Accuracy: The degree of conformity of a measured or calculated value to its definition or with respect to a standard reference (see uncertainty).
Aging: The systematic change in frequency over time because of internal changes in the oscillator. For example, a 100 kHz quartz oscillator may age until its frequency becomes 100.01 kHz (see drift). NOTE: Aging is the frequency change with time when factors external to the oscillator such as environment and power supply are kept constant.
Allan Variance or Allan Deviation: The standard method of characterizing the frequency stability of oscillators in the time domain, both short and long term.
Ambiguous time: Condition of having more than one possible value. For example, if a 24 hour clock displays a time of 15 hours, 5 minutes and 8 seconds, it is ambiguous as to the day, month, and year.
Atomic Time (TA) scale: A time scale based on atomic or molecular resonance phenomena. Elapsed time is measured by counting cycles of a frequency locked to an atomic or molecular transition. Other scales use mechanical reference devices such as quartz crystals or are based on the rotation rate of the earth.
Calibration: The process of identifying and measuring time or frequency errors, offsets, or deviations of a clock/oscillator relative to an established and accepted time or reference frequency standard such as UTC - National Institute of Standards and Technology (NIST) or UTC - Bureau International des Poids et Mesures (BIPM).
Clock: A device for maintaining and displaying time.
Clock time difference: The difference between the readings of two clocks at the same instant. NOTE: To avoid confusion in sign, algebraic quantities should be given, applying the following convention. At time T of a reference time scale, let a denote the reading of the time scale A, and b the reading of the time scale B. The time scale difference is expressed by A-B = a-b at the instant T. The same convention applies to the case where A and B are clocks. There is no universally accepted convention for the significance of the sign. If A-B is measured electrically, a positive value usually implies that a given tick from A arrives before the same tick in B, whereas, the reverse is usually true if A and B are calendar dates.
Coherence of phase: Phase coherence exists if two periodical signals of frequency M and N resume the same phase difference after M cycles of the first and N cycles of the second, where M/N is a rational number.
Coordinated clock: A clock synchronized within stated limits to a spatially separated reference clock.
Coordinated time scale: A time scale synchronized within stated limits to a reference time scale.
Coordinated Universal Time or Universal Time Coordinated (UTC): A coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM), which forms the basis of a coordinated dissemination of standard frequencies and time signals. NOTE: A UTC clock has the same rate as a Temps Atomique International (TAI) clock or international atomic time clock but differs by an integral number of seconds called leap seconds. The UTC scale is adjusted by the insertion or deletion of seconds (positive or negative leap seconds) to ensure approximate agreement with UT1 (also known as the Julian Date).
Date: A unique instant defined in a specified time scale. NOTE: The date can be conventionally expressed in years, months, days, hours, minutes, seconds, and fractions. Also, Julian Date (JD) and Modified Julian Date (MJD) are useful dating measures (see Julian Date and Modified Julian Date).
Disciplined oscillator: An oscillator with a servo loop that has its phase and frequency locked to an external reference signal.
Drift (frequency): The linear (first-order) component of a systematic change in frequency of an oscillator over time. Drift is due to aging plus changes in the environment and other factors external to the oscillator (see aging).
DUT1: The approximate time difference between UT1 and UTC, expressed to the nearest 0.1s. DUT1 = UT1 + or - UTC. NOTE: DUT1 may be regarded as a correction to be added to UTC to obtain a better approximation to UT1. The values of DUT1 are given by the International Earth Rotation Service (IERS) in integral multiples of 0.1s.
Ephemeris Time (ET): An astronomical time scale based on the orbital motion of the earth around the sun (see Terrestrial Time).
Epoch: Epoch signifies the beginning of an era (or event) or the reference date of a system of measurements.
Error: The difference of a measured value from its known true or correct value (or sometimes from its predicted value).
Frequency: The rate at which a periodic phenomenon occurs over time.
Frequency analysis techniques: Analysis techniques in the frequency domain , where signals are separated into their frequency components and the power at each frequency is displayed.
Frequency deviation: The difference between frequency values of the same signal at two different times or the difference between the instantaneous signal frequency and the average signal frequency.
Frequency difference: Difference between the frequencies of two different signals.
Frequency drift: See drift and aging.
Frequency offset: The frequency difference between the realized value and a reference frequency value. Offset is often not referenced to the nominal. For example, during irradiation testing the offset is referenced to the frequency before irradiation.
Frequency shift: Change in frequency from a standard reference.
Frequency stability: Statistical estimate of the frequency fluctuations of a signal over a given time interval.
Long term stability usually involves measurement averages beyond 100s.
Short term stability usually involves measurement averages from a few tenths of a second to 100s.
NOTE: Generally, there is a distinction between systematic effects such as frequency drift and stochastic frequency fluctuations. Special variances have been developed for the characterization of these fluctuations. Systematic instabilities may be caused by radiation, pressure, temperature, and humidity. Random or stochastic instabilities are typically characterized in the time domain or frequency domain. They are typically dependent on the measurement system bandwidth or on the sample time or integration time.
Frequency standard: A precise frequency generator such as a rubidium, cesium, or hydrogen maser whose output is used as a frequency.
Primary frequency standard: A standard whose frequency corresponds to the adopted definition of the second with its specified accuracy achieved without external calibration of the device. Currently, only the cesium frequency standard is defined as a primary standard. Rubidium gas cells, hydrogen masers, and other types of atomic standards are not, by definition, considered primary standards.
Secondary frequency standard: A frequency standard which requires external calibration. For example, a crystal oscillator might be considered a secondary frequency standard.
Global Positioning System (GPS): A highly accurate, global satellite navigation system based on a constellation of 24 satellites orbiting the earth at a very high altitude. In addition to navigation, the system also provides very precise time.
GPS signals {broadcast signals of GPS and their functions are as follows}:
L1 - 1575.42 MHz
- primary navigation signal
- C/A and P codes and navigation data
L2 - 1227.6 MHz
- second frequency provides higher accuracy ionospheric delay calibration
- P code and navigation data
L3 - 1381.05 MHz
- global burst detector
- SBand command channel
GPS C/A code: The standard GPS code known as the coarse/acquisition code or "civilian code." The code is a series of 1023 pseudorandom binary byphase modulations on the carrier and has a chip rate (bit transition time) of 1.023 MHz (often called "Standard Positioning Service").
GPS Pcode: This is called the precise code or "protected code" and is a series of pseudorandom, binary byphase modulations on the carrier and has a chip rate of 10.23 MHz. The P code repeats about every 267 days. Each 1week segment of the code is unique to a particular GPS satellite and is reset each week (on saturday).
Differential GPS: The precise measurement of the difference in the positions of two receivers tracking the same GPS signal. One of the receivers may be a stationary reference point (precise benchmark) for position and the other could be a roving receiver for determining the position of a remote location.
GPS common view: A technique which involves two separated receivers, whose positions are accurately known, tracking the same GPS satellite for precise time determination. Most satellite, atmospheric and ionospheric errors in GPS are reduced using this technique.
Greenwich Mean Time (GMT): A 24 Hour system based on mean Solar time plus 12 hours at Greenwich, England. Greenwich Mean Time can be considered approximately equivalent to Coordinated Universal Time (UTC), which is broadcast from all standard time and frequency radio stations. However, GMT is now obsolete and has been replaced by UTC.
Instant: A specific time.
International Atomic Time or Temps Atomique International (TAI): An atomic time scale based on data from a worldwide set of atomic clocks. It is the internationally agreed upon time reference conforming to the definition of the second, the fundamental unit of atomic time in the International System of Units (SI). It is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium - 133 atom. The TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in France. Although TAI was officially introduced in January 1972, it has been available since July 1955. Its epoch was set so that TAI was in approximate agreement with UT1 on 1 January 1958 (see second).
Julian Day: Obtained by counting days from the starting point of noon on 1 January 4713 B.C. (Julian Day zero). One way of telling what day it is with the least possible ambiguity.
NOTE: The Julian Date is conventionally referred to UT1, but may be used in other contexts, if so stated.
Julian Date (JD): The Julian Day number followed by the fraction of the day elapsed since the preceding noon (1200 UT). { Example: The date 1900 January (1) 0.5 day UT corresponds to JD = 2 415 020}.
Julian Day Number (JDN): The number of a specific day from a continuous day count having an initial origin of 1200 UT on 1 January 4713 BC, the start of Julian day zero. { Example : The day extending from 1900 January (1) 0.5 day UT to 1900 January 1.5 days UT has the number 2 415 020}.
Modified Julian Day (MJD): Equal to the Julian day. Shifted so its origin occurs at midnight on 17 November 1858. The MJD differs from the Julian date by exactly 2 400 000.5 days.
Modified Julian Date (MJD): Julian date less 2 400 000.5
Truncated Julian Day (TJD): The JDN 2 440 000.5 occurred on 24 May 1968 and defines the origin of the TJD time scale used in the PB5 time code. NOTE: The TJD is used by the scientific community for recording astronomical and historical events and for archival data storage and is useful in the space sciences area. The TJD has an epoch of 24 May 1969 with a repetition period (recycle time) of 10000 days (27.379 years) and recycled on 9 October 1995. The TJD is currently equal to MJD minus 50000. TJD = MJD truncated to four digits.
Leap second: An intentional time step of one second used to adjust UTC to ensure approximate agreement with UT1. An inserted second is called a positive leap second, and an omitted second is called a negative leap second. A positive leap second is presently needed about once per year.
Nominal value: The ratio of a value to a reference value. NOTE: In a device that realizes a physical quantity, it is the specified value of such a quantity. It is an ideal value and free from tolerance.
Normalized frequency difference: The ratio between the actual frequency (f1) minus the nominal frequency (f2) over the nominal frequency.
Offset: The difference between the realized value and a reference value.
On time: The state of any bit (in a time code) that is coincident with the Standard Time Reference (U.S. National Institute of Standards and Technology).
Phase: A measure of a fraction of the period of a repetitive phenomenon, measured with respect to some distinguishable feature of the phenomenon itself. In the standard frequency and time signal service, phase time differences such as time differences between two identified phases of the same phenomenon or of two different phenomena are mainly considered.
Phase jump: A sudden phase change in a signal.
Phase shift: An intentional change in phase from a reference.
Phase deviation: The difference of the phase from a reference.
Phase signature: A deliberate phase offset for the purpose of signal identification. For example, NIST's radio station WWVB broadcast is deliberately phase shifted at 10 minutes after the hour, so a person knows that WWVB is being tracked and not some other signal.
Precision: The degree of mutual agreement among a series of individual measurements. Precision is often, but not necessarily, expressed by the standard deviation of the measurements.
Proper time: The local time, as indicated by an ideal clock, in a relativistic sense. NOTE: Proper time is distinguished from a coordinated time which involves theory and computations. If a time scale is realized according to the proper time concept, it is called a proper time scale. {Examples (a) for proper time: the second is defined in the proper time of the cesium atom; (b) for proper time scale: a time scale is produced in a laboratory and not transmitted outside the laboratory}.
Reproducibility: With respect to a set of independent devices of the same design, it is the ability of these devices to produce the same value.
With respect to a single device, it is the ability to produce the same value and to put it into operation repeatedly without adjustments.
Resettability: The ability of a device to produce the same value when specified parameters are independently adjusted under stated conditions of use.
Resolution: The degree to which a measurement can be determined is called the resolution of the measurement. The smallest significant difference that can be measured with a given instrument. For example, a measurement made with a time interval counter might have a resolution of 10 ns.
Resolution of a time code: The smallest increment of time or least significant bit which can be defined by a time code word or subword.
Second: The basic unit of time or time interval in the International System of Units (SI) which is equal to 9 192 631 770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133 as defined at the 1967 Conference Generale des Poids et Mesures.
Sidereal time: The measure of time defined by the apparent diurnal motion of the vernal equinox; hence, a measure of the rotation of the Earth with respect to the reference frame that is related to the stars rather than the sun. Two types of sidereal time are used in astronomy: mean sidereal time and apparent sidereal time. One sidereal day is equal to about 23 hours, 56 minutes, and 4.090 seconds of mean solar time. Also, 366.2422 mean sidereal days equal 365.2422 mean solar days.
Standard frequency: A frequency with a known relationship to a reference frequency standard. NOTE: The term standard frequency is often used for a signal whose source is from a reference standard frequency.
Standard frequency station or time-signal station: A station which provides a standard frequency or time signal emissions such as NIST's radio station WWV.
Standard-frequency emission: An emission which disseminates a standard frequency at regular intervals with a specified frequency accuracy. NOTE: In Recommendation 460, the Comite Consultative International du Radio (CCIR) recommends a normalized frequency deviation of less than 1 x 10 -10 . The CCIR is now known as the International Telecommunications Union-Radio (ITU-R).
Standard frequency satellite service: A radio communication service using earth satellites for the same purpose as those of the terrestrial standard frequency service.
Standard-time-signal emission: A broadcast which disseminates a sequence of time signals at regular intervals with a specified accuracy, for example, NIST's radio station WWV. NOTE: In Recommendation 460, the ITU-R recommends standard time signals to be emitted within 1 ms with reference to UTC and to contain DUT1 information in a specified code.
Synchronization: The process of measuring the difference in time of two time scales such as the output signals generated by two clocks. In the context of timing, synchronization means to bring two clocks or data streams into phase so that their difference is 0 (see time scales in synchronism).
Syntonization: Relative adjustment of two frequency sources with the purpose of canceling their frequency difference but not necessarily their phase difference.
Stratum clocks: Accuracy requirements placed on clocks in four strata. Accuracy of stratum clocks refers to clock performance when the clock receives no input reference.
Terrestrial Time (TT): The new 1991 International Astronomical Union replacement for what was once called Ephemeris Time. On 1 January 1997, TT = TAI + 32.184 seconds, and the length of the second is chosen so that it agrees with the International Second (SI) on the geoid. The TT scale differs from the old Ephemeris Time in its conceptual definition. Practically, however, it is realized by means of International Atomic Time (TAI).
Time code: A system of symbols (digital or analog) used for identifying specific instants of time. An information format used to convey time information. NOTE: Time is used to specify time of day or a measure of time interval.
Time comparison: The determination of a difference between two time scales.
Time interval: The duration between two instants read on the same time scale.
Time marker: A reference signal enabling the assignment of dates on a time scale.
Time reference: The basic repetition rate chosen as the common time reference for all instrumentation (usually 1 pulse per second (pps)).
Time scale: A system of unambiguous ordering of events. A time scale is meant to be stable and homogeneous.
Time-scale difference: The difference between the readings of two time scales at the same instant (see clock time difference).
Time scales in synchronism: Two time scales are in synchronism when they assign the same date to an instant. NOTE: If the time scales are produced in spatially separated locations, the propagation time of transmitted time signals and relativistic effects, including the reference coordinate frame, are to be taken into account.
Time-scale reading: The value read on a time scale at a specific instant. NOTE: The reading of a time scale should be qualified by giving the time scale a name.
Time-scale unit: The defining basic time interval in a time scale. NOTE: This unit is different from the realized time scale unit.
Time signal satellite service: A radio communication service using Earth satellites for the same purpose as those of the time signal service.
Time standard: Device used for the realization of the time unit.
Continuously operating device used for the realization of a time scale in accordance with the definition of the second and with an appropriately chosen origin.
Time step: A discontinuity in a time scale at some instant. NOTE: A step is positive (+) if the time scale reading is increased and negative (-) if the reading is decreased at that instant.
Uncertainty: The limits of the confidence interval of a measured or calculated quantity. NOTE: The probability of the confidence limits should be specified, preferably as one standard deviation.
Universal Time (UT) Family: Universal Time (UT) is the general designation of time scales based on the rotation of the Earth. In applications in which a precision of a few tenths of a second cannot be tolerated, it is necessary to specify the form of UT such as UT1 which is directly related to polar motion and is proportional to the rotation of the Earth in space. The UT1 is further corrected empirically for annual and semiannual variations in the rotation rate of the earth to obtain UT2.
Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis. The UT is sometimes designated GMT, but this designation should be avoided. Communicators use the designation (Z) or (Zulu). Timekeepers should use UTC of the national standard, for example, UTC(USNO) rather than GMT.
Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun.
UT0: UT0 measures UT with respect to the observer's meridian (position on earth) which varies because of polar motion.
Binary coded decimal: A numbering system which uses decimal digits encoded in a binary representation.
Binary number system: A numbering system which has 2 as its base and uses two symbols, usually denoted by 0 and 1.
Bit: An abbreviation for a binary coded digit of which a word or subword is composed.
Bit transition time: The time required for a bit in the time code or subword to change from one logic level to the next such as a logical 0 to a logical 1 or vice versa.
Identification bit (ID): A bit with a fixed state (logic level) used for time code identification and other information.
Inhibit/read bit: A bit generated with the time code which can be used to prohibit a user from reading the code during the time code update.
Parity bit: A bit derived from and generated with the bits in the time code word or subword to facilitate error detection and correction.
Subword: A subdivision of the time code word containing only one type of time unit such as days, milliseconds, or microseconds.
Time-code word: A specific set of time code symbols which identify one specific time. A time code word may be subdivided into subwords.
