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U.S. Naval Observatory Washington, DC 20392-5100 USA March 1984 "KEEPER OF THE MASTER CLOCK" Do you need to know the precise time? If you're ever told you are late for an appointment, how would you like to say: "My clock tells me the correct time to within one ten millionth of a second". Of course, your clock costs $30,000 and weighs 150 pounds, but it is as correct as man and technology can currently make it. This is perhaps a frivolous way to look at the vital function of precise timekeeping. The U.S. Naval Observatory has been in the forefront of timekeeping since the early 1800's. In 1845, the Observatory offered its first time service to the public - a time bell was dropped at noon. Beginning in 1865 time signals were sent daily by telegraph to Western Union and others. In 1904 a U.S. Navy station broadcast of the first worldwide radio time signals was based on a clock provided and controlled by the Observatory. A special telescope, known as a Photographic Zenith Tube (PZT) was installed in 1915 to observe the variation of latitude. Beginning in 1935, it was used to determine Universal Time (UT), also called Greenwich Mean Time. Today we have a method of keeping very precise time - the atomic clock. The principle of operation of the atomic clock is based on measuring the microwave resonance frequency (9,192,631,770 cycles per second) of the cesium atom. At the Observatory, the atomic time scale (AT) is determined by averaging 20 to 24 atomic clocks placed in separate, environmentally controlled vaults. Atomic Time is a very uniform measure of time (1 billionth of a second per day). The Photographic Zenith Tube is still used to determine UT based on the rotation of the Earth about its axis. This is one method we use to set the faces of our atomic clocks. The largest PZT in existence (13 meters focal length) is in use at the Observatory. PZTs are mounted in a fixed vertical position so that a star may be photographed as it crosses the meridian near the zenith. The meridian is defined as an imaginary line drawn from the north celestial pole to the south celestial pole and passing through the observer's zenith. The time that each photographic image is made is recorded by a computer. Measurement of the position of the star images on the photographic plates provides data to determine the clock reading when the star crossed the meridian. The position of the star is known, and the Universal Time when it was on the meridian may be computed. Because the Earth does not rotate at a uniform rate, UT rate is not constant. Adjustments of clock time are necessary about once a year and are due to a slow decrease in the rotation of the Earth with respect to atomic time. These adjustments are called "leap seconds". On June 30, and sometimes on December 31, one extra second may be added to the official U.S. time scale. When this adjustment is necessary, the "leap second" is added to the minute beginning at 23:59 U.T.C. (7:59 p.m. EDT). Precise time measurements are needed for the synchronization of clocks at two or more stations. Such synchronization is necessary, for example, for communications systems. Precise time is also used for precise position determination using electronic signals from satellites or shore-based stations. Radio and television stations are also users of precise time - the time of day - and precise frequencies in order to broadcast programs. Many programs are transmitted from coast to coast to affiliate stations around the country. Without precise timing, the stations would not be able to synchronize the transmission of these programs to local audiences. The military and industry also use precise time to synchronize communications and navigation systems, calibrate machinery, and meet timetables for rail and air traffic. While precise time is not necessarily for everyone, what a reply it would be for one of the most commonly heard lines in daily life - "When do we eat?" A Time of Day announcement can be obtained by calling 202/653-1800, locally in the Washington area. For long distance callers, the number is 900/410-TIME. The latter call is a commercial service for which the telephone company charges 50 cents for the first minute and 35 cents for each additional minute. The 900 service will allow anyone in the United States to dial up direct access to the Master Clock at the Naval Observatory for US$0.50. Australia, Hong Kong end Bermuda can also access this service at International Direct Distance Dialing rates. The official United States time is determined by the Master Clock at the U.S. Naval Observatory. The advantages of the new service in the USA include: 1. Use of dedicated landlines. Long distance callers will obtain the full accuracy of the TIME announcement. Landline delays in the USA are less than 25 milliseconds, and are constant. (By using regular long distance calling to area code 202, satellite routing can introduce delays of up to 250 milliseconds without the caller being aware of it.] 2. 8,000 calls can be handled simultaneously. (With the local service, only 10 calls can be handled at a time. Many are lost because of saturation. Callers may get a busy signal.) 3. Callers can stay on the line for as long as necessary. (With the local service, when lines are busy, there is an automatic cut-off after 15 seconds.) Visitors to the Time Service Building wishing more definitive information should feel free to speak to any Time Service staff member or may call 202/653-1460. ***************************************************** U.S. Naval Observatory Washington, DC 20392-5100 USA 6 April 1992 THE U.S. NAVAL OBSERVATORY MASTER CLOCK The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination. Modern electronic systems, such as, electronic navigation or communication systems, depend increasingly on precise time and time interval (PTTI). Examples would be the ground based LORAN-C navigation system and the satellite based Global Positioning System (GPS). These systems are all based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of about 10 feet. In fast communications, time synchronization is equally important. All of these systems are referenced to the USNO Master Clock. Thus, the USNO must maintain and continually improve its clock system so that it can stay one step ahead of the demands made on its accuracy, stability and reliability. The present Master Clock of the USNO is based on a system of some 24 independently operating cesium atomic clocks and 5 to 8 hydrogen maser atomic clocks. These clocks are distributed over 12 environmentally controlled clock vaults, to insure their stability. By automatic hourly intercomparison of all clocks, a time scale can be computed which is not only reliable but also extremely stable. Its rate does not change by more than about 1 nanosecond per day from day to day. On the basis of this computed time scale, a clock reference system is steered to produce clock signals which serve as the USNO Master Clock. The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen Masers are extremely stable clocks over short time periods (less than one week). They provide the stability and reliability needed to maintain the accuracy of the Master Clock System. MASTER CLOCK REFERENCE SYSTEM #2 The electronic system in this room is Reference System #2, which is part of the U.S. Naval Observatory Master Clock. The major parts of this system are: Hydrogen Maser #18 (first cabinet from the right) Hydrogen Maser #19 (second cabinet from the right) Electronic Control System (cabinet on the left side) Environmental Control System (behind the door in the rear) Masers are atomic clocks which have outstanding short-time stability. This is due to the fact that Masers directly use the atomic microwave radiation coming from Hydrogen atoms which emit the frequency of 1420MHz when they change the state of their single electron. The radiation (which is very weak, 1E-11 Watts approximately) can be used to phase lock a quartz crystal clock to it. This type of atomic clock differs from the Cesium clocks where one only detects the presence of a transition. The main components of a Maser are the hydrogen gas supply, a controllable "leak" for the gas into the high vacuum system, a gas discharge to produce atomic Hydrogen, and a state selector which rejects atoms in the lower energy state so that only the higher state can enter the resonance cavity. Once the atoms enter the resonance cavity, they find other atoms radiating and they fall in step. They "start to talk to each other" and echo what they hear. This produces a highly coherent oscillation. This is the signal to which the crystal oscillator is phase locked. All of that is packaged in the cabinets on the right, including power supplies, temperature control, and magnetic shielding. Great care is required to keep environmental disturbances small so that the full performance potential of these sophisticated clocks can be realized. The masers have been built by a group under Dr. Robert F. C. Vessot at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. The two Masers operate jointly, one of them being the "lead" Maser and the other being phase locked to the lead to within 10 picoseconds (ps). This is done so that we can switch from one to the other without making any noticeable steps in time or in rate (light travels just 0.3mm in 1ps). The actual difference between the two Masers is displayed on the oscilloscope on top if the Electronic Control System. The line's ragged appearance is caused by the control actions of the computer which makes the very small frequency changes once per hour in the follower Maser's frequency synthesizer. The computer has blinking lights; they blink as long as the computer is idling. The control computer also measures temperatures, humidity and a host of diagnostic indication in order to alert the operators to any possible trouble. If the problem is associated with the lead Maser, then this Maser will be replaced with the other Maser by electronically switching the input to the clock designated in Reference System #2 in the cabinet on the left (the electronic control system). The control computer serves as a local data node. It reports the settings and measurements to the main Time Service Data Acquisition and Control System (DAS). It also receives instructions from the DAS on how to keep the system on the computed time scale for a protracted period. Essentially then, this system is a type of flywheel which implements and extends the computed time scale of the Observatory. For this purpose, the driving oscillators, i.e., the Masers, must have the very best shorttime stability obtainable. They must also be very reliable. This latter point can only be achieved with redundancy. That is why two Masers are combined in this system. The stability obtained with a single Maser is better than one part in ten to the 14 (1E-14) for periods from a few hundred seconds out to several days. 1E-14 allows a timing precision of better than 1 nanosecond over one day. ******************************************************** U.S. Naval Observatory Washington, DC 20392-5100 USA 1 May 1992 The U.S. Naval Observatory and The Global Positioning System by Mihran Miraman The U.S. Naval Observatory (USNO) monitors the timing of the Global Positioning System (GPS) for the purpose of providing a reliable and stable coordinated time reference standard for the satellite navigation system. Monitoring the timing of the GPS has been a mission requirement of the USNO since the first satellites were launched, more than ten years ago. During this period, the GPS has evolved into a nearly complete network of satellites which provides accurate time as well as precise position. Likewise, the USNO GPS operations have also evolved to accommodate the increased timing requirements of both military and civilian users. These users rely on the GPS as a means for time synchronization with the USNO and as a common source for the purpose of precise time transfer. As more satellites are added to the constellation and the costs of user equipment decrease, the number of users will steadily increase. The data provided to GPS operations are usually of a classified nature and not available to unauthorized users. However, the USNO also operates GPS receivers that are unkeyed and function in the unauthorized mode. The data from these receivers are available to users via USNO publications and the USNO Automated Data Service (ADS). The USNO has several GPS timing receivers in constant operation in Washington, DC; the primary units are a STel-502 and a dual frequency receiver (DFR) also produced by STel. The STel-502 receiver has been in continuous operation since 1984 and is the source of all GPS data that appear in USNO publications and the USNO ADS. The DFR is a modified version of a GPS monitor site receiver. It is capable of tracking both frequencies emitted from the satellites and corrects for ionospheric effects. The DFR is an encryption keyed receiver, which operates in a classified mode. Consequently, the data from this receiver are not available to unauthorized users. The USNO also operates GPS timing receivers at remote Data Acquisition Sites (Dabs). These sites are located at strategic military installations and civilian agencies for the purpose of time synchronization and for monitoring the timing of the Loran-C radio navigation system. These receivers provide a means for determining the offset of the remote site clocks from the USNO Master Clock via the GPS. These receivers are not encryption keyed and the data are therefore unclassified. This permits the data to be transferred to the USNO by conventional means, i.e., telephone lines. The time differences provided by the USNO for the GPS represent the difference between the USNO Master Clock (MC) and the GPS Composite Clock (CC) as derived from each individual satellite. Currently, precise timing to approximately +/-35 nanoseconds (billionths of a second) relative to the USNO Master Clock may be obtained by monitoring any Block I satellite, and any Block II satellite so long as encryption measures such as Selective Availability (SA) and Anti-Spoofing (AS) have not been activated. These measures degrade and or deny the precise positioning capability of the GPS to unauthorized users. Block I satellites are prototypes and do not have the encryption capability of Block II operational satellites. Daily average values for Block I, Block II, and the entire GPS constellation of satellites are also determined. These values represent a 2-day filtered smoothing of measurements from those satellites considered reliable. These single values are an estimate of the difference between MC(USNO) and CC(GPS), with a precision of timing to about +/-10 nanoseconds for Block I satellites, and for Block II and the entire constellation when SA is not implemented. When SA is implemented, the precision of timing is about +/-60 nanoseconds for Block II satellites and the entire constellation. The two receivers at the USNO in Washington are scheduled to track satellites according to a recommended common-view tracking schedule for international time comparisons. This schedule is provided by the Bureau International des Poids et Mesures (BIPM) in Paris, France. The primary link between the BIPM and its time scale contributors is the GPS. Satellite track times are chosen to minimize elevation angles between pairs of stations. Open periods are filled with the emphasis on providing a balanced coverage of all usable satellites. In addition to precise timing, the USNO also supports GPS operations with information regarding the orientation of the Earth in space. The very long baseline interferometry (VLRI) program, of the USNO, determines daily parameters for the position of the Earth's rotational pole. These parameters correct the dynamical reference frame of the GPS satellites to an inertial reference frame. This information is necessary for determining improved orbits for the satellites. The USNO GPS database provides a reliable, near real-time data set which military and civilian users rely on for precise clock synchronization and Earth orientation. Access to the GPS timing data via the USNO ADS is user friendly and available worldwide over telephone lines and computer networks. As the GPS reaches its full potential, USNO GPS operations will continue to expand to provide the precise timing support essential to both military and civilian users. ********************************************************* U.S. Naval Observatory Washington, DC 20392-5100 USA [Not dated] LEAP SECONDS Civil time is occasionally adjusted by one second increments to insure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is the basis for our civil time. In 1956, following several years of work, two astronomers at the U.S. Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the frequency of the Cesium atom (the standard of time) and the rotation of the Earth at a particular epoch. As a result, they defined the second of atomic time as the length of time required for 9,192,631,770 cycles of the Cesium atom at zero magnetic field. The second thus defined was equivalent to the second defined by the fraction 1 / 31,556,925.9747 of the year 1900. The atomic second was set equal, then, to an average second of Earth rotation time near the turn of the 20th century. The Sub-bureau for Rapid Service and Predictions of the International Earth Rotation Service (IERS), located at the U.S. Naval Observatory, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation. The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the deceleration of the Earth to be roughly 1-3 milliseconds per day per century. This is an effect which causes the Earth's rotational time to slow with respect to the atomic clock time. Since it has been nearly 1 century since the defining epoch (i.e. the ninety year difference between 1990 and 1900), the difference is roughly 2 milliseconds per day. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously. In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive. This reflects the general slowing trend of the Earth due to tidal braking. Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds as a measure of the rate at which the Earth is slowing. The one-second increments are, however, indications of the accumulated difference in time between the two systems. As an example, the situation is similar to what would happen if a person owned a watch that lost two seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by two seconds tomorrow. At the end of a month, the watch will be roughly a minute in error (thirty days of two second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again. This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock while it is not possible to alter the Earth's rotational speed to match the atomic clocks. Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be one second. Instead of allowing this to happen a leap second is inserted to bring the two times closer together. The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to international agreements, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used. [The leap second is inserted at 23:59:59 UTC. It is designated 23:59:60, and is followed by 00:00:00 UTC.] ******************************************************** U.S. Naval Observatory Washington, DC 20392-5100 USA July 22, 1991 TIME SERVICE PUBLICATIONS Series 4 - Daily Time Differences - Provides time differences between the U.S. Naval Observatory Master Clock and LORAN-C, Television and satellite (GPS) systems. Also provides information on the operational status of each system including VLF (Navy and Omega). General notices of interest for precision timekeeping are also given. (Issued weekly/ADS/MARK III) Series 7 - Earth Orientation Bulletin (International Earth Rotation Service (IERS) Bulletin A) Lists Earth orientation data (x, y, UT1-UTC, nutation) as available from the International Earth Rotation Service. Predictions of Earth orientation information for up to one year from the date of publication are given. (Issued weekly/ADS/MARK III) Series 8 - Times of Coincidence (NULL) Ephemeris Tables for Television - At present, these tables are applicable only for WTTG, Washington, DC. They may be of interest in countries operating on the NTSC system. (Issued as necessary) Series 9 - Times of Coincidence (NULL) Ephemeris Tables and General Information for LORAN-C - Individual tables are issued for the master station of each LORAN chain. Coordinates and emission delays are updated. (Issued as necessary) Series 14 - Time Service Announcement - Includes general information pertaining to time determination, measurement and dissemination. (Issued as necessary) Series 15 - Earth Orientation Bulletin (IERS Bulletin B) and Bureau International des Poids et Mesures (BIPM) Circular T - Bulletin B lists monthly Earth orientation data. Circular T gives all differences between the atomic time scales (UTC and TA1) maintained by independent laboratories. (Issued monthly to U.S. addresses only) Series 16 - Precise Time Transfer Report - Lists the time difference UTC(USNO MC)-UTC(reference clock) for Defense Satellite Communications System terminals, adjustments to reference clocks and precise time synchronization measurements. The time differences are obtained via satellite time transfers and television. (Issued quarterly) ********************************************************* International Standards of Time and Frequency --------------------------------------------- by Gernot M. R. Winkler U.S. Naval Observatory Washington, DC. 20392 STANDARDS OF TIME Before the introduction of modern means of communications, there was no world standard of time. Each locality kept its own local time, realized by the local observatory or, in most cases, sundials. The railroads were one of the first that needed some form of regional time and they promoted the introduction of standard time. Soon thereafter, international coordination became necessary. The history of international standards of time and frequency begins with the Washington Conference of 1884. This conference recommended the following: 1. That a Standard Meridian be adopted by all countries. 2. That this meridian be the meridian going through the transit instrument at the Greenwich Observatory. 3. That East longitude should be counted plus from this standard meridian, and west negative. 4. That a Universal Day be adopted, and that this day be a mean solar day, to begin at mean midnight of the initial meridian, coinciding with the beginning of the civil day and date of this meridian, and that it be counted from zero up to 24 hours. 5. That the astronomical and nautical days should also begin at mean midnight (this was implemented in 1925). 6. That it is hoped that the decimal system be extended to the division of angular space and to time. With the exception of point six, all of these recommendations were eventually adopted. But more had to be done. With the advent of radio time signals, the need arose for coordination of these signals, which differed often by several seconds. In October 1912, by invitation of the French, a conference was convened at the Paris Observatory to study this problem. As an eventual result of these deliberations, the Bureau International de l'Heure (BIH) was formed at the Paris Observatory. After the formation of the International Astronomical Union in 1919, its newly established Time Commission was charged with supervision of the BIH. It was a natural consequence of the need to coordinate the time determinations of the contributing observatories that the BIH established a reference system of longitude that was based on all of the instruments that made observations, thereby conceptually separating the Standard Meridian from the original transit instrument in Greenwich. In addition, the Universal Time (UT), as a measure of the Universal Day, was different from the old Greenwich Mean Solar Time because it started the day at midnight while old GMT started at Noon. For a while, the English-speaking countries continued to refer to this new UT also as Greenwich Mean Time (GMT), thereby contributing to some confusion because GMT could mean either the old or the new convention. And with time, more details had to be considered: UT is actually a system of slightly different times and one distinguishes: UT0 which is Mean Solar Time at the Standard Meridian (near the old Greenwich meridian but not identical with it) plus 12 hours. This UT0 as it is observed by transit instruments must be corrected for Polar Motion (PM). That gives UT1 This UT1 is still somewhat variable due to the seasonal variations of the rotation of the Earth. These have been empirically determined and can be applied as a correction to UT1. The result is UT2 This is nearly uniform time. Modern high precision atomic clocks, however, keep time still much more uniform, about 1 million times more. The need arose, therefore, to distinguish the time of the clocks and time signals (UTC) from the astronomically determined UT2. A leap second is introduced whenever the difference UT2 - UTC goes beyond 0.5 seconds (s). The maximum difference allowed is 0.9s. On the average, a leap second is introduced every one to three years. This will probably change in the future because there is a gradual (secular) slowing down of the rotation of the Earth. The day is for these reasons not equal to 86400s but slightly longer. The accumulated difference is taken out by means of the leap second. UTC is, with the exception of the leap seconds, strictly uniform clock time that is coordinated internationally by the Bureau International de Poids et Mesures (BIPM), which has taken over the functions of the old BIH in regard to atomic time. The other, astronomical times, UT1 and UT2, fall under the responsibility of the International Earth Rotation Service (IERS), the central bureau of which is at the Paris Observatory and various sub-bureaus at other major time centers. E.g., the sub-bureau for Rapid Service and Predictions is at the USNO in Washington. International coordination, as it concerns the radio time signals, falls under the auspices of the International Telecommunications Union (ITU), and its consultative body, the CCIR. The details that concern the radio time signal standardization, including details of the leap second implementation and fixing the UTC frequency in 1972, have been published as CCIR Study Group VII Recommendations in the CCIR Green Books. The BIPM defines and coordinates the International Atomic Time (TAI), which is a pure atomic time, never adjusted. It is the basis of UTC that differs from TAI only in consequence of the leap seconds. STANDARDS OF FREQUENCY Until 1955 the unit of time has been the Second as a subunit of a mean solar day. Later, one tried to overcome the known variability of the speed of rotation of the Earth by using its orbital motion around the Sun as the time standard. This produced the old "Ephemeris Second" as standard. Frequency is given as cycles per second, and whenever the second changed, so did frequency. This was a very undesirable situation, and work to refer frequency to an invariable unit of time started as soon as the first practical atomic clock was created by Dr. Louis Essen at the National Physical Laboratory (NPL) in Teddington, UK. Joint measurements by the NPL and the USNO started in 1956 and established a connection between the astronomical unit of time and the cesium resonance that served as a practical standard. In 1960, agreement was reached to accept this measurement as a provisional standard. The frequency of cesium was given as 9,192,631,770 cycles per Ephemeris Second. The formal designation of UTC was also introduced at that time by agreement between the British and American time keeping authorities. The Hertz is the unit of frequency given as cycles per standard second. Since 1967 we use as the unique, official standard of time (and by implication also of frequency) the duration of 9,192,631,770 cycles of a microwave resonance frequency of the cesium atom under specified conditions. The new definition has been set to agree, within the errors of measurement, with the traditional measure of time which is dictated by our dependency on the Sun. This new standard, however, has the great advantage that it can be easily realized anywhere; i.e., we can carry our time measure with us wherever we go. In using it we must, however, pay attention to the necessity of making certain transformations of our time measures of events in systems which are in motion and/or in different gravitational potential relative to us. This necessity arises precisely because of the purely relational (as a relation between processes) nature of our time concept. The mathematical relations constitute what is known as relativity theory. THE INTERNATIONAL SYSTEM OF UNITS SI The system of measurement units that has been internationally adopted as the successor of the "metric" system is the Systeme International (SI), which is in its ideal principles a system for local use, the same way as the concept of time itself. This locality has been specifically recognized in the case of the time measures, albeit only gradually. At the time of the adoption of the present SI Second in 1967, it was implied that the definition had to be in the concept of proper time. Only much later, when the exact meaning of TAI had to be considered, was the distinction between local (proper) time and coordinate time included in the actual wording of a recommendation. Regarding the other base units of the SI, the evolution of the system is not yet completed. The Second has become the most important unit: by having defined the speed of light once and for all, the unit of length is now also based upon the Second. In addition, the Second is based not on a prototype, but on a postulated constant of nature - the energy levels of the cesium atom. Efforts are under way to continue in this direction: away from prototypes towards constants of nature. In practical terrestrial timing applications, the need to reduce remote time measurements relativistically became very obvious with the first satellite timing experiments. However, it took some time to make this part of relativity a part of the timing engineer's tool box (see [2] through [10] for a discussion of details). As an official act, the CCDS has at first clarified the principles involved, and has defined in 1970 the International Atomic Time (TAI) as a coordinate (and coordinated) time [8]. The wording (translated from the French) is: "The TAI is the temporal reference coordinate established by the Bureau International de l'Heure (BIH) on the basis of the readings of atomic clocks that operate in various establishments in conformance with the definition of the Second, the unit of time of the International System of Units (SI)." (Recommendation S 2, 1970). This definition of TAI has been augmented with a declaration in 1980 that clarified the meaning in the relativistic context. This became necessary because the above wording only implies the reference time TAI to be coordinated, since TAI is defined on the basis of the clock readings in the contributing establishments. The unit of time, as it is defined as a base unit of the SI, however, necessarily refers to proper time. Wherever we operate a cesium frequency standard, it gives us the time measure as a proper unit, even though this also was not explicitly spelled out in the original definition. This CCDS declaration is as translated [8]: "TAI is a coordinate time scale, defined in a geocentric reference frame with the SI Second as scale unit as it is realized on the rotating geoid. Therefore, it can be extended to a fixed or moving point in the vicinity of the Earth with sufficient accuracy at the present state of the art by the application of the first order corrections of the General Theory of Relativity; i.e., the corrections for the differences in the gravitational potential and the differences of speed, in addition to the rotation of the Earth." The CCIR Study Group VII has issued a report [9] that deals in some detail with the cases of a portable clock near the surface of the Earth, and with electromagnetic signals used for remote synchronization. The report is in essential agreement with the CCDS documents with some additional detail of importance for terrestrial applications. THE PRESENT SITUATION IN ASTRONOMY REGARDING STANDARDS OF TIME The old Ephemeris Time, as it was used as the argument for orbital computations, was replaced in 1977 by Dynamical Time, which included relativity considerations with scaling (changing the rate of the clocks to compensate for the gravitational potential at the point of origin for the purpose of avoiding a secular runoff). Two main guiding principles were used in this replacement. First, the Moon was to be replaced with the more accurate cesium standard (even though there was ambiguity in the wording!). And second, continuity with the past ET was considered essential. However, the new space applications of precise time, particularly in pulsar research, suggest further evolution. This is very significant because by far the most stringent requirements for long-term clock stability and accuracy in the relativity corrections that must be applied in the reduction of observations come from astrodynamics and astronomical research. The 1977 decisions regarding Dynamical Time were to some degree premature and, therefore, unfortunate. The name was entirely confusing but even in the text, an unfortunate ambiguity existed in respect to the choice of the scale unit: was it defined at the epoch or for all time? In addition, no agreement could be reached about the role of TAI: was it, as the CCDS later spelled out, a coordinate time, or in astronomical context, a proper time? And last but not least: the scaling! At the time, we obviously did not see clearly enough the importance of consistency in our system of measurement. In fact by scaling we gain nothing if - and that is a crucial point - we do not disseminate the time. In that case, we do not have to worry about runoff! On the other hand, dropping the scaling allows us to use our physical system of measurement consistently. IAU Colloquium 127 has now provided a long overdue clarification. Relativity has been included in all concepts from the beginning. Recommendation G2 [10] specifically mentions two coordinate systems, the (solar) barycentric and a geocentric system; it specifies that the SI (i.e., the second, the meter, c, etc.) should be extended to outer space without scaling factors; and it links the time coordinates to atomic clocks that operate in conformance with the definition of the Second. Recommendations T1 and T2 establish a consistent nomenclature for the various time-like arguments that need to be distinguished as a consequence of the above recommendations. This nomenclature takes into account the previously introduced time-like arguments that are used in the ephemerides: Terrestrial Dynamical Time TDT, and Barycentric Dynamical Time TDB, the former originally taken as TAI + 32.184s, and usually considered as the relativistic successor to ET. TDB is reckoned at the SI rate with scaling (which effectively assigns a number to the cesium standard frequency that is different from the SI). T1 and T2 also introduce new arguments in conformance with G1 and G2: Geocentric Coordinate Time TCG, and Barycentric Coordinate Time TCB. And lastly, it also introduces an ideal Terrestrial Time TT that is practically TAI but without the very small errors of implementation. First of all, it must be noted that the French names determine the abbreviations. And even though this seems like a bewildering profusion of different time scales, the method is clearer by grouping them in the following way: Dynamical times were conceived for the sole purpose of providing relativistic successors to the ET. This historical origin, together with the urgency with which they were introduced before a systematic position could be reached, as it exists now, together with the 32.184- second offset inherited from ET, explains why better distinctions and definitions were needed. Moreover, the name Dynamical is misleading because it refers to the intended use, while the scale is really an atomic time. The scaling of the rate means that the standard units (e.g., of mass) must also be scaled if they are connected with TDB, a serious complication that is avoided with the introduction of the "Coordinate" times. These are not scaled but adopt the SI at the origins. This is in the spirit of the SI, which is tacitly assumed to be a proper system of units. Another problem we face is that we need to be able to conceptually separate TAI as an established, operational time, from its ideal concept, which can be better approximated after the fact by reprocessing and the inclusion of additional information (possibly, pulsar observations). That is the reasoning behind TT. It is practically identical with TDT except that the separation of the realized from the ideal time was not explicitly included in exactly this sense in the definition of TDT, which was rather vague on this point [10]. In addition, the possibly misleading implications arising from the name Dynamical are now avoided with TT. SUMMARY The present world standard for common use is UTC. It is very nearly Greenwich Mean Solar Time (old style) plus 12 hours. The difference is kept to less than 0.9 s with adjustments in the form of leap seconds. The regional standard times are usually offset by an integer number of hours. The standard of frequency is the Hertz, meaning one cycle per second. The second is the SI second based on the atomic definition using the relationship between ET and the cesium resonance as determined by the NPL-USNO measurements of 1956 to 1960. For applications in space, relativity corrections have to be applied and the astronomers have in addition, introduced a number of standard concepts that should assist in their applications. NOTES AND LITERATURE: [1] Derek Howse (1980) "Greenwich Time and the Discovery of the Longitude", Oxford University Press, ISBN 0-19-215948-8. Excellent extensive review of the early history. [2] Louis Marton (ed.) (1977) Advances in Electronics and Electron Physics. Chapter 2. Academic Press NY. ISBN 0-12-014644-4 LCC# 49-7504. Gives an overview of modern time measurement with many references. [3] C. O. Alley, "Introduction to Some Fundamental Concepts of General Relativity and to Their Required Use in Some Modern Timekeeping Systems", Proceedings of the Thirteenth Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, pp. 687-727, 1981 (NASA Conference Publication 2220). [4] J. C. Hafele and R. E. Keating, "Around the World Atomic Clocks", Science, vol. 177, pp. 166-170, 14 July 1972. This is the report on the first measurement of relativistic time changes with actual clocks. [5] C. O. Alley, "Relativity and Clocks", Proceedings of the 33rd Annual Symposium on Frequency Control, pp. 4-39A, 1979. [6] N. Ashby and D. W. Allan, "Practical Implications of Relativity Radio Science, vol. 14, pp. 649-669, 1979. This is a particularly important paper for timing applications: it discusses many specific examples, from clock trips to satellite time comparisons. [7] D. W. Allan, M. A. Weiss, and N. Ashby, "Around-the-World Relativistic Sagnac Experiment" Science, vol. 228, pp. 69-70, 5 April 1985. [8] Comite Consultatif pour la Definition de la Seconde, "Declaration et Note", 9e Session, Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, F-92310 Sevres, France, 1980. [9] Recommendations and Reports of the CCIR, "Relativistic Effects in a Terrestrial Coordinate Time System", ITU Geneva, Report 439-4, Volume VII, pp. 134-138, 1986. A slightly modified version will be published in the "Green Book" of the XVIIth Plenary Assembly, Duesseldorf, Germany, 1990. [10] IAU Colloquium 127: Reference Systems, Virginia Beach, Virginia, USA, 14-20 October 1990. Eds. J. A. Hughes, G. Kaplan, and C. Smith, U. S. Naval Observatory, 1991. *********************************************************** 25 April 1992 DECIMAL TIME SYSTEMS The question is often asked why we do not use the decimal system for counting and measuring time. The answer is that we an use the decimal system for time measurements and, indeed, that in science we use such a system. However, there is a difficulty in doing so which prevents us from adopting a purely decimal system for everyday use. Actually there are several different ways in which we can go about reckoning time decimally. It all depends on what we adopt as the unit of time. If we adopt the year as the unit then we count years and fractions of a year. As I write this it is 1986.724061 as example for a date that is expressed as a fraction of the year beginning with 1 January, 0h UT. However, there are several difficulties which preclude the general adoption of this method of time reckoning for all purposes. First of all, people are not likely to be satisfied with a measure which obscures the connection with the day measure because the day is of basic importance for our life. But there is a second problem in that the year and the day are not commensurate. A year (let us take the tropical year, i.e., equinox to equinox; there are others, of slightly different length) is equal to 365.2422 days. For a continuous measure, therefore, the beginning of the year will not coincide with the beginning of a day but will move roughly one quarter day into January 1st every year until the occurrence of a leap day will bring the beginning of the year back one day. For this reason one has to fix the beginning of this decimal year reckoning in a way which makes it independent of the day count. This "Besselian" year starts when the mean sun reaches the right ascension of exactly 18h 40m (mean equinox). The beginning of these Besselian Years (BY) is given with the help of the Julian Day as follows: JD 2433282.423 + 365.2422(year - 1950.0) For 1986 the BY started January 0.643 i.e., it started before the beginning of the calendar year (January 1, 0h UT). This latter epoch is also the reference for the above given fraction of the year which, therefore, must be carefully distinguished from the BY. Our date given in this form is 1986.725038 (BY). But all this is clearly not practical and we must look for a different fundamental time unit. A shorter unit will have the benefit of being also more precise with the same number of fractional digits. Nevertheless, the problems of incommensurability will remain because none of the "natural" time units are commensurable with each other. Unless we wish to allow discontinuities when we go from one year to the next (or from one day to the next) which would sacrifice all advantages of a decimal counting system, this incommensurability is the main obstacle to a generally useful decimal time measure. Certain people use the month which creates even greater problems with the lunar calendars and we will not discuss it further. (For more information see the articles on Calendar in any encyclopedia). However, why not use the day? This count is in use in the form of the Julian Day count (or the Modified Julian Day as an abbreviated form). Unfortunately there is also a problem with this choice which precludes its general acceptance. If I give the time of day in fractions of a day (e.g., 0.333333) then people again will not make much sense of it (0800 given in hours is more understandable). But beyond the problems of making people accept such a new thing (which would be formidable, indeed!), there is still a second problem. We would have to establish also a different system of measurements altogether. The present system as it is used in science, technology and for legal purposes, is the successor of the "metric" system and has been internationally accepted as the Systeme International. It is based on the second as a base unit and as the unit of time. Other base units are the meter, the kilogram, etc. In this system only strictly dekadic, i.e., multiples and submultiples of powers of ten, are permitted and are designated with the "metric" prefixes. Examples for these are T, G, M, k, for Tera, Giga, and kilo. A decimal day time measure would be completely incompatible with the SI because 86400 is not a simple dekadic multiple of the base unit, and such a system could not, as may be believed, be adopted without the most protracted international negotiations ever seen. (The diplomats needed 6 months at the beginning of the Vietnam peace conference just to reach agreement on the shape of the conference table). But then, why not count time in seconds? That would avoid this problem, or would it? Yes indeed, it would avoid this problem (at the expense of creating a new one) because the second is the generally accepted unit of time and is a base unit of the SI. However, I doubt that the public would be more willing to accept this because now, we have again lost the connection with the day. A continuing decimal second count would mark the end of the first day as second 86400, the end of the second day as 172800, the third as 259200 etc, and very soon a time check would have to be given in 7, then in 8 digits, and then in 9, etc. Each year has about 32 million seconds and these numbers rapidly become unmanageable. The above given date would correspond with 906.461.953 (906 Million seconds!) on the seconds count from the epoch of atomic time (1 January 1958, 0h UT) including the 23 leap seconds which were introduced between that epoch and 1986. This is certainly not a meaningful time check. We must conclude that the problem is not simple and the field is wide open for the world improvers to solve! At the USNO all of these time measures are somehow in use, each for the purpose best suited. We can do that because there is no problem for computers. They have no prejudices and can convert from one style to any other without the slightest difficulty. It is only the people who make the fuss and have problems in adjusting to anything new, or getting used to different measures. After all, this country is still resisting the full acceptance of the SI, on purely traditional grounds. Of all these styles the most useful for many purposes is without question the Modified Julian Day with fractions of a day. It has been adopted by several international scientific - technical bodies. Its intended used, however, must remain restricted to special purposes. The date given above is in this style 46695.457532 MJD which corresponds with 1986 SEP 22 (Mo) 10:58 UT. It is day of the year 265 and a time of 264.457532 days has elapsed since the beginning of the calendar year. The difference of one day is due to the difference between ordinal numbers (our days of the month) and cardinal numbers (with which we count elapsed time). Midnight of the first day of January is the beginning of the year (The day starts with midnight! It is, however, advisable to avoid ambiguity if you specify midnight) but at this moment zero days have elapsed since the beginning of the year. The command @DAT gives also the parameter T which counts time from January 0, 0UT (This epoch is used in some formulae) and this is now T = 265.457532 days Caution is indeed required if one uses any of these non "standard" ways to give date and time! History: It is most likely that our system of dividing the day into hours, minutes, and seconds is of Babylonian origin. The numbers 12, 30, 60, and 360 must have attracted attention as number bases very early. The month is about 30 days and the year has about 360 days, i.e., 12 months. In addition, these numbers excel in that they are divisible by more factors than the number 10 which contains only 2 and 5. In fact, one can say with justification that the choice of ten as base of our number system has not been the optimum choice. It was based on the purely accidental fact of our ten fingers and ten toes, very important for the counting by mathematically naive people. Eight would be much better and is indeed used extensively in computer work. But, of course, for the well schooled person almost any choice would do particularly today when conversion by computers is completely automatic. The importance of standardization in many, but not all, areas recedes as the population becomes more sophisticated. This is true even for languages. Just see the proliferation of computer languages which bother the beginners much more than the experts. In fact, one could make the case that using different systems keeps the mind fresh! The Babylonians were careful and early observers of the heavens and they divided the circle into 360 degrees, corresponding with the daily steps the sun makes on the sky. At that time Astronomy and Astrology were practically the same thing. These 360 steps of the sun were divided into 12 main areas, the zodiacal signs which are also related to the moon. Both, Sun and Moon take on different characters depending on in which of these signs they are. This led also to the concept of celestial houses (again 12), etc. The division into 12 units became, therefore, an important and indispensable step, which led to the division of the day and night into 12 hours each. The subdivisions into minutes and seconds came much later, seconds have appeared only in the later medieval. Apparently Man is prolific in giving names to inventions of his mind. Most recently another unit of time has become popular in computer circles, particularly those concerned with digital video work. It is the Jiffy which now re-appears as the name for 1/60 of a second, the time for one picture scan. In one thousand years nobody will probably know where this unit came from! G. Winkler. ************************************************************* THE MERCURY ION FREQUENCY STANDARD. ---------------------------------- The Mercury Ion Frequency Standard is a novel kind of atomic frequency standard. By using ions which are electrically charged it is possible to confine them in a small region of space by the use of an electromagnetic field trap. This allows the observation of the particles for a long time (many seconds) without having them collide with the walls which would disturb the atomic resonance one wishes to use as a frequency reference. The mercury isotope Hg 199 has an extremely narrow microwave resonance line at 40,507 MHz. A narrow line at a high frequency is desirable because this defines a reference frequency very precisely. To give a comparison with the widely used Cesium clock we list a few numbers in the table below: Cesium 133 Hg 199(+) ----------------------------------------------------------- Atomic weight 133 199 Resonance frequency 9,192 MHz 40,507 MHz Line width 500 Hz 800 mHz Quality factor (f/Df) 1.8E7 5E10 Improvement over Cs 2,700 Principle of Operation: By firing electrons into the Hg vapor, ions are generated and are kept in the RF trap. About 2 million ions form a cloud. Their motion is gently slowed down by the presence of low pressure Helium (1E-6 Torr). At that pressure the Helium pressure shift, i.e., the frequency change of the Hg resonance is slight and can be accurately calibrated. The actual resonance, or clock transition is observed in the following way: In addition to Hg 199 which has a hyperfine structure there is also another isotope of Hg, which does not have a hyperfine structure (Hg 202). A very strong U.V. line at 194 nm is generated with this isotope in a mercury discharge. This light is absorbed by the upper state of the clock transition in Hg 199(+) to bring the ions into a higher (optical) state from which they then relax into both lower states. Through the pumping action, however, the upper (clock) state will be quickly de-populated and all ions end up in the lower (clock) state which does not absorb the pumping light. From this moment on the vapor is transparent and no relaxation fluorescence can be observed. If, however, one exposes these ions to the clock resonance frequency, then they will be moved into the upper clock state and can again absorb light. The fact that they absorb light is observed through the fluorescence when they fall into the lower optical levels. The fluorescence, therefore, is the means to detect the clock transition. The frequency measurement is performed in a steadily repeating sequence of pumping (discharge is on), exposure with the microwave resonance frequency, and observation of the fluorescence at the next pumping cycle. Each cycle lasts 2.5 seconds. By averaging such measurement cycles over a few hours, a resolution of a few parts in ten to the fifteen can be achieved. A prototype instrument of this kind has been constructed by the Hewlett-Packard Laboratories in Palo Alto, Cal. and has been delivered to the USNO in July 1986. It is now a part of the USNO master clock system. A second unit has been delivered by the HP. Laboratories in May 1987 and is now being evaluated. Its design has been slightly modified to utilize the experience obtained with the first unit. Up until now, we cannot see any secular drift between the two units above about 3E-17 per day. This is the limit of resolution with the data at hand. Eventually this will be lower as more data will be accumulated. For more details see the paper by Cutler et al. in the 13th PTTI Proceedings (1981, p. 563) and a more recent report by Cutler et.al. given at the last Frequency Control Symposium. (Available upon request) G. Winkler ************************************************************ EXPLANATIONS CONCERNING CLOCK READINGS AND CLOCK DIFFERENCES. ------------------------------------------------------------- A clock is principally an interpolation device. Its readings must therefore be corrected to an established time scale by adding the clock correction to the reading. A more general method of referring clock readings to time scales or other clocks consists in the algebraic method of giving clock differences. The difference between two clocks A and B is given by the difference of their readings for the same event. In practice this is done by measuring the time interval between two corresponding markers of the two clocks with clock A connected to the start input and clock B connected to the stop input of a time interval counter (meter). Let A and B be the corresponding readings of the two clocks (for the same event). Then their difference will be A - B. If UTC is the true time of the corresponding event then the clock correction for clock A will be UTC - A. The clock error is A - UTC. The error is therefore the amount one has to subtract from a clock reading whereas the correction is to be added to refer the reading to the reference time scale. The rate of a clock is the change of its clock correction per unit of time. In practice this is often given in ns/day (in the case of precision clocks). The frequency of the clock's frequency generator, on the other hand, is related to the error because if the clock error increases then its frequency is high in respect to the reference. Frequency is usually given as relative or fractional frequency, i.e., in parts in a billion, or whatever. Fractional frequency, therefore, has the opposite sign from the clock rate. A plus rate corresponds with a slow rate or a low frequency. The bulletins of the USNO, particularly SER 4, in general give the phase value (or the time readings) in the sense MC - Signal. This is a convenience which allows the readings to be interpreted as signal delays. On chart recordings of the arriving signal the increasing readings correspond with the phase values as given in the bulletin. With this "convention" the readings increase if the receiver delay is larger. Example: Phase value = time of arrival = MC - Antenna = MC - Transmitter + prop.delay + receiver delay Clock Performance and Performance Measures. ------------------------------------------ The quality of a clock is not dependent upon its error or its rate. It is the rate variations from interval to interval (usually the standard interval is a day) which determine the quality. If these variations are irregular then the clock's behavior can only be described statistically. If the rate changes systematically, i.e., if it increases by nearly the same amount every day then we talk about a drift of this clock. Quartz crystal clocks and (much less so) Rubidium vapor cells typically have such a drift. Cesium clocks, unless there is something wrong with them, or they have been misadjusted, show no drift. Regarding the performance of pocket or wrist crystal watches, the most important disturbance comes from the temperature variations to which the watch is exposed. As a rule of thumb, crystals have a temperature coefficient of about 1 part per million. That amounts to a rate change of 0.1s per day per degree temperature change. All measurements of clock performance, or clock stability, start with a set of regularly executed measurements of the clock correction. With these measurements a table is constructed with the time of measurement (or the day number in the series), the measurement, and first and second differences. The table looks like this: n Clock Error First Diff. Second Doff. Sec.Dif.Square ms ms/d ms/d/d --------------------------------------------------------------- 0 325 0 1 350 25 2 377 27 2 4 3 401 24 -3 9 4 430 29 5 25 5 461 31 2 4 6 494 33 2 4 7 529 35 2 4 8 566 37 2 4 9 601 35 -2 4 10 636 35 0 0 11 673 37 2 4 12 710 37 0 0 13 749 39 2 4 14 790 41 2 4 15 835 45 4 16 etc. The squares column will be needed in a moment. We have assumed that we observe a quartz crystal clock which is temperature controlled. The units are milliseconds and the rates are given in ms/day. In the case of atomic clocks, the unit would probably be in nanoseconds because of the much greater stability of these clocks. Our example clock would be a very good crystal clock because the rate variations as shown in the 4th column are small. Nevertheless, the rate shows a systematic increase of 20ms in 14 days, i.e., the clock has a noticeable drift. This drift is also visible as the average second difference (sum = 20, divide by 14 ---> average drift = 20/14 = 1.43ms/d/d). The widely adopted and by far the most simple measure of clock stability is the co called Allan Variance, internationally known as two sample sigma. It is computed as follows: Form the squares of the second differences, add them, divide by 2 times the number of terms and form the square root. This gives 86/28 = 3.07; the square root finally gives 1.75ms/day as the measure of stability. Such stability measures are also often expressed in relative terms, i.e., as parts per million, etc. One finds the translation between these two styles by remembering that one day has 86400 s. Therefore 1ms rate change per day corresponds with (1.0E-3)/8.64E4 = 1.157 parts in 10 to the eight Our test clock, therefore, exhibits a frequency instability of 2.02 parts in 10 to the eight from day to day (1.157x1.75). Remember: A clock error is given in units of time (s, ms, ns), whereas a rate difference is given as a relative number or as ms/day, ns/day, etc. More information is available in literature upon request. ***************************************************************** HOW DO WE FIND THE VERY BEST CLOCK? The question is frequently asked how one can decide which of the best clocks available to us is really the very best. Because we assume that we have selected the best clocks available, there is nothing better to compare our clocks with. We seem to have uncovered a principal difficulty. However, there is an answer to this perplexing question. We only have to consider our basic principles: Time is not only the most basic but also the most abstract measure which we use to bring order into nature. Because of its abstractness, and because we all think we know what it is we become easily confused. Therefore one has to develop clear and distinct ideas. Time is a measure which we bring into nature according to our definitions. We postulate that the same interval of time elapsed if we can demonstrate that the same process has taken place. Now how do you establish identical processes and how do you decide your specific question of which of the best clocks is the very best? The answer is given in the literature concerning time keeping, especially in the documentation of our time computation algorithms. This literature is available on request. The essence is this. The best clock is the one which shows the smallest residuals in its errors in reference to a time scale which is computed on the basis of a clock set. And the second best clock is the one which shows the second smallest residuals in respect to the computed time scale which is statistically better than any contributing clock. In practice, however, you use only clocks for the set which have otherwise been shown to be acceptable. We do not use Mickey Mouse watches because they would not be cost effective. One would have to use zillions of them and it would not be practical! Everybody knows that Mickey Mouse watches agree very poorly among themselves whereas atomic clocks agree within nanoseconds (ns) from day to day. Of all the Mickey Mouse watches at most one of them can be right and most likely none is. The situation and reasoning is identical with the reasoning which we apply when we judge anything else. If we find substantial disagreement then probably nobody is right or at most one can be correct. Which one it is we can only find out gradually as more information becomes available. This is the scientific way which is not different from common sense, only more systematic. We do not arbitrarily assign credence to one clock on the basis of intuition. Basically the answer is, therefore, that clocks can only be judged by the degree of conformity with other clocks. The best time measure is that which agrees with the consensus of a set of other processes. As you probably will agree, nothing else could be expected to be useful in science. This is the basis of our time keeping: We have a group of nominally 24 standard clocks and we compute from hourly measures a best time scale which is then used to produce a table of corrections for each of the clocks. As you see, time keeping is intrinsically dependent on statistics and probability. If 10 clocks agree to within, let us say 10 ns, and one differs by 100 ns, then it is overwhelmingly unlikely that this one clock has been right and the other 10 wrong. It is the same procedure as it is used in a court of law. One interrogates and cross examines the witnesses who each will tell you a slightly different story. One or two may tell you a completely different story. The proceedings are designed to establish a core of facts which correspond with the consensus of the witnesses and that is accepted as "truth". Of course, what really happened may still be different but we have no other way to arrive at an acceptable definition of truth. And this consensus can be evaluated in terms of probability theory. In science the procedure is exactly the same. We try to establish a consistent measure and consistent theories so that they are applicable in the largest possible domain. With clocks the ideal reference cannot be realized but only be approximated by finding the consensus of a large number of different standard processes. That is the reason why scientists are so interested in comparing clocks based on different principles. Up to now all such tests have revealed no surprises. It turns out that a time scale constructed on the basis of undisturbed standard processes in the form of our atomic clocks is superbly applicable in the description of other natural processes such as in astronomy (pulsars), or in modern technology, where one has to do the same thing independently at the same time but at different places. On the basis of these principles it has also been discovered in 1934 that our earth does not rotate uniformly but shows small seasonal variations in the length of the day. But for more details I again refer you to the papers mentioned above. Gernot M. R. Winkler. ************************************************************* STANDARD TIME: In the U.S. the standard time is governed by the Uniform Time Act of 1966 with amendments of 1972. This law specifies the times of change-over to advanced time and back to standard time. The amendment of 1972 gives permission to a state to exempt the most easternmost portion of that state from the time change. For example, the easternmost part of Indiana is not on Daylight Saving Time. Areas or states presently not on advanced time are Arizona, Hawaii, Puerto Rico, the Virgin Islands, American Samoa and, the previously mentioned eastern part of Indiana. The Uniform Time Act is administered by the Department of Transportation. In 1986 another amendment has been passed by congress which changes the dates of change-over. The new rule is: Spring change to advanced time on first Sunday in April (0200 local time). Fall return to standard time on last Sunday in October (0200 local time). THE DOT POINTS OF CONTACT ARE: GENERAL INFORMATION . . . . 202-366-4723 FOR DETAILED LEGAL QUESTIONS : JOANNE PETRI . . . . . . . 202-366-9306 *************************************************************** 26 July 1985 THE U.S. NAVAL OBSERVATORY MASTER CLOCK. The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination. Modern electronic systems, such as electronic navigation or communications systems, depend increasingly on precise time and time interval (PTTI). Examples would be the navigation systems LORAN and Transit. The reason for these is that they are all based on travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one billionths of a second) corresponds to a position accuracy of 10 feet. In fast communications, time synchronization is equally important. All of these official systems are referenced to the USNO master clock. Thus, the USNO must maintain and continually improve its clock system so that it can stay one step ahead of the demands made on its accuracy, stability, and reliability. The present master clock of the USNO is based on a system of independently operating cesium beam and hydrogen maser clocks, distributed over a number of temperature controlled clock vaults. This assures a high reliability of operation but also makes the small disturbances, which are unavoidable even in the best clocks, statistically independent. By automatic hourly intercomparison of all clocks, a time scale can be computed which is not only very reliable but also extremely stable. Its rate does not change by more than about 1 nanosecond per day from day to day. On the basis of this computed time a clock reference system can be steered to produce clock signals which serve as reference for portable clocks, the telephone time announcement, and an extensive monitor system which is the interface with the above mentioned timed electronic systems. These reference systems (there are several, again in the interest of absolutely essential reliability) are driven by "fly-wheel" oscillators, i.e., extremely stable clocks, which produce the short-time stability and accessibility of the system. The clocks used for this purpose are Hydrogen Masers, the same type of clock which is also used by radio astronomers as local standard. Developments are under way to possibly replace some of these devices with even more capable clocks in the future because the USNO, in order to fulfill its mission, must continually improve its methods and instrumentation. More detailed information about the USNO timing activities is provided below. USNO Activities in the Timing Area 1. Master Clock Three reference systems are used to realize the coordinated clock timescale of the Observatory. All time interval measurements are made against these reference systems which are designated MC1, MC2 and MC3. Each system is driven by a hydrogen maser directly. The frequency synthesizers of these masers are set once per day, if necessary, to keep the reference systems close to the computed mean timescale UTC(USNO), which in turn is close to the predicted UTC(BIPM). The unsteered internal reference is designated as A.1, while the reference of the actual Master Clock(s) is UTC(USNO). UTC(USNO) is kept within 100 ns of UTC(BIPM). An estimate of the slowly changing difference UTC(BIPM) - UTC(USNO,MC) is computed daily and published on the Automatic Data Service (ADS). 2. Timescale The USNO timescale is generated as described by L. A. Breakiron, 1991, Proceedings of the 23rd Annual Time and Time Interval (PTTI) Applications and Planning Meeting, pp. 297-305, except for revisions in the weighting and steering. The ensemble consists of about 9 masers and 30+ cesium clocks. The clocks are included in the actual ensemble or rejected on the basis of both long-term and short-term performance. Most of the cesium clocks are of the new HP5071 type. The clocks are distributed in 11 vaults, all of which are temperature-controlled. Some are also humidity-controlled. The weights change with time so that the masers are completely deweighted 90 days in the past in order to prevent any residual drift from affecting the timescale. The reference clocks are steered to the timescale by no more than 400 ps/day and with a time constant that varies from 10 to 60 days. Reference system #2 is designated as lead reference, or MC2, to which all measurements can be corrected if necessary. However, most of the time the differences between these systems are about 1ns or less. Measurements between all clocks are made every hour; a second, independent high precision system measures the high performance clocks every 100s. The various clock vaults are located in several buildings that are separated by as much as 300m. The connecting cables are either low loss coaxial cables or fiber optic links. All are installed underground. 3. Time Dissemination Various timed systems are being kept within narrow tolerances of the USNO Master Clock. The LORAN chains covering North America have been within about 100ns(rms), whereas the overseas chains had larger tolerances, the largest in the case of the Mediterranean chain (1micros). The Transit navigational satellite system has been within 100us of UTC(USNO). Most of the timing users will find it advantageous to switch now to the GPS as a worldwide source of precision time. The GPS, with the correction given in the navigation message (A0 and A1), has been within 15ns rms during the period April through July 5, 1993. This refers to observations with the selective availability removed. Including selective availability, observed with a single frequency receiver, the rms error has been 69ns, with a maximum error of 291ns. These measurements include all available satellites with a 13-minute observation per pass. By obtaining the small residual difference between UTC(USNO,MC) and UTC(BIPM) from the Automatic Data Service (ADS) of the USNO, a near real time access to UTC is, therefore, possible via the GPS at the level of accuracy given above. By averaging over all available satellite passes per day, a fixed station with a cesium frequency standard can increase this precision to below 10ns with appropriate filtering. The obtainable accuracy will usually be limited by the stability and calibration of the local antenna-receiver delays. For highest accuracy, the USNO has extended the use of its two-way satellite time transfer instrumentation. Regular time transfers have been continued with the NIST in Boulder, Colorado, with the NRC in Ottawa, Canada, and with the USNO station in Richmond, Florida. During 1992, experiments have also been conducted with the Technical University in Graz, Austria, and with OCA in Grasse, France. An additional high precision time reference station has been established on the island of Oahu, Hawaii, and initial two-way time transfers have been started with that station. Some problems with the spread-spectrum modems have limited the obtained precision of these measurements to about 3ns. The mobile Earth station has been used to make relative delay calibration between USNO and several other sites. It is currently being planned to resume the time transfers to Europe, but approval from INTELSAT has not yet been received. The instrumentation at the USNO consists currently of two 4.5m VERTEX antennas, the mobile Earth station, one VSAT, and a new "Fly-Away" small terminal that will be used for the quick calibration of remote stations because this terminal can be easily transported by air and assembled by one person in a few hours. Thu Jul 22 14:09:46 utc 1993