US Naval Observatory: Providing Precision Time & Location Data
The U.S. Naval Observatory (USNO), the historic origin of METOC, today performs some of the American Military’s most technical, far-reaching, and critically-important work. While not directly involved in meteorological and oceanographic observation and prediction, the USNO provides precise time and location data, which is used not only by other components of METOC and the Navy, but by the entire Department of Defense—and the entire world.
The Observatory, founded in 1830 in Washington D.C. as the Naval Depot of Charts and Instruments, was established to ensure the proper function of all the Navy’s navigation equipment, notably chronometers. Accurately functioning chronometers were vital to maritime travel at that time as they are necessary to gauge longitude while at sea. Calibrating and gauging the reliability of each instrument was performed by comparing its function against the timing of the rotation of the earth. To do this, the observatory used a “transit telescope,” an optical system that precisely measures positions of celestial bodies, notably stars, as they visually intersect the meridian passing through the observatory’s location as the planet rotates.
From these beginnings, the small facility soon began disseminating time and celestial body information.
In 1845, personnel at the Depot began dropping a large ball at noon every day, visible to citizens of Washington D.C. as well as to ships plying the Potomac River. In later years, Western Union transmitted USNO coordinated time throughout the United States. Starting in 1852, the Depot began publishing the “American Ephemeris and Nautical Almanac,” an annual publication giving precise positions of celestial bodies, necessary for maritime navigation and a number of terrestrial applications, like surveying. The information is still published each year, although it has been divided into two publications, the Nautical Almanac and the Astronomical Almanac, since 1981.
Today, the USNO continues to provide time and positioning information, and the instruments the facility uses—descendants of its earliest telescopes and chronometers—are the most advanced and precise in the world. The U.S. Naval Academy continues to teach celestial navigation, and the Navy considers it a backup form of navigation on ships. It is also a form of navigation on another type of Navy vehicle, a Trident II D5 submarine launched ballistic missile. Once launched, a special star tracking camera isolates a star in the sky and compares its location to digital tables the USNO produces, and then the missile makes adjustments to align itself onto the proper course to the target (such missiles do not use GPS as this system can possibly be jammed or “spoofed”). The science is called astrometry (related to astronomy). With astrometry, which the USNO continues to pioneer, specialists take extremely precise measurements of celestial bodies.
“USNO has two main jobs that can be summarized in two words: reference frames,” explains Geoff Chester, the public affairs officer of the USNO. Position information comes from radio telescopes, located throughout the globe, that observe quasars. Quasars, extremely bright galactic cores, are some of the most distant objects known in the universe. Although the universe is expanding, their great distances make them essentially stationary to an observer on earth, and provide ideal “fixed” positional references. “Perhaps the most important part of these measurements are called Earth Orientation Parameters (EOPs),” explains Chester, who has a background in astronomy and physics (and whose great grandfather was the superintendent of the observatory from 1902 to 1906). “When measured against the celestial reference frame, we can see the small but significant changes in the Earth’s speed of rotation as well as the changing angle of the planet’s rotational poles. These parameters figure into the position solutions of GPS.”
GPS, or Global Positioning System, relies on the USNO’s time calculations to function. The foundational process of GPS is called multilateration, in which precise distance measurements from each of three or more orbiting satellites to a position on the surface of the planet gives a precise coordinate for that position. With the speed of light known, distances are calculated by time-of-travel from each satellite to the position in question, hence the importance of precision time.
Called the Department of Defense Master Clock, the USNO system that coordinates this process is an ensemble of more than 100 specialized atomic clocks. Atomic clocks, initially developed by Britain’s National Physical Laboratory in the early 1950s, rely on counting oscillations of the single valence electron of certain atoms (cesium, rubidium, and hydrogen). Chester explains that when these atoms are stimulated by microwave radiation, the electron oscillates between two “hyperfine” states at a specific frequency. “Working with astronomers here at USNO, a relationship was determined that linked an astronomically defined second, the Ephemeris Second, with a specific frequency of oscillation in the cesium-133 atom.
Since 1967, the second has been defined by this frequency as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium-133.” Clocks that use cesium, Chester explains, are formally called “cesium beam frequency standards,” as their frequency defines the second. Each GPS satellite has a number of onboard atomic clocks, some that use cesium and some that use rubidium. Just as the USNO originally analyzed chronometers and noted any deviations from locally-observed timing, personnel at the USNO today carefully check the onboard clocks on GPS satellites daily against the Master Clock, which has a precision of a few hundred picoseconds (a picosecond is a trillionth of a second). Any deviations in specific clocks in satellites from the Master Clock are disseminated so that receivers can automatically adjust and provide location information of the highest possible accuracy and precision. Department of Defense GPS satellites transmit frequencies usable by any GPS-enabled device on earth, and furthermore, USNO adjustments are disseminated globally. USNO-sourced timing data are critical in a number of nonmilitary applications. Virtually all global electronic financial transactions and computer and telephonic networking systems rely on the USNO’s continuously updated timing standards.
Just like the early days when it provided daily noontime ball drops for residents of Washington D.C. and those on the Potomac, the modern USNO keeps the entire U.S. Military and much of the entire world synchronized.