The ACES program aims to develop portable, battery‐powered atomic clocks with stability, repeatability, and environmental sensitivity approaching that of laboratory‐grade cesium beam frequency standards. “The history of human knowledge is reflected in our clock technology,” says Robert Lutwak, program manager for the agency’s Atomic Clocks with Enhanced Stability (ACES) project. “Over thousands of years, you can track human engineering by looking at the quality of our clocks. It’s always been at the cutting edge of technology. And really, the most accurate instruments on the planet right now are clocks.”
Precision timing and synchronization is essential to DoD communications, navigation, reconnaissance, and electronic warfare systems. The requirements for timing precision and stability have grown increasingly demanding as systems have evolved towards higher data rates, increased spectrum congestion, and time-dependent encryption algorithms. This demand will continue to grow over the next decade, particularly due to emerging requirements for precision timing in GPS-denied environments and synchronization between system-of-systems components on distributed platforms, says DARPA. “The requirements for timing precision and stability have grown increasingly demanding as DoD systems have evolved towards distributed engagement and surveillance architectures,” says DARPA.
DARPA’s Atomic Clock with Enhanced Stability (ACES) program
The Atomic Clock with Enhanced Stability (ACES) program will provide unprecedented frequency and timing accuracy on low SWaP platforms. ACES should create a new generation of palm-sized, battery-powered atomic clocks that perform up to 1,000 times better than the current generation, Lutwak says. These include, but are not limited to laser‐cooled and magneto‐optically trapped atomic samples, and RF‐trapped ion samples, as well as interrogation of less environmentally‐sensitive microwave and optical transitions.
Of particular interest are portable battery-powered clocks, which require minimal time for calibration after power-on and which maintain accurate time and frequency over extended duration in relevant DoD operating environments. The program consists of three phases including proof of concept, physics integration, and clock integration.
Among their myriad potential advantages, better clocks could reduce one of the more worrisome modern-day national security vulnerabilities: a deep and growing dependence on the Global Positioning System (GPS), not just within the military but among numerous civilian sectors of the economy. That’s because satellite-based atomic clocks—whose precision and accuracy reside in super-uniform, high-frequency oscillations of atomic energy states (typically those of cesium or rubidium atoms) rather than the mechanical oscillations of pendulums or the quartz crystals inside modern watches, cell phones and computers—provide the key reference signals that are pivotal to GPS. The longer that clocks on Earth or on aircraft can maintain extreme accuracy in the absence of satellite reference signals, the lower the impact of any loss of satellite contact, whether caused by natural forces or adversarial activities.
Success will require record-breaking advances that counter accuracy-eroding processes in current atomic clocks, among them variations in atomic frequencies that result from temperature fluctuations and subtle frequency differences that can occur if the power shuts down and then starts up again.
In general, precise timing for DoD tactical systems is provided by Global Positioning System (GPS) receivers, which provide a relatively low size, weight, and power (SWaP) local timing source that is directly traceable to the DoD Master Clock. The local GPS receiver is typically augmented by a local timing reference (clock) in order to provide improved short-term stability (clean up) and flywheel operation (holdover) between relatively noisy and potentially intermittent GPS readings. Depending on platform application requirements, deployed local tactical clocks necessitate a trade-off between performance and size, weight, and power (SWaP), ranging from low-cost, low-SWaP, mechanical oscillators to relatively high-cost, high-SWaP atomic clocks.
Chip-Scale Atomic Clock (CSAC)
High accuracy atomic clocks have traditionally not been suited for smaller mobile electronic systems, where weight, size and power consumption are critical. This changed with the development of the Chip-Scale Atomic Clock by the Defense Advanced Research Projects Agency (DARPA), the National Institute of Standards and Technology (NIST), Symmetricom and others. In 2004 NIST demonstrated the first CSAC, manufactured with standard microfabrication and microelectromechanical systems (MEMS) technology.
In 2011 Symmetricom (now Microsemi) in 2011 launched the world’s first commercially available Chip-Scale Atomic Clock (CSAC), as an enabler for mobile systems to achieve better timing performance. Their Quantum SA.45s CSAC achieved specified short time stability (Allan deviation) of 2.5 × 10−10 at 1 second integration time, less than 120 mW power consumption, and a package size of 17 cm3.
Achieving the best possible oscillator performance has always been dictated by the practical limits of the available technology and components. Regular crystal oscillators are often troubled by being sensitive to temperature changes, although solutions like temperature compensated crystal oscillators (TCXOs) and oven controlled crystal oscillators (OCXOs) are available for better performance. In the upper scale of performance, time and frequency standards utilizes caesium or rubidium atomic clocks.
Like quartz oscillators and clocks, atomic clocks function by generating a very stable frequency from a stable reference. The main difference is that a quartz oscillator derives its frequency from a mechanically vibrating reference, which makes the frequency sensitive to long-term changes in mechanical dimensions and stress.
An atomic clock, on the other hand, derives its frequency from the energy difference between the hyperfine states of an alkali metal atom, which is a constant of nature, and thereby, much more predictable and stable. Unfortunately, however, the alkali metal atoms must be maintained at sufficient density in a vapor state to operate the atomic clock. This means excess power must be consumed to heat the atomic vapor cell. For a tabletop atomic clock, this takes tens of watts of power. But when one shrinks the atomic cell to less than 10mm3 using the MEMS technology, the amount of power needed to keep the atoms in a vapor state can be reduced to less than 10mW in a properly designed thermal control system. In effect, the smaller the mechanical structure, the less power is needed to heat up to a given temperature.
The output signal in a CSAC is is generated by a 10 MHz TCXO which is continuously compared and corrected to the ground state hyperfine frequency of the cesium atoms in the physics package. The physics package consists of a cesium vapor cell, heater, and laser optics, among other components. The main clock output is a 10 MHz square wave in addition to a one pulse per second (1PPS) output signal.
Challenges in DARPA Chip-Scale Atomic Clock (CSAC)
Existing battery-powered timing sources limit the mission application space of electronic systems due to power-on to power-on frequency changes (“retrace”) and due to longterm frequency drift and frequency sensitivity to temperature (“tempco”). Today, the best battery-powered clocks, developed under the DARPA Chip-Scale Atomic Clock (CSAC) program, provide 100X superior performance to mechanical oscillators of comparable SWaP.
Nonetheless, they require 6-12 hours of calibration after turn-on, due to retrace error, and support limited mission durations of 3-6 hours, due to tempco and drift. With the proliferation of CSACs among DoD systems suppliers, it is evident that even greater opportunities for advanced capabilities will be enabled by a battery-powered atomic clock with enhanced stability. Of particular value are clocks with significantly reduced temperature coefficient of frequency (“tempco”), long-term frequency variation (“drift”), and frequency repeatability under power cycling (“retrace”).
Tempco, drift, and retrace are the primary performance limitations of conventional CSACs. These limitations result from the fundamental device architecture, specifically CW laser interrogation of microwave transitions of buffer-gas confined atoms. Variations in the laser spectrum and vapor cell characteristics over temperature and time, as well as power cycling, lead to variations in the measured atomic transition frequencies, which impart instability to the clock frequency and timing accuracy.
Alternative approaches to atomic confinement and interrogation, which are less subject to these perturbations, are well-known and have been demonstrated in laboratory-scale and high-SWaP industrial instruments. These include, but are not limited to, laser-cooled and magneto-optically trapped atomic samples, RF-trapped ion samples, as well as interrogation of less environmentally-sensitive microwave and optical transitions.
The objective of the Atomic Clock with Enhanced Stability (ACES) program is to leverage these modern atomic physics techniques to develop portable, battery-powered atomic clocks with stability, repeatability, and environmental sensitivity approaching that of laboratory-grade cesium beam frequency standards. This will be accomplished through research, development, and integration of reduced SWaP components and technologies for advanced atomic physics interrogation techniques.
This will be accomplished through research, development, and integration of reduced SWaP components and technologies for advanced atomic physics interrogation techniques. “It will take a collaboration of teams with skillsets from diverse fields, including atomic physics, optics, photonics, microfabrication and vacuum technology to achieve the unprecedented clock stability that we seek,” program manager Robert Lutwak said.
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