An Assessment of Precision Time and Time Interval Science and Technology (2002)

Finding: Accurate time and frequency have been, are, and will be critical to the Navy’s (and indeed all of DOD’s) war-fighting capability.

The original driver for significant improvement in knowledge of time was the need to determine longitude in navigation of the seas, which led to the development of the chronometer. Today, PTTI technology has proven to be of enormous leverage in war fighting. Comparison of signals between ground- and space-based clocks is the foundation for GPS and its myriad applications to precision force location and operations and to precision-guided munitions. Today, precise time synchronization is needed to determine efficiently the start of a code sequence in secure communications and the precise location of emitting targets, and precise frequency control is required for spectrum utilization and for frequency-hopped, spread-spectrum communications. The recent upgrade of the rubidium standards flown on GPS satellites has decreased the user equivalent range error achieved with GPS by a factor of one-half, which can improve the probability of kill and reduce the number of sorties, casualties, and unwanted collateral damage.

Advances in PTTI, including a reduction in size, weight, and power required for mobile devices, an improvement in the ruggedness of field systems, and more precise and accurate time dissemination, will lead to improvements in DOD capabilities that rely on PTTI. For example, rugged chip-scale atomic clocks with 10⁻¹¹ accuracy would significantly improve networking and weapons system effectiveness by enabling better four-dimensional coordination of systems. Reliable oscillators would improve the jamming resistance of GPS-guided munitions by enabling faster direct acquisition of the GPS signal in cases of jamming. Increased speed of disseminating command knowledge on the battlefield, combined with smaller, more precise weapons, would reduce collateral damage and, in turn, operational and political costs. In addition to improving existing systems, the historical record strongly suggests that further advances in PTTI technology will enable advances in systems autonomy and weapons accuracy, leading to new war-fighting capabilities that are only dimly foreseen today.

Finding: Internationally, the United States is an important player in PTTI research but no longer the dominant one.

Increased foreign participation in major PTTI conferences attests to the fact that the United States has more competition in PTTI research than was true historically. Countries such as Australia, China, Finland, France, Germany, and Russia all train students specifically in PTTI technology.

Recommendation: While it is important to retain U.S. leadership in PTTI, ONR and USNO should consider whether opportunities exist to take advantage of growing foreign competence in PTTI by cooperating with allies. This might defray the cost of some current programs, freeing resources to pursue advances in other areas of PTTI.

Finding: Training in precision frequency control and timing per se is rare in the United States, and training in the fields relevant to PTTI is mixed—some is adequate, some is not.

U.S. researchers in atomic frequency standards and clocks are generally drawn from the ranks of students trained in atomic physics and precision measurements. Years of on-the-job training then turn these researchers into PTTI experts. A number of U.S. universities provide training in atomic physics and precision measurements relevant to PTTI, and a number of federal laboratories support training opportunities that focus on development of advanced atomic clocks and frequency standards.

In particular, however, there is a dearth of U.S. training opportunities in areas relevant to high-precision quartz resonators and oscillators, which play a key role in most applications that utilize PTTI in the field. There are a number of areas for potential improvements in the performance of these oscillators that would translate directly into improved systems for the warrior. Without trained researchers in this area, the United States risks losing out on such improved systems.

Finding: There is little incentive for commercial firms to produce PTTI devices for defense systems.

A large market exists for inexpensive quartz crystal oscillators for applications such as watches, cell phones, and other relatively low-precision consumer goods. The market for defense-quality quartz oscillators is miniscule in comparison with this lower-quality market. The situation is similar for atomic clocks, where the largest market for moderate-quality clocks is commercial telecommunications. Relative to this market, the purchase of high-precision clocks for defense systems is infrequent and involves a small number of clocks.

Most defense applications also require oscillators and clocks that can withstand the harsh operating environments of the battlefield and space. Such environmental factors pose severe engineering challenges for device designers. Products for the consumer market or for the telecommunications industry are not made to operate in these environments, so commercial firms that manufacture for civilian applications cannot readily shift into production of devices for defense purposes.

Industry needs a steady source of funding in order to maintain precision frequency sources for special applications, such as space-qualified, severe environments and very small size and power requirements. In particular, without sustained support, precision quartz oscillators may no longer be available from U.S. sources.

Finding: Past Navy funding for basic research in atomic and molecular physics has led to significant advances in PTTI.

For example, ONR was a supporter of the research that led to three Nobel Prizes in physics— Norman Ramsey in 1989, Bill Phillips in 1997, and Carl Wieman, Eric Cornell, and Wolfgang Ketterle in 2001. ONR-funded spectroscopic studies by Ramsey (Harvard University) in the 1940s led to the design of the microwave interrogation method that remains an essential part of high-accuracy clocks today. ONR was an important early funder of work in the 1980s by Phillips (NIST) in laser cooling and trapping of atoms, a technique that is at the heart of today’s most accurate clock, the cesium fountain clock. ONR was also an important early funder of work by Cornell, Wieman (JILA), and Ketterle (Massachusetts Institute of Technology) in the 1990s that led to the realization of the Bose-Einstein condensate, a form of ultracold matter that had been predicted theoretically some 70 years earlier but never realized. This breakthrough has not yet been applied to PTTI, but just as PTTI built on the scientific groundwork laid by Ramsey and Phillips to further advance basic physics, Cornell, Wieman, and Ketterle’s work is also expected to lead to PTTI applications. These investigators all had as their primary goal advancing our fundamental scientific understanding of nuclei, atoms, and molecules, and the judicious support of their research ideas by ONR program officers led to fundamental advances in PTTI as well.

Finding: The U.S. Navy has been the principal developer of advanced atomic PTTI technologies for DOD applications.

ONR funding was key to the development and commercialization of cesium atomic beam clocks, rubidium gas cell clocks, and the hydrogen maser. These three technologies, together with precision crystals (supported mainly by the U.S. Army), account for all commercially available precision clock technology. The NRL Timation project and subsequent Navy developments in satellite navigation and orbiting precision clocks led to today’s GPS and resulted in NRL funding being DOD’s main source for the development of advanced atomic clocks for space. USNO has been a key player in the development of precise time coordination and transfer techniques such as GPS common view, GPS carrier phase, and two-way satellite time transfer.

Finding: PTTI is a Critical National Defense Technology, and advances in PTTI that are most relevant to defense will only be developed under military sponsorship. PTTI is of benefit to all the Services and is essential to the Navy.

DOD has unique needs in the area of PTTI. These include applications operating under environmental extremes, with low power consumption and light, compact design, and autonomous submarine operation over long periods. PTTI technologies enable

• Rapid direct Y-code acquisition of GPS and coded communications,

• Precise positioning and maneuver of assets,

• GPS-guided munitions,

• Network-centric warfare capability using high-speed, broadband data transmission to war-fighting units, and

• Greater precision low-cost weapons with improved antijam capabilities.

Finding: Only the Navy possesses the technical capabilities and sustained interest in PTTI that will meet DOD needs for operational atomic clocks.

The Navy has historically played a large role in PTTI. The depth and scope of its S&T budget for PTTI research over the past 30 years exceed the depth and scope of the U.S. Army and Air Force investments. USNO is the designated agency for providing a real-time estimate of Coordinated Universal Time (UTC). Additionally, DOD directive 5160.51 recognizes the Navy as the PTTI manager. The Navy has served as a resource for others in the DOD requiring PTTI. Traditionally, the ONR has taken the lead in supporting research and development in PTTI because of its critical importance for navigation.

While other services do fund PTTI-related research and development, there are certain aspects of PTTI that depend directly on Navy capabilities and investments. These include but are not limited to

• Maintenance of state-of-the-art atomic clock stability in space military environments,

• Maintenance of a fundamental time reference with at least an order of magnitude better timing accuracy than that used by any DOD application, and

• Global dissemination of precise timing signals for military use.

Recommendation: Based on the above considerations, the committee recommends that PTTI should be established as a National Naval Responsibility.

Finding: Significant opportunities to advance PTTI science and technology do exist and can be pursued if resources are directed at them.

Pursuit of advanced concepts such as those utilizing optical frequencies should advance the precision available today in laboratory clocks by one or more orders of magnitude. Local oscillator improvements can be obtained by better understanding of quartz crystal systems and by pursuit of optoelectronic oscillators. Materials development can better isolate sensitive clock elements from noise and decrease the size and weight of devices, enabling higher-precision PTTI applications on the battlefield. MEMS, nanoscale devices, and coherent population trapping may also play a role in enabling smaller, lighter devices with greater precision than is currently available on the battlefield.

Finding: Some areas of science and technology important to DOD PTTI applications are currently given insufficient attention, such as research to improve synchronization, local oscillators, and the ruggedness of small, low-power clocks.

Researchers have achieved orders of magnitude improvement in the accuracy of laboratory time measurements in the last two decades. We are not yet able to utilize this accuracy in fieldable systems, partly because not enough research has been devoted to areas critical to development of operational devices and partly because research, especially applied research, has not been effectively transitioned into fieldable capabilities.

Finding: Continuity of programs is required to ensure that expertise is transferred from one generation of PTTI researchers to the next.

The technology underlying PTTI is reliant on a level of precision not found in most other technologies. It places requirements on practitioners that cannot be learned without several years of training. To successfully build PTTI systems requires an understanding not just of the fundamental physics involved in high-precision frequency or time measurements, but also of all of the engineering factors that go into building a system that delivers that high precision in the real world. No course of academic training can produce an accomplished PTTI scientist; rather, an apprenticeship of sorts is required, in which the early career researcher works side by side with the accomplished PTTI scientist to learn the nuances of producing real systems of such incredibly high precision. For this reason, PTTI expertise, if lost, could not be readily rebuilt and certainly could not be rebuilt in a short (5 years) time.

Recommendation: ONR should stabilize its 6.1 funding of PTTI-related research at its current level or higher. Means should be sought to broaden basic research beyond atomic, molecular, and optical physics so that it includes enabling advances in materials science, chemistry, photonics, and other relevant areas. Support for quartz crystal research is of particular importance.

Recommendation: To improve the insertion of PTTI advances into DOD capabilities, ONR should increase 6.2/6.3 funding in PTTI, focusing on improvements in ruggedness; decreasing the size, weight, and power consumption of PTTI devices; and developing new and improved techniques for precise time dissemination and coordination.

Finding: Coordination and planning of PTTI activities between the Services, with DARPA, and between DOD and other agencies is fragmented. Both internal and external guidance are needed.

PTTI is important to all four branches of the military. Each has slightly different needs in PTTI, and three of the four branches, as well as DARPA, support their own program in PTTI research and development. However, the Department of the Navy’s program supports more fundamental research than do those of the other Services. Based on the committee’s data gathering and on its members’ experiences, there seems to be little coordination of PTTI research and development activities between the Services. Of particular concern is an apparent lack of the planning required to leverage advances at both the basic and applied level and translate them into better military system performance. Coordination between the Services and with DARPA would result in a quicker and more complete capture of the benefits that come from PTTI advances. Similarly, coordination between DOD and other federal agencies with significant interest in PTTI technologies, such as NASA and NIST, could be strengthened.

The Department of the Navy, as the DOD PTTI manager, is responsible for coordinating the development of PTTI techniques among DOD components. As already noted, the committee finds that such coordination is lacking. This may be due in part to the difficulty of one Service (Department of the Navy) understanding and addressing the needs of the others. Not only is PTTI broader than the Department of the Navy, it is also broader than any one Department of the Navy or DOD program. For example, in the technology area review and assessment (TARA) process overseen by the DOD Director of Defense Research and Engineering (DDR&E), different aspects of PTTI research fall under different TARAs. Within ONR, PTTI-related 6.1, 6.2, and 6.3 research and development are found in several different program codes. They are also found within NRL. The committee could find no instance in which the PTTI program as a whole is regularly reviewed and no instances of external review. The committee is aware that the DOD PTTI manager convenes an annual review of advances in PTTI science; while this is useful, it does not replace review of the DOD strategy in PTTI. The committee has

already noted research areas that promise advances in PTTI and its applications that are not currently receiving significant attention from the Department of the Navy or DOD. The committee believes that regular advice from external technical experts could help the Department of the Navy construct its investment strategy and address these problems of breadth and coordination as it carries out its responsibilities as PTTI manager.

Recommendation: DDR&E should coordinate PTTI research DOD-wide and develop an insertion plan to ensure that 6.2/6.3 advances in PTTI are transitioned into operational military systems. To help DDR&E and to ensure that the Navy’s responsibilities as PTTI manager are met, the Department of the Navy should convene regularly (at least annually) an outside, independent group of experts to address DOD needs in PTTI and opportunities in PTTI research.