The Criticality of Time

Today, virtually every advance in modern technology demands a highly accurate, stable and reliable time standard. Atomic clocks are at the top of the timekeeping ``food chain,'' setting the standard by which all other precision instruments are calibrated. These instruments, in turn, are used to design, manufacture and operate virtually all modern technologies.

One of the earliest, most vital, and universal needs for precise time information was, and still is, as a basis for place location. Navigators of ships at sea, planes in the air, and even small pleasure boats depend constantly and continuously on accurate time information to know where they are and to chart their course.

Hail Britannia

By the beginning of the 18th century, ship building had improved to the point where larger, stronger vessels made ocean trade -- as well as ocean warfare -- a practical reality. But too often ships laden with precious cargos were lost at sea, driven off course by storms, with the crew unable to determine where they were or to chart a course to a safe harbor.

Navigators had long been able to read their latitude north of the equator by measuring the angle formed by the horizon and the North Star. But east-west navigation was almost entirely a matter of ``dead reckoning.'' If there were a clock aboard that could accurately tell what time it was at the zero meridian in Greenwich, England, however, it would be easy to determine a position east or west of there.

Realizing the potential benefits of improving its ships' ability to navigate the seas, both militarily and commercially, the British government in 1713 offered an award of £20,000 to anyone who could build a clock that would serve to determine a ship's longitude to within 1/2 degree. It was this critical need for accurate, dependable clocks, underscored by the enormous award, that pushed inventors of that time into developing better and better timepieces.

Among those seeking this handsome award was an English clock maker named John Harrison. Harrison spent more than 40 years trying to build a clock that had the required accuracy and would be stable enough to cope with the rolling seas, temperature changes that caused intolerable expansion and contraction of delicate metal springs, and salt spray that corroded everything aboard ship. When finally he developed a chronometer he considered nearly perfect, the government commission was so afraid of losing it at sea that testing was delayed until Harrison built a second unit identical to the first to provide a pattern.

Finally, in 1761 Harrison's son William tested the instrument on a voyage to Jamaica. In spite of a severe storm that lasted for days, driving the ship far off course, the chronometer proved to be amazingly accurate, to within 54 seconds over a 5-month period, or about 1/3 second per day, and making it possible to determine his longitude at sea within 18 minutes of arc, or less than 1/3 of one degree. Harrison claimed the £20,000 award, part of which he had already received, and the remainder was paid to him in various amounts over the next two years -- just three years before his death.

The Need for Greater Precision

Today, the most accurate clocks in the world are atomic clocks. Originally developed for commercial applications by Hewlett-Packard Company in 1964, atomic clocks achieve their accuracy by synchronizing a microwave oscillator with the vibrations of the cesium 133 atom, an accepted worldwide definition of time. Beating more than 9 billion times per second, these atomic clocks are the yardsticks against which all other clocks in the world are measured.

The demand for clocks with ever greater levels of precision is now being driven by the needs of high technology. Crafted by physicists and electrical engineers, atomic clocks play a vital role in modern communications, synchronizing the rapid movement of information through telephone systems and computer networks. They provide the accuracy required to properly generate the signals transmitted by virtually every radio and television station in the world. And state-of-the-art satellite-based navigation systems used by everything from cruise ships to space shuttles rely on the precise timing provided by atomic clocks. Celestial navigation and communication, as well as the work of astronomers to determine the age of the universe and the motion of stars and galaxies, require a degree of exactness that already pushes the limits of today's atomic clocks.

Secure Communications

Many government agencies, including the military, require secure, jam-proof radio communications channels. One sophisticated technique currently being used is the so-called spread-spectrum or frequency-hopping radio that provides jam-resistant voice communications without the risk of eavesdropping. Eventually, this technique may be applied to cellular telephones to reduce line drops caused by interference from buildings and other structures.

A conventional radio signal is transmitted at single frequency. The listener merely tunes a receiver to that frequency to hear the signal. Obviously, anyone with a receiver capable of tuning to that frequency can also listen in, or set up a transmitter at that frequency to jam the original signal.

A secure, frequency-hopping transmitter changes frequencies at irregular, ``pseudo-random'' time intervals, choosing specific combinations of frequencies that depend on a code word of the day. The listener must have a receiver that follows an identical choice of frequencies in precise time synchronization with the transmitter.

Some of the secure radio systems in use today are first synchronized to Coordinated Universal Time (UTC), derived from atomic clocks. The more exotic frequency-hopping radios even contain their own atomic clocks, enabling them to maintain the required accuracy independently for many weeks without being synchronized again. These clocks can, in turn, be used to synchronize other radios that may have drifted too far to operate properly.

Telephone Networks

Public and private communications networks require precise timing at their switching points to accurately convey voice, data and video signals in digital form. Recently, all three major U.S. carriers announced upgrades to their network synchronization methods to provide higher quality transmission to customers of high-speed data circuits and services. The impetus for the upgrades is the increasing use of the networks to carry information in digital form and the anticipation of the Synchronous Optical Network (SONET), the proposed international standard for fiber-optic, high-speed data transmissions and formats.

Carriers must synchronize their networks to ensure the smooth flow of information across circuits and numerous switches without garbling the data or dropping the connection altogether. While voice communications are rather tolerant of timing errors in the networks, resulting in small clicking sounds, digital data transmissions can come to a screeching halt under the same conditions. As data rates increase above 1 megabit per second (one million bits per second), the possibility of errors due to timing problems increases dramatically. When future services like switched video enter the scene, even higher data rates will be required, making the need for highly accurate timing all the more evident.

To provide this synchronization, all the carriers use atomic clocks to provide timing signals to their networks. But even these clocks have a tendency to drift off frequency, and so they must be checked and controlled by some outside source, such as the Department of Defense's Navstar Global Positioning System (GPS), or U.S Coast Guard's LORAN (Long Range Navigation) signals, which in turn are controlled by other atomic clocks.

AT&T announced the most ambitions program comprising a 16-node distributed network. Synchronization for the network will be provided by atomic clocks calibrated and timed with signals from the GPS, which provides precise timing signals generated by atomic clocks. The new clocking scheme is expected to be accurate to one part in 10E13, or one error in 10 trillion events.

Radio Telescopes

Scientists are using the radio signals transmitted by distant stars to measure to a few centimeters the distance between widely-separated parts of the surface of the earth. Such measurements may give new insights into earth crust movements (like continental drift) and deformations that may be crucial for the prediction of earthquakes.

An antenna set up at each of two points to be measured on the earth receives the radio signal from the star. Each antenna receives the signal at a slightly different time because the star is not directly overhead and the signal must travel a slightly different distance. Both antennas receive the same signal, but one lags the other in time, and these signals are recorded on tape along with time signals from two synchronized atomic clocks. If the clocks are synchronized to a very high degree of accuracy and the position of the star is known, the separation between the two receiving antennas can be determined mathematically based on the measured time lag.

The same technique is used to determine the distance the stars. By knowing the distance between two radio telescope antennas and the time lag between the signals they receive, the distance to the star can be calculated.

Traffic Control

In 1988 Los Angeles County embarked on an innovative project to synchronize its traffic lights with the atomic clocks at the National Institute for Standards and Technology (NIST) in Boulder, CO. The precision of the official U.S. timekeeping signals, broadcast by radio station WWV, improves the timing of stoplights on major roads in the county, allowing traffic to flow more smoothly at the posted speeds.

The $13-million, five-year plan will install radio receivers tuned to WWV at more than 1000 intersections along 43 of the county's busiest roads. These will replace the current system of synchronization that uses buried telephone lines connecting lights at one intersection with others. Under the old system, signal controllers at a dozen intersections failed to operate in step on any given day, wreaking havoc with the entire system. Additional funding is being sought to add another 1200 intersections to the system.

Officials claim motorists will save 55,000 hours a day in driving time and 22 million gallons a year in fuel otherwise spent slowing down, stopping, and speeding up again because of improperly synchronized traffic signals.

Uniformity of Time

The atomic clocks of today achieve a precision unimaginable to clock makers of just 100 years ago. Yet the quest for clocks with even greater accuracy goes on. New technologies based on lasers and mercury atoms could be as much as 100,000 times more accurate than today's atomic clocks.

The history of timekeeping has been the search for systems that keep time with greater and greater uniformity. Ever since the Egyptians brought the sundial into use nearly 3000 years ago, the ability to measure time accurately has played a pivotal role in the advancement of civilization. By knowing the time, the movements of people, armies and assembly lines can be coordinated, the seasons, plantings and harvests become manageable, and oceans and heavens can be navigated. Today, modern technologies -- from garage door openers and remote control TVs to radio telescopes and data communications networks -- are the primary forces driving the need for commercially available clocks of increasingly high precision.

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