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The Navy and PTTI

THE NAVY’S ROLE IN DEVELOPING PTTI

Naval operations and commercial shipping have been drivers for many technological developments in precision time and time interval science and technology (PTTI). In the Age of Exploration, the inability to determine longitude accurately made navigation on the open seas difficult and treacherous. Determining longitude required comparing the time at the current location with the time at a known location, say the Greenwich meridian. No shipboard clocks could determine time to an accuracy sufficient for navigational purposes. Heads of several seafaring nations offered great prizes for a solution to the problem of longitude. In the early 18th century, the Longitude Prize offered by Britain led to the development of the ship’s chronometer. This device was so amazingly workable that it remained in use unchanged in its essential elements until the electronic era of the early 20th century.1

Following World War I and the development of the electronic oscillator and radio communications, the U.S. Navy took an ever more active role in the development of emerging PTTI technologies. The U.S. Naval Observatory (USNO), the Naval Research Laboratory (NRL) and, after World War II, the Office of Naval Research (ONR) were important players in the development of the technology that makes up the current state of the art in PTTI. Although the defense applications of PTTI now go well beyond navigation, the U.S. Navy has maintained its leadership role in the field. The discussion that follows highlights those events in the history of PTTI that have had significant Navy support.

Atomic Clocks

The advances that had been made in high-frequency electronics during World War II radar research set the stage for the development of atomic clocks.2 In 1942 the Joint Chiefs of Staff established a

1  

The story of the search for a practical method of determining longitude at sea is told by Dava Sobel in Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Walker and Co., New York, N.Y., October 1995.

2  

For a more complete discussion of the history of atomic clocks, see Norman F. Ramsey, “The Past, Present, and Future of Atomic Time and Frequency,” Proceedings of the Institute of Electrical and Electronics Engineers, Vol. 79 (1991), No. 7, pp. 921-926.



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An Assessment of Precision Time and Time Interval Science and Technology 1 The Navy and PTTI THE NAVY’S ROLE IN DEVELOPING PTTI Naval operations and commercial shipping have been drivers for many technological developments in precision time and time interval science and technology (PTTI). In the Age of Exploration, the inability to determine longitude accurately made navigation on the open seas difficult and treacherous. Determining longitude required comparing the time at the current location with the time at a known location, say the Greenwich meridian. No shipboard clocks could determine time to an accuracy sufficient for navigational purposes. Heads of several seafaring nations offered great prizes for a solution to the problem of longitude. In the early 18th century, the Longitude Prize offered by Britain led to the development of the ship’s chronometer. This device was so amazingly workable that it remained in use unchanged in its essential elements until the electronic era of the early 20th century.1 Following World War I and the development of the electronic oscillator and radio communications, the U.S. Navy took an ever more active role in the development of emerging PTTI technologies. The U.S. Naval Observatory (USNO), the Naval Research Laboratory (NRL) and, after World War II, the Office of Naval Research (ONR) were important players in the development of the technology that makes up the current state of the art in PTTI. Although the defense applications of PTTI now go well beyond navigation, the U.S. Navy has maintained its leadership role in the field. The discussion that follows highlights those events in the history of PTTI that have had significant Navy support. Atomic Clocks The advances that had been made in high-frequency electronics during World War II radar research set the stage for the development of atomic clocks.2 In 1942 the Joint Chiefs of Staff established a 1   The story of the search for a practical method of determining longitude at sea is told by Dava Sobel in Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Walker and Co., New York, N.Y., October 1995. 2   For a more complete discussion of the history of atomic clocks, see Norman F. Ramsey, “The Past, Present, and Future of Atomic Time and Frequency,” Proceedings of the Institute of Electrical and Electronics Engineers, Vol. 79 (1991), No. 7, pp. 921-926.

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An Assessment of Precision Time and Time Interval Science and Technology Radio Propagation Laboratory at the National Bureau of Standards (NBS), now the National Institute of Standards and Technology (NIST). The Radio Propagation Laboratory developed the world’s first atomic clock in 1948. This clock was based on the measurement of a spectroscopic absorption line in ammonia. Because its stability was no better than that of high-quality quartz oscillators, the ammonia system was quickly abandoned for the greater potential accuracy of the cesium atomic beam device. At the heart of this device, brought into operation at the NBS in 1951, was a microwave cavity design developed in 1948 by Norman Ramsey of Harvard University, with funding from the ONR nuclear physics branch. This cavity design and the interrogation method developed with it have proven so essential to high-accuracy atomic clocks that they remain part of all advanced clocks today. (Ramsey received the Nobel Prize for this work in 1989.) The National Physical Laboratory of Britain (NPL) had developed this cesium beam standard into an operable clock by 1955. NPL then teamed with William Markowitz of the USNO to measure the frequency of the cesium transition relative to Ephemeris Time. The result of this measurement now defines the fundamental unit of time, the second. In a remarkable effort led by Jerrold Zacharias of the Massachusetts Institute of Technology and partially funded by ONR, in 1955 the technology of the cesium atomic beam clock was transformed into a commercial product, the Atomichron, at the National Company. The NRL took delivery of the first unit produced. In the late 1950s the rubidium cell clock was developed at NBS. It was tested in collaboration with NRL, using NRL’s Atomichron and a classified NRL microwave synthesizer that NBS researchers were not allowed to examine. This rubidium clock technology is the workhorse of our space-based clocks today. In 1960, with ONR funding, Ramsey developed the hydrogen maser. Subsequently, with funding and technical support from NRL, the hydrogen maser clock was brought into semicommercial production. Forty years later, this clock technology still produces the best short- to medium-term clock stability commercially available. These three clock types—the cesium atomic beam, the rubidium gas cell, and the hydrogen maser—make up the totality of our commercially available atomic clock technology. Navy involvement has also been vital to the development of the most advanced laboratory atomic standards and the technology that will produce our next generation of high-performance atomic clocks. NRL funded the development of the buffer-gas-cooled mercury ion frequency standard at Hewlett-Packard. Several units were produced and delivered to the USNO. ONR funded basic research in the laser cooling of ions and atoms, which has led to our most accurate laboratory standards today—the cesium atomic fountain and the emerging optical clocks. (This work also led to the 1997 Nobel Prize in physics for Bill Phillips of NIST.) ONR-funded work in Bose-Einstein condensates (for which the 2001 Nobel Prize in physics was awarded to Eric Cornell of NIST, Carl Wieman of the University of Colorado, and Wolfgang Ketterle of the Massachusetts Institute of Technology) and the area of quantum entanglement is expected to find application in future, still higher performance atomic clocks. Space Clocks, Timation, and GPS Immediately following the launch of the first artificial Earth-orbiting satellite, Sputnik, by the Soviet Union in 1957, the Navy set up the Naval Space Surveillance System (NAVSPASUR) to track satellites, and shortly afterward a group at Johns Hopkins University’s Applied Physics Laboratory (JHU/APL) began to track satellites by Doppler shift. Operated in reverse, this technique allowed simple two-dimensional navigation. The concept led APL to develop the first satellite navigation system, Transit, in the early 1960s, with Navy and Advanced Research Projects Agency (ARPA) funding. In 1964, Roger Easton of the NRL put forward a concept for an improved system that would

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An Assessment of Precision Time and Time Interval Science and Technology 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. Time Coordination 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. IMPORTANCE OF PTTI TO THE NAVY AND MODERN WARFARE 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; 3   For an overview of the Global Positioning System and its operations, see NRC Committee on the Future of the Global Positioning System, Global Positioning System: A Shared National Asset, Aeronautics and Space Engineering Board, National Academy Press, Washington, D.C., 1995.

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An Assessment of Precision Time and Time Interval Science and Technology electronic warfare; missile guidance; and identification, friend or foe (IFF) systems. DOD systems such as GPS, military strategic and tactical relay (MILSTAR), joint surveillance target attack radar system (JSTARS), Patriot, advanced medium-range air-to-air missile (AMRAAM), joint tactical information distribution system (JTIDS), and many classified programs are based on clocks with much greater accuracy and oscillators with much less noise than are required for commercial applications. The most precise methods of vehicle positioning and navigation rely on accurate time, and many aspects of communications (synchronization, encryption) rely on precise time. From the general perspective of the DOD, precise time synchronization is needed primarily to efficiently determine the start of a code sequence in secure communications, to perform navigation, and to locate the position of signal emitters by means of time difference of arrival (TDOA), as is done, for example, in GPS positioning. Similarly, precise frequency control is required in communications, particularly for spectrum utilization and for frequency-hopped spread spectrum. Certain approaches to position location also rely on precise frequency determination. Frequency difference of arrival (FDOA) positioning techniques measure Doppler-induced frequency differences produced by a moving source and detected at spatially separated sites simultaneously or at a fixed reception site over time. The latter approach was used in the TRANSIT satellite navigation system. Conversely, FDOA can be based on frequency differences observed over time by a moving receiver locating a fixed source, as might be used in electronic intelligence activities. Finally, radars require precise frequency for moving target indication. DOD applications require increasingly precise time and frequency information distributed to widely separated sites, necessitating effective means of time and frequency dissemination; GPS is currently the most widely used means. These needs for PTTI are shared among the four Services, but the most precise requirements are those of the intelligence community. While there are similarities between the PTTI requirements of the Services, there are also significant differences. The Army and the Marine Corps are oriented toward small size and very-low-power time and frequency devices to serve the needs of the soldier in the field. The Air Force has both space-based requirements, which are strategic, and aircraft-based requirements, which are tactical. Space-based applications of time and frequency devices demand long-term performance, environmental sensitivity, and reliable clock technology. Tactical applications usually have moderate performance requirements (in terms of stability and precision) but severe operating condition requirements, such temperatures from −55°C to +85°C and operation in high humidity or in highly dynamic environments requiring performance under conditions of acceleration and vibration. The Navy has both tactical (e.g., aircraft and ships) and strategic (e.g., submarine) needs and is unique in its need to communicate and synchronize between highly distributed assets. Consideration of conflicts that the United States has participated in over the last 35 years illustrates the importance PTTI has on the battlefield. Many Navy pilots were lost in attempts to destroy a bridge over the Red River in North Vietnam during the late 1960s. Weapons could not be targeted precisely enough to destroy the target. In contrast, recent conflicts have seen tremendous reductions in casualties and new capabilities because of improved targeting. During the Gulf War, Iraq could not comprehend an attack from the featureless desert, but the famous End Run maneuver, enabled by GPS and atomic clocks, was exactly that. In Bosnia, GPS enabled all-weather and nighttime delivery of precise weapons. Local air defenses forced high-altitude delivery of the weapons, yet this enhanced weapon effectiveness, since there was more time for the guided munitions to acquire GPS signals. The media coverage of the Bosnian conflict enhanced sensitivity to collateral damage. This called for consideration of the bounded inaccuracy of weapons—an estimate of the bound on the probability of an event (for example, the probability of a weapon system miss beyond a certain distance from the target). GPS allows the

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An Assessment of Precision Time and Time Interval Science and Technology weapons operator to place usable bounds on this parameter, allowing bomb drops close to troops in contact with or near civilian targets, while avoiding fratricide or civilian casualties. Afghanistan continues the trend with weapons launched from long-distance, remotely piloted vehicles (RPVs). News photos have illustrated the awesome capabilities for precision strikes that have been enabled by precise timing capabilities. Chapter 5 provides concrete examples of how PTTI affects the warfighter and what battlefield capabilities might be enabled by improvements in PTTI. THE NAVY’S OPERATIONAL RESPONSIBILITIES IN PTTI Navy responsibility in PTTI is currently designated in DOD Instruction 5000.2, Part 7, Section C, which calls for the Navy to Maintain the DOD reference standard through the USNO, and Serve as the DOD precise time and time interval (frequency) manager, with responsibilities for Developing an annual DOD-wide summary of precise time and time interval requirements and Coordinating the development of precise time and time interval techniques among DOD components. SECNAV Instruction 4120.20 assigns responsibilities within the Department of the Navy for implementation of the Navy’s responsibilities in PTTI. It names the Assistant Secretary of the Navy (Research, Engineering and Systems) as the Department of the Navy PTTI Coordinator and the Superintendent of the USNO as the DOD PTTI Manager. It further states that the Chief of Naval Operations shall program funds necessary to maintain the DOD reference standard and the means for dissemination to the accuracies required by the components and user agencies. In addition to maintaining the DOD Master Clock for the DOD, USNO has an active research effort in clock development, time-scale algorithms, and time transfer through both GPS carrier phase and TWSTT. NRL maintains the Navy expertise in space clock technology, providing services and advice to Navy and DOD programs related to the space-based clocks used in GPS and other systems. NRL uses both in-house programs and commercial vendors to develop and test space clock technology. ONR sponsors basic research and development intended to enable future advances in PTTI technologies.