INNOVATION GPS and Leap Seconds Time to Change? Dennis D. McCarthy, U.S. Naval Observatory William J. Klepczynski, Innovative Solutions International Since ancient times, we have used the Just as leap years keep our calendar approxi- seconds. While resetting the GLONASS Earth’s rotation to regulate our daily mately synchronized with the Earth’s orbit clocks, the system is unavailable for naviga- activities. By noticing the approximate about the Sun, leap seconds keep precise tion service because the clocks are not syn- position of the sun in the sky, we knew clocks in synchronization with the rotating chronized. If worldwide reliance on satellite how much time was left for the day’s Earth, the traditional “clock” that humans navigation for air transportation increases in hunting or farming, or when we should have used to determine time. Coordinated the future, depending on a system that may stop work to eat or pray. First Universal Time (UTC), created by adjusting not be operational during some critical areas sundials, water clocks, and then International Atomic Time (TAI) by the of flight could be a difficulty. Recognizing mechanical clocks were invented to tell appropriate number of leap seconds, is the this problem, GLONASS developers plan to time more precisely by essentially uniform time scale that is the basis of most significantly reduce the outage time with the interpolating from noon to noon. civil timekeeping in the world. The concept next generation of satellites. As mechanical clocks became of a leap second was introduced to ensure Navigation is not the only service affected increasingly accurate, we discovered that UTC would not differ by more than 0.9 by leap seconds. Spread-spectrum systems that the Earth does not rotate “like second from UT1, the time determined by the also rely on time synchronization for effec- clockwork,” but actually has a slightly rotation of the Earth — a choice made pri- tive communications. When sychronization nonconstant rotation rate. In addition marily to meet the requirements for celestial is lost, so too is coherent communication. to periodic and irregular variations navigation. Thus, while a leap second is being intro- caused by atmospheric winds and the To determine longitude and latitude using duced, and until sychronization is estab- interaction between the Earth’s core a sextant and observations of stars, one needs lished, communications can be disrupted and the mantle, the tidal interaction to know UT1 at the instant of the observa- between some systems. of the Earth and the Moon causes a tions. An error of one second in time could In view of these emerging problems, user secular slowing down of the Earth’s translate into an error of about 500 meters in dissatisfaction with leap seconds is beginning rotation. So rather than use the position. Celestial navigation was one of the to surface, and concern is growing that users variable time scale based on the major navigation techniques used throughout will construct time scales independent of Earth’s rotation, we now use time the world in 1972, when the first leap second UTC that they perceive are more suited to scales based on extraordinarily was introduced. With the recent proliferation their individual requirements. This would precise atomic time, the basis for all of satellite navigation, however, it is appro- lead to an increased number of nonstandard the world’s civil time systems — priate to reconsider this historical position, as time scales. Coordinated Universal Time (UTC). the leap second may in fact be detrimental to Although we have accurate estimates of However, because of the desire some systems, possibly creating life-threat- the deceleration of the Earth’s rotation, sig- to keep UTC more or less in ening situations. nificant variations prevent the prediction of synchronization with the Earth’s Modern commercial transportation sys- leap seconds beyond a few months in rotation as an aid in determining tems now almost entirely depend on satellite advance. This inability to predict leap sec- navigation fixes using astronomical navigation systems. In the future, commer- onds, coupled with the growing urgency for a observations, leap seconds are added cial aviation is expected to rely on various uniform time scale without discontinuities, to UTC — currently about every 18 augmentation systems to improve satellite makes it appropriate to re-examine the leap months. navigation accuracy, reliability, integrity, second’s role. Later in this article, we will In contrast, the time scale used to and availability beyond current capabil- outline several possible scenarios for dealing regulate the Global Positioning System ities. The introduction of a leap second with leap seconds in the future, but first, let’s — GPS Time — is a “pure” atomic does not affect GPS operations because its review how the present system came to be. time scale without leap seconds. In time system is GPS Time, which is not this month’s column, Drs. Dennis adjusted to account for leap seconds. But A BRIEF HISTORY McCarthy and William Klepczynski GPS does provide UTC by transmitting the Historically, humans used astronomical phe- suggest that the practice of adding leap necessary data in its navigation message, nomena to keep time. The passage of the Sun seconds to UTC be done away with or permitting a receiver to compute UTC from across a north–south (meridian) line deter- at least modified, as more and more GPS Time. mined noon. Until 1960, the average solar navigators adopt Global Navigation By contrast, GLONASS uses UTC as its day was used as the basis for timekeeping, Satellite Systems as their primary time reference (actually UTC ` 3 hours with the second defined as 1/86,400 of the means of positioning. which is Moscow Time), and so its satellite mean solar day. Under this system, the length clocks must be reset to account for any leap of the second depended on the Earth’s rate of 50 GPS WORLD November 1999 www.gpsworld.com INNOVATION rotation, because its rotation causes the Sun to appear to move across the sky. 80 During the mid-1930s, astronomers con- cluded the Earth did not rotate uniformly, basing their findings on measurements of the 60 most precise clocks then available. We now know that a variety of physical phenomena affect the Earth’s rotational speed. This 40 caused the second to be redefined in 1960 in terms of the Earth’s orbital motion around 20 the Sun. The new second was called the “Ephemeris” second and the time scale 0 derived from this definition was called (seconds) T Ephemeris Time (ET). D The name called attention to the fact that -20 the definition depended on the position and motion (ephemeris) of the Sun (or Moon) used in the astronomical determination of -40 time. It was originally thought that the new definition would provide a more uniform -60 measure of the second’s length. ET is very 1600 1700 1800 1900 2000 difficult to measure and observe, however, Year requiring a series of accurate astronomical observations stretching over years. In 1967, the second was again redefined, Figure 1. Observations and quadratic fit of the difference between a uniform time this time in terms of the resonance frequency scale and one based on the Earth’s rotation of the cesium atom, which had already been calibrated with respect to ET. By the early we would have to add some time to the face can vary either way. So far, all leap seconds 1960s, cesium frequency standards became of our atomic clock, which is exactly what have been positive. known as reliable, uniform, and accurate we do when we add a leap second to our Astronomical observations show a near- clocks. Defining the second this way pro- clocks. We are bringing the faces of our lab- constant deceleration of the Earth’s rotation vided a uniform standard that easily could be oratory clocks back into agreement with rate caused by the braking action of the tides. measured in a laboratory with greater preci- the time kept by the rotating Earth, so that This deceleration explains why the length of sion and accuracy than any astronomical sunrise will occur at the “correct” time from the astronomical day is approximately two phenomena. year to year. milliseconds longer today than at the begin- Increasing Accuracy. Subsequently, ET was The Move to Cesium. With the introduction of ning of the twentieth century. With the differ- superseded by a set of dynamical time scales the cesium atom–based definition of the sec- ence between UTC and UT1 growing at the defined to meet special relativistic require- ond, it was known that there would be a time rate of two milliseconds per day, or 0.7 sec- ments. At the level of accuracy with which varying discrepancy between a clock running ond per year, currently about one leap second ET could be determined (approximately at a uniform rate and a theoretical one using a per year must be inserted in UTC. 0.001 second), these new time scales are second defined solely by the Earth’s rotation The astronomical observations provide a equivalent to ET and include Terrestrial rate. Beginning in 1961, the Bureau Interna- clear estimate of the magnitude of the decel- Dynamical Time (TDT), Barycentric tional de l’Heure (the forerunner of the Inter- eration of the Earth’s rotation rate. Figure 1 Dynamical Time (TDB), Terrestrial Time national Earth Rotation Service) accounted combines data from as far back as the 1600s (TT), Geocentric Coordinate Time (TCG), for these observed variations by making with data recently acquired using modern and Barycentric Coordinate Time (TCB). small adjustments, on the order of a few mil- space geodetic techniques to show the The advantage of these scales over ET is the liseconds, to our civil time clocks and by difference between astronomical time and incorporation of relativistic effects and a making occasional small adjustments to the uniform time, along with a parabola fit to the defined relationship to atomic time.