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

Knowledge of time is essential to precise knowledge of location, and for this reason the Navy, with its need to navigate on the high seas, has historically played an important role in the development and application of advanced time realization and dissemination technologies. Discoveries coming from basic research funded by the Office of Naval Research (ONR) lie at the heart of today’s highest performance atomic clocks, Naval Research Laboratory (NRL) expertise played a role in developing the space-qualified atomic clocks that enable the Global Positioning System (GPS), and the U.S. Naval Observatory (USNO) maintains and disseminates the standard of time for all of the Department of Defense (DOD). The Navy has made major investments in most aspects of precision time and time interval (PTTI) science and technology, although specific PTTI-related research has also been funded by the Defense Advanced Research Projects Agency (DARPA) and non-DOD agencies such as the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), and the Department of Commerce. Navy funding, largely through ONR, has a history of being an early enabler of key new developments. Judicious funding decisions by the Navy—particularly by ONR program officers—have underpinned most of the major advances in PTTI science and technology (S&T) in the last 50 years. Chapter 1 describes the Navy’s contributions to PTTI S&T (hereinafter referred to simply as “PTTI”) in detail.

PTTI is important to modern naval needs, and indeed to all the armed Services, for use in both navigation and communications. Precise time synchronization is needed to efficiently determine the start of a code sequence in secure communications, to perform navigation, and to locate the position of signal emitters. Precise frequency control is required in communications for spectrum utilization and frequency-hopped spread-spectrum techniques. There are many examples of essential military operations that depend on PTTI and could benefit from improvements in PTTI technology. These include:

• GPS clocks and autonomous operations,

• Weapon system four-dimensional coordination,

• GPS antijamming,

• Network-centric warfare, and

• Secure military communications.

Realizing reductions in the size, weight, and power requirements and increases in the ruggedness of PTTI devices without sacrificing performance would put more accurate and precise timekeeping in the hands of the warrior, improving capabilities in all of the above operations. Chapter 5 discusses applications of PTTI techniques and devices to real military scenarios.

Research areas related to PTTI that, if pursued, should lead to improved military capabilities include the following:

• Atomic, molecular, and optical physics. Recent advances such as the realization of Bose-Einstein condensates hold promise for pushing the limits of our fundamental laboratory time standards.

• Materials science. Advanced materials can help isolate the high-performance elements of atomic clocks from environmental effects and reduce the size and weight of these devices. Materials advances will be necessary, for example, to successfully produce a chip-scale atomic clock.

• Chemistry. Advances that improve the availability of high-perfection quartz would have immediate impact on devices that currently rely on quartz crystal oscillators as clocks or as local oscillators. This includes most PTTI devices used by the DOD.

Chapter 4 discusses these and other research areas that can be pursued to advance DOD capabilities in PTTI.

The health of the U.S. PTTI infrastructure, discussed in Chapter 3, is mixed. The United States has been a leader in PTTI science and technology since at least the time after World War II, but in the last two decades its dominant position in PTTI has been eroded by increased foreign investment in PTTI sciences. University programs in atomic, molecular, and optical (AMO) physics and precision measurement produce sufficient numbers of scientists to enter PTTI work, but these researchers require 5 or more years apprenticeship after their formal studies before they become independently productive PTTI researchers. This is unlike the situation in several other countries, where there are special programs of study in key aspects of PTTI science and technology. Certain aspects of the U.S. PTTI infrastructure are in jeopardy, especially U.S. expertise in high-performance crystal oscillators. Industrial support for DOD needs in high-performance clocks and frequency standards is narrow and shallow. Most industrial firms focus their efforts on the much larger and less demanding low-performance commercial markets and could not readily shift their product development and production into high-performance devices, which are highly complex. The handful of firms that is capable of producing high-performance devices for defense purposes finds the demand for these devices to be too small and intermittent to support a business. Private industry currently has no incentive to conduct R&D related to high-performance devices suitable for military use.

Based on its analysis of the above factors and the current status of U.S. PTTI research and development, the committee has formulated a dozen findings and five associated recommendations.

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

PTTI has proven to be of enormous leverage in war fighting. Advances in a number of areas, including reduction in the size, weight, and power requirement of mobile devices, improved ruggedness of field systems, and better precision and accuracy for 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. Advanced GPS satellite clocks would greatly improve ranging accuracy, reduce collateral damage, and enhance system survivability. Reliable oscillators would improve the jamming resistance of GPS-guided munitions by enabling faster direct acquisition of the GPS signal in cases of jamming. Faster dissemination of command knowledge on the battlefield, combined with smaller, more precise weapons, would reduce collateral damage and thus reduce operational and political costs.

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 in the past. 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 PTTI 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.

In particular, 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 in which the performance of these oscillators could be improved, which would translate directly into improved systems for the warrior. Without researchers trained in crystal oscillators, the United States risks losing out on these advances in capabilities.

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

A large market for inexpensive quartz crystal oscillators exists for applications such as watches 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, with the largest market for moderate-quality clocks being commercial telecommunications. Relative to this market, the purchase of high-precision clocks for defense systems is infrequent and involves a small number of clocks.

Industry needs a steady source of funding to maintain production capability for 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 at Harvard University in the 1940s led to the design of the microwave cavity that remains an essential part of high-accuracy clocks today. ONR was an important early funder of work in the 1980s by Bill Phillips (at the National Institute of Standards and Technology (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. These investigators all had as their primary goal advancing fundamental scientific understanding of the nature of nuclei, atoms, and molecules, but 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. The NRL Timation project and subsequent Navy developments in satellite navigation and orbiting precision clocks led to today’s GPS and led to NRL becoming the DOD’s lead developer 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 PTTI needs. These include applications operating under environmental extremes, with low power consumption and light, compact design, as well as clocks for autonomous submarine operation over long periods.

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

The Navy has always played a large role in PTTI. The depth and scope of its S&T budget for PTTI research over the past 30 years exceeds that of the Army and Air Force budgets. USNO is the designated agency for providing Coordinated Universal Time (UTC). Additionally, the Navy is recognized through DOD directive 5160.51 as the PTTI manager. The Navy has served as a resource for others in the DOD requiring PTTI. Traditionally, ONR has taken the lead in supporting research and development in PTTI owing to PTTI’s critical importance for navigation.

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

• 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 concepts utilizing optical frequencies or coherent population trapping should improve the precision available today in laboratory atomic clocks by one or more orders of magnitude. Local oscillator improvements can be obtained by better understanding quartz crystal systems and by pursuing optoelectronic oscillators and other high-performance microwave oscillators. Materials development can improve the isolation of sensitive clock elements from noise and decrease the size and weight of devices, enabling higher-precision PTTI applications on the battlefield. Microelectromechanical systems (MEMS) and nanoscale devices may also enable smaller, lighter devices with higher 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 do not yet have the capability to utilize this accuracy in fieldable systems, partly because of insufficient research devoted to areas critical to the development of operational devices and partly because research, especially applied research, is not being 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 deals with a level of precision not found in most other technical areas. It places requirements on practitioners that cannot be learned without several years of training. No course of academic training can produce an accomplished PTTI scientist; rather, an apprenticeship of sorts is required, in which an early career researcher works side by side with an accomplished PTTI scientist to learn the nuances of producing real systems of incredibly high precision. For this reason PTTI expertise, if lost, could not be readily rebuilt and certainly not in a short (5-year) time frame.

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 support 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 insertion of PTTI advances into DOD capabilities, ONR should increase 6.2/ 6.3 funding in PTTI, focusing on achieving 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.

Based on the committee’s data gathering and on its members’ experiences, there seems to be little coordination of PTTI research or 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 levels to improve 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 the 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 PTTI manager for DOD, currently has responsibility for coordinating the development of PTTI techniques among DOD components. Not only is PTTI broader than the Department of the Navy, it is also broader than any single Department of the Navy or DOD program. The committee could find no instance in which the PTTI program as a whole is regularly reviewed and no instances of previous 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 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.

Recommendation: The Director of Defense Research and Engineering (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.