service

WASHINGTON, April 11, 2013 - While providing the deterrence to protect the United States from a strategic attack, U.S. Strategic Command is playing a very real, yet often unrecognized, role in operations in Afghanistan and around the globe, its commander, Air Force Gen. C. Robert Kehler, reported.

"I joke to theater combatant commanders and tell them, 'There isn't anything you do that Stratcom doesn't touch,'" Kehler told American Forces Press Service during an interview here.

"At first they would push back on that," he said, not immediately recognizing Stratcom as the behind-the-scenes force that drives many of the capabilities they rely on every day.

Kehler said he reminds them that Stratcom is the driving force behind satellites that allow them to communicate, cyber defenses that protect their networks, and GPS capabilities that help them navigate and, when necessary, lock in on and engage targets. In addition, the command coordinates the intelligence, surveillance and reconnaissance capabilities that give U.S. and coalition forces a decisive edge on the battlefield that saves lives.

"We are in the fight everywhere U.S. military people operate, communicate, have global awareness and local awareness," Kehler said. "In all those cases, there is some piece of that that is either provided by or enabled by Strategic Command."

Despite being central to military operations, that support largely is transparent to users, he acknowledged.

"We are providing real-time, day-to-day capability for space and for cyber. We are providing the ballistic missile defense system. We are providing the synchronization for combating weapons of mass destruction," Kehler said.

"We are providing the synchronization activity for intelligence, surveillance and reconnaissance on a global basis," he continued. "We are providing analysis and targeting on a global basis, to include the cruise missile support activities for the Atlantic and Pacific. We are providing the long-range global fires through global strike, if those are required in the theater."

For example, Stratcom provided global-strike capability for U.S. Africa Command during the opening days of Operation Odyssey Dawn in Libya, Kehler said.

In addition, Strategic Command provides the oversight and tactics, techniques and procedures to ensure military operators have uncontested access to the electromagnetic spectrum.

That access, required for almost every modern technical device, "provides us the opportunity to communicate with one another and to share data across long distances," Kehler said. "It's the glue that binds us all together."

Stratcom's challenge, he said, is to ensure all U.S. forces have access to and control of this spectrum that provides the a vital military advantage, while protecting against vulnerabilities adversaries might try to exploit through jamming or "dazzling" that makes sensors inoperable.

Kehler offered high praise for the men and women of Stratcom for their behind-the-scenes contributions to the wartime mission and to every other military operation around the world.

"We believe we are standing in the theaters, shoulder-to-shoulder, with theater combatant commanders," Kehler said. "We are essential to the function of the geographic combatant commands. And we are critical in the fight."

Meanwhile, Stratcom continues to provide what Kehler called the ultimate form of support for those charged with defending the nation: deterrence that prevents conflict from breaking out in the first place, and if it does, from escalating.

"We don't want to fight a war. We don't want to get there. We would rather be in some place where we have prevented one," Kehler said. "And we think that deterrence and assuring our allies, contribute to the prevention of conflict, which is where we would rather be."

HURRICANE HUNTERS

 FROM: NATIONAL SCIENCE FOUNDATION
September 24, 2012

The Deep Convective Clouds & Chemistry (DC3) Experiment, which began in mid-May, explores the influence of thunderstorms on air just beneath the stratosphere, a region that influences Earth's climate and weather patterns. Scientists used three research aircraft, mobile radars, lightning mapping arrays and other tools to pull together a comprehensive picture. Credit: NOAA

Dropsondes--Work Horses in Hurricane Forecasting
Small cylinders dropped from airplanes gather atmospheric data on their way down

Inside a cylinder that is about the size of a roll of paper towels lives a circuit board filled with sensors. It's called a dropsonde, or "sonde" for short. It's a work horse of hurricane forecasting, dropping out of "Hurricane Hunter" airplanes right into raging storms. As the sonde falls through the air, its sensors gather data about the atmosphere to help us better understand climate and other atmospheric conditions.

"Dropsondes have a huge impact on our understanding of hurricanes and our ability to predict hurricanes," explains electrical engineer Terry Hock at the Earth Observing Laboratory in the National Center for Atmospheric Research (NCAR), located in Boulder, Colo.

With support from the National Science Foundation (NSF), Hock and his colleagues at NCAR have been designing, building and improving dropsonde technology for more than 30 years. "Our most current development is a fully automated dropsonde system for NASA's unmanned Global Hawk aircraft," says Hock.

Compared to earlier models, today's sondes are lighter weight, relatively inexpensive and loaded with sensors.

"We have a lot of electronics and, on the back side, a battery pack to operate the sonde. We have a temperature and two humidity sensors, and we have a GPS receiver," explains Hock, as he points out the different circuit board components. "As the sonde moves, we're using that GPS receiver to track the sonde's movements very precisely, which is then telling us the wind speed and wind direction. At the top of the sonde is a parachute which slows down the descent."

Electrical engineer Dean Lauritsen, a member of Hock's team, developed the system software on the aircraft, which controls the aircraft data system and process, and also displays dropsonde data during the sondes free fall to earth. There's such a system on the HIAPER, the NSF/NCAR Gulfstream V Research Aircraft, which uses sondes for scientific research, and a similar system used by the U.S. Air Force Reserve Hurricane Hunters in Biloxi, Miss., and the NOAA Hurricane Hunters in Tampa, Fla. On board each aircraft are a computer and a rack of electronic equipment to monitor and receive information from sondes. "The system is capable of tracking as many as eight dropsondes in the air at the same time. Each one of them is transmitting data on a separate frequency as it falls." says Lauritsen.

From the time the sonde leaves the aircraft, it is checking surroundings two times a second and sending information back to the aircraft, including pressure, temperature, humidity, wind speed, and wind direction. Future developments are expected to include sensors for chemicals such as ozone.

"We're taking vertical slices of the atmosphere constantly as the sonde falls," says Hock. "We're seeing very precise single measurements show up immediately on the computer screen."

Researchers process the information using NCAR-developed custom software, and then send it to weather forecasters and researchers around the world. In the case of the Hurricane Hunters, the information goes to the National Hurricane Center in Miami.

NCAR software engineer Charlie Martin develops custom software called ASPEN, which stands for Atmospheric Sounding Processing Environment. ASPEN helps make sense of all the dropsonde data. "Once the dropsonde has fallen through the atmosphere and the data has come back to the aircraft, that raw data needs a little more treatment before we send it to weather services around the world," explains Martin.

Martin points to a map showing a compilation of dropsonde wind data collected in August 2011, as Hurricane Irene was churning its way toward the Florida coast. "The winds are in a circular pattern," says Martin, as he identifies small triangles on the map that represent the wind and wind direction. "The center of the hurricane is clearly depicted in the center of the circular pattern. The National Hurricane Center uses this data along with other data to classify the hurricane and assign a category to it."

Hock and his team also custom fit aircraft with launchers to deploy the sondes, including one system for helium-filled balloons. In 2010, American and French researchers deployed balloons over Antarctica that dropped 600 sondes over a four-month period to study atmospheric conditions and the shifting ozone layer. "There is now a very dense set of measurements that came out of this project that has mapped the Antarctic atmosphere like it has never been done before," notes Martin.

"Atmospheric conditions above the Antarctic continent are hard to study since only a handful of sounding stations are regularly maintained there," says Peter Milne, program manager for ocean and atmospheric sciences within NSF's Office of Polar Programs. "Fortunately, the Antarctic polar vortex, a huge cyclone that sets up above the entire continent, is like the NASCAR of long distance ballooning, with balloons sweeping around the continent for as long as they stay aloft. Using these drifting platforms provided a unique data set."

Such "inside information" is helping scientists learn more about climate and hurricanes. Data from dropsondes is also giving scientists a better understanding about atmospheric conditions that spawn any number of weather conditions. Hock expects this will help forecasters make earlier and more precise hurricane predictions, giving people in the path of a killer storm more time to get out of harm's way.