Small Atomic Clocks Chart New Horizons

April 2008
By Henry S. Kenyon

 
The goal of the Defense Advanced Research Projects Agency’s (DARPA’s) Chip-Scale Atomic Clock (CSAC) program is to provide warfighters with enhanced radio communications and jam-resistant navigation systems.
Chip-scale time keepers offer accurate frequency location, lower power requirements for messaging, detection and navigation equipment.

A tiny device the size of a sugar cube may revolutionize military communications and sensor systems. The technology is a micro-scale atomic clock designed to help spectrum-hopping radios synchronize their frequencies and access signals from navigation satellites. This prototype time keeper is undergoing testing to determine its readiness for military applications.

The goal of the Defense Advanced Research Projects Agency’s (DARPA’s) Chip-Scale Atomic Clock (CSAC) program is to create miniaturized, low-power atomic time and frequency reference devices for high-security ultrahigh frequency communications, jam-resistant global positioning system (GPS) receivers, sensors and guided munitions. To achieve these objectives and to enable the technology to fit in handheld military radios and other small form factors, the Arlington, Virginia-based agency’s researchers have designed a miniature atomic clock the size of a 1-centimeter cube. Their work tapped a range of micromechanical technologies and nanotechnologies to develop the ultra-miniaturized device.

An atomic clock keeps highly accurate time, which is essential for modern military communications, explains CSAC program manager Amit Lal. He notes that most modern radios operate by transmitting data in small packets. If many users in a group such as an infantry platoon are communicating, differing times are allocated to the radios to allow transmission on the same frequency. The data packets have guard bands that protect individual packets from overlapping. This timing feature ensures communications, even if all of the radios are not synchronized. A CSAC can provide such accurate timing per radio that it reduces the guard bands, allowing twice the information to be transmitted, Lal shares. “Radios need to be synchronized. The better time you keep, the faster the network can be kept in synchronicity,” he says.

Jam-resistant navigation is another use for the small atomic clocks. Blocking or interfering with GPS transmissions is a challenge for navigation systems. Current equipment requires access to four GPS satellites for an accurate position fix. If only one or two satellites are available because of jamming or interference, it may not be possible for many systems to determine their location. For example, soldiers entering a large building will lose GPS signals. When they emerge, they may want to know their position to request assistance. But if GPS is unavailable, the troops must use maps or deduce their location.

But CSAC technology permits devices to keep such accurate time that soldiers’ equipment will remain synchronized with GPS clocks even after losing the signal for several hours. This accurate timing allows communications equipment to reacquire GPS transmissions within milliseconds. Lal adds that a potential benefit of CSAC is that future warfighters may need to acquire signals from only one or two GPS satellites to determine their position.

These clocks also enable a fast, accurate GPS lock. Lal shares that military GPS systems use very long pseudo random codes for signal recognition. These long codes prevent civilian GPS devices from detecting military codes and their positioning data. But the disadvantage of lengthy codes is that it can take up to two minutes for military systems to decrypt the code and access the GPS signal. With a CSAC device, Lal notes, detection time is cut to seconds or milliseconds.

 
DARPA researchers reduced an atomic clock down to a
1-cubic-centimeter cube that can fit on a circuit board. By keeping highly accurate time, it can permit troops to reacquire navigation satellite data rapidly and can allow communications systems to frequency hop more quickly at higher bandwidths.
DARPA’s clocks also are designed for low-power operations in handheld communications and personal navigation equipment. Lal says that commercially available atomic clocks consume between 5 to 10 watts of power. In addition, size is a critical issue. The smallest available atomic clocks have a volume of 200 cubic centimeters (CCs), but he points out that high-quality atomic clocks are larger, with volumes of  300 CCs or more, and have an average power consumption of 10 watts. The CSAC program reduced the size of these clocks by 200 times down to 1 CC and cut power requirements down to 30 milliwatts.

This extreme size reduction is enabled by the use of vertical cavity surface emitting lasers to replace the lamps used in commercial clocks. The clock’s physics package is a 1-millimeter cube gas cell containing alkaline metals such as cesium or rubidium. The laser heats these metals to 90 degrees centigrade using only 5 milliwatts of power and measures the resonation of the excited atoms. He notes that a commercial atomic clock would require 3 or 4 watts of power to heat the metals to the same temperature.

All clocks keep track of time by counting the “ticks” of a resonator. In mechanical clocks, the resonator is a pendulum resonating at a frequency of one swing per second. The accuracy of a time-keeping device depends on the accuracy of its resonator at a specific frequency. An atomic clock uses the frequencies of atoms as a resonator. Atoms resonate at highly consistent frequencies, providing extremely accurate time keeping.

These tiny atomic clocks do not conduct any heat to the surrounding circuitry. This thermal damping is achieved by isolating the CSAC from the surrounding electronics by long, thin “tethers.” These structures are tiny supports with a cross-section of 5 x 5 microns, almost 20 times thinner than the diameter of a human hair. The tethers attach the cube to the chip. When the cube is heated, the tethers’ extreme thinness prevents heat from conducting away from the physics package.

Micromachining processes are a driving technology behind the CSAC program. Lal explains that techniques for bonding glass and silicon wafers together allow scientists to control the properties of the cube’s interior precisely. Coating the cube’s interior is critical because it is exposed to highly reactive metals for months or years. Cesium or rubidium will burn if exposed to oxygen.

State-of-the-art commercial atomic clocks use glass tubes to hold their metals. But this large size requires more power to heat the contents. Micromachining allows the tiny CSAC devices to be heated with low power and to retain low heat conductivity to the surrounding electronics.

Because the CSAC uses only 30 milliwatts of power, it can greatly extend the battery life of the systems it is embedded in. Lal notes that before the program it would have been impossible to put an atomic clock in every soldier’s kit. Even in applications where power is not important, size is still a consideration. The U.S. Army’s Future Combat Systems (FCS) program is developing a family of advanced, highly computerized combat vehicles. Lal says that Army planners envision using four or five CSACs in each vehicle to synchronize sensors. He adds that FCS vehicles may spend time with their engines off or in camouflage, but they will still need to operate their sensors, making low power consumption important.

The large class of sensors designed to measure frequency shifts also would benefit from built-in CSACs providing time, frequency and distance data, Lal says. Because its laser is tuned to atomic resonances and fixed to a sharply defined wavelength, the clock provides meter reference, distance reference and time reference. The difficulty with many of these sensors is calibrating them, but he adds that a CSAC would provide its own built-in calibration. “This technology may allow you to make sensors out of different materials that are not as expensive. The assumption is that the CSAC will completely calibrate a sensor,” he says.

Lal believes that CSAC represents a major step forward for communications systems. He explains that communications systems need specific frequencies to operate. These frequencies are typically generated by phase-locked loops and oscillators. He predicts that in the future, radio receiver architectures could use CSACs to generate precision frequencies while putting much less burden on phase-locked loop gains. This combination would provide lower power consumption and higher reliability for radio reception.

DARPA also plans to embed CSAC and other advanced time-keeping technologies in future GPS satellites. If the small atomic clocks are successful, Lal believes it may be possible to begin launching small, inexpensive “nano” navigation satellites (nanosats). He adds that in a future conflict, an adversary could harm U.S. military communications by destroying or jamming dozens of GPS satellites. However, if hundreds or thousands of nanosats were in orbit, such a constellation would prove very difficult to overcome, he says.

The CSAC program is now in Phase 4—operational testing—of its development. This stage is rare for DARPA programs because most efforts end at Phase 3, but Lal shares that DARPA director Anthony Tether is so confident about the technology’s great potential that he cleared the effort for additional development. Lal says this additional phase will ensure that there are no reliability issues with the clocks that might emerge in a full production development program. “The program was very successful in achieving the original milestones. But rather than just leaving it at this point, it would be worthwhile to explore the reliability of these atomic clocks,” he says.

Phase 4 testing is taking place at the U.S. Army’s Communications Electronics Research Directorate in Fort Monmouth, New Jersey. The devices will be tested to military specifications, what Lal refers to as “shaking and baking,” to measure the technology’s ability to withstand the vibration and heat encountered in airborne and ground-based platforms. The devices also must withstand temperature variations from zero to 50 degrees Celsius and must provide the frequency stability necessary for applications such as radio synchronization. If the clocks are able to meet military standards, they will spiral into radios and other applications.

Although the core CSAC package works well in the laboratory, Lal cautions that testing under operational conditions is critical. One potential challenge might be the ability of the thin tethers that attach the device’s physics package to the chip to withstand vibration and acceleration. Another concern is that vibration frequencies may affect clock accuracy.

High altitudes and low atmospheric pressure may cause other difficulties. Unlike conventional atomic clocks, the CSAC’s tiny physics package has very thin walls. At lower pressures, increased diffusion of the cesium and rubidium gases will occur, which could affect the clock’s frequency over time. However, Lal is confident that the device will perform well on ground and low-altitude platforms.

Phase 4 will last roughly a year, and its success will determine whether the clock moves on to a manufacturing program to develop mass production methods or if the program returns to DARPA for additional fine-tuning. If the tiny clocks prove ready for deployment, the agency will conduct manufacturing research to bring the price of an individual CSAC down to less than $100.

Web Resource
DARPA Chip-Scale Atomic Clock program: www.darpa.mil/MTO/Programs/csac/index.html