orbit precision clocks. Signals from such a satellite could provide more precise navigation as well as precise time signals that were available worldwide. To achieve this goal, NRL started programs to develop improved quartz frequency standards suitable for spaceflight. Soon thereafter, the Timation program, which involved atomic clocks in space, was established. These space-qualified atomic clocks were then used in the Global Positioning System (GPS). GPS became a joint Service program in 1973, with the Air Force designated executive agent for the system. NRL became a key participant in the development of advanced atomic clocks for flight in GPS satellites. This NRL program nurtured industrial development of space-qualified atomic clocks, developed alternative sources for clocks, supported advanced clock development, provided testing services for the space qualification of clocks, and, ultimately, provided an in-house expertise base from which all DOD space clock programs can draw.

The USNO Time Service provided its first electrically delivered time signals in 1865, when a telegraph signal was used to synchronize clocks at a number of naval facilities. In 1904 the Navy was the first to broadcast a time signal via radio. In the World War II period, the Navy was involved in the development of radio navigation systems that used time of arrival of signals rather than a less precise radio direction method. These long-range navigation (LORAN) systems were used until recently for time transfer and coordination.

LORAN suffered from unknown time propagation delays that needed to be calibrated for the most precise uses. The Navy sponsored a number of flying-clock experiments to provide this calibration, among other things. Despite LORAN’s limitations in precise time transfer, it was well suited for frequency comparison. In the late 1950s, in collaboration with Britain’s NPL, USNO used the LORAN system to determine the frequency of the cesium transition relative to the Ephemeris second. In a similar experiment in the early 1960s, USNO and Varian Associates used the LORAN system to determine the frequency of the transition in the hydrogen maser.

The GPS system provides one of the best and the most ubiquitous time coordination systems ever.³ Since the essence of the system is a time-encoded signal, it was a simple matter to use the system for time transfer. The USNO, in collaboration with various timing labs around the world, contributed to the development of the three most common ways to transfer time or time difference via the GPS system: the one-way, common-view, and carrier-phase techniques. The USNO also helped develop two-way satellite time transfer (TWSTT). TWSTT involves the use of communication satellites and active transmission from ground sites, making it complex and expensive to use. But it can give short-term stability for determination of time difference similar to GPS carrier phase and absolute time synchronization that may be better than GPS common view, and so is the method of choice for certain applications.

Warfare has always been four-dimensional. Both location (latitude, longitude, altitude) and time play a critical role in defense and battle. Highly accurate clocks and frequency sources are of vital importance to DOD, because the accuracy and stability of these devices are key determinants of the performance of command, control, communications, and intelligence (C3I); navigation; surveillance;

Front Matter (R1-R12)

Executive Summary (1-6)

1. The Navy and PTTI (7-11)

2. State of the Art of PTTI Physics and Devices (12-22)

3. State of PTTI Research and Infrastructure (23-33)

4. Research Opportunities in PTTI (34-38)

5. Defense Needs for PTTI (39-43)

6. Findings and Recommendations (44-50)

Appendix A: Committee Biographies (51-55)

Appendix B: Tutorial on PTTI Frequency Standards (56-71)

Appendix C: Acronyms and Abbreviations (72-74)

Appendix D: Glossary (75-76)