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GLOSSARY TIME TERMS

 


Accurate & Precise
Accurate - Not Precise      Not Accurate -  Precise Not Accurate -  Not Precise
Low Resolution of Time

Please take a look to above pictures to understand a difference between time (phase) accuracy, precision, resolution,


Above pictures presents interpretation of time (frequency) stability and accuracy


Accuracy - The degree of conformity of a measured or calculated value to its definition or with respect to a standard reference time. In the meaning of NTP (Network Time Protocol) the accuracy determines how close the PC clock is to UTC reference (GNSS or external atomic clock).

Aging - the change in frequency with time due to internal changes in the oscillator.  It is the frequency change with time while factors external to the oscillator (environment, power supply, etc.) are kept constant.  Generally aging, rather than drift, is what one measures during oscillator evaluation.

Atomic Time Scale (TA or TAI) - 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. Earlier time scales were based on the rotational rate of the earth.

BEIDOU –  see COMPASS

Calibration - The process of identifying and measuring time or frequency errors, offsets, or deviations of a clock/oscillator relative to an established standard, such as UTC(NIST).

Clock - a device for maintaining and displaying time.

GOMPASS (BEIDOU)– is Chinese satellite navigation system. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System.

Coordinated Universal Time (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. 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 leap seconds to ensure approximate agreement with UT1.

Drift (frequency) - the linear (first-order) component of a systematic change in frequency of an oscillator over time.

Frequency - the rate at which a periodic phenomenon occurs over time.

Frequency drift - see drift. Frequency offset - the frequency difference between the measured value and the defined value.

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.

Frequency standard - an oscillator such as a rubidium (Rb), cesium (Cs), or hydrogen (H) maser who’s output is used as a frequency.

GALILEO – is a global navigation satellite system (GNSS) currently being built by the European Union (EU) and European Space Agency (ESA). One of the aims of Galileo is to provide a high-precision positioning system upon which European nations can rely, independently from the Russian GLONASS, US GPS, and Chinese COMPASS (BEIDOU) which can be disabled in times of war or conflict. Galileo is compatible to US GPS (see GPS).

GLONASS – acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a space-based satellite navigation system operated by the Russian Aerospace Defence Forces. It provides an alternative to Global Positioning System (GPS) and is the only alternative navigational system in operation with global coverage and of comparable precision. Glonass use L1-1575.42MHz with additional frequency margin between 1597.50-1609.50Mhz.

GPS (Global Positioning System) - a highly accurate, global satellite navigation system based on a constellation of at 24 satellites orbiting the earth at a very high altitude 20000 km. GPS signals are: L1-1575.42MHz; L2-1227.6MHz; L3-1381.05 MHz

GMT (Greenwich Mean Time) - 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.

International Atomic Time (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.

Network Time Protocol (NTP) - Protocol for distributing time and maintaining clocks via TCP/IP network. Contrary to popular belief, NTP is not based on the principles of synchronizing machines with each other. NTP is based on the principles of having all machines get as close as possible to the correct time. The effects may be similar, but this is a subtle but important distinction.

Precission Time Protocol (PTP/IEEE1588) - Ethernet protocol for time synchronization at LAN (can not be used for Internet).

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).

Synchronization - Relative adjustment of two frequency sources with the purpose of canceling their frequency difference but not necessarily their phase difference.

Stability (frequency) - 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.

Stratum - indicates how fare from cesium ref. the clock is in the chain of synchronization.  

Time code - a system of symbols (digital or analog) used for identifying specific instants of time. An information format used to convey time information. IRIG-B is example of Time Code.

Time interval - The duration between two instants read on the same time scale.

Time scale - a system of unambiguous ordering of events. A time scale is meant to be stable and homogeneous.

Time standard - a continuously operated 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. A step is positive (+) if the time scale reading is increased and negative (-) if the reading is decreased at that instant.

 

ACRONYMS

1PPS

One Pulse Per Second (Freq. reference)

ITU

International Telecom. Union

AIV

Assembly, Integration and Validation

KPI

Key Performance Indicator

BIPM

Bureau International des Poids et Mesures

MCT

Modular Coherent Transfer

CA

Certificate Authority

NMI

National Metrological Institutes

CCTF

Consultative Committee for Time & Freq.

NTA

National Time Authority

CIF

Core Infrastructure Facilities

NTP

Network Time Protocol

COTS

Commercial Off-the-Shelf 

OFT

Optical Fibber Technology

CV

Common View

OSC

Oscillators

DAB

Digital Audio Broadcasting

PPP

Point Precise Positioning

DEMETRA

Demonstrator for EGNSS services

PTP

Precise Time Protocol

DB

SQL Database

QR

Quality Report

DVB

Digital Video Broadcasting

RINEX

Receiver Independent Exch. Format

EP

Elproma (TSI#2 developer)

RMO

Regional Metrological Organization

EBU

European Broadcast Union

SFN

Single Frequency Network

EGNOS

European Geostationary Nav Overlay System

SIS

Signal In Space

EGNSS

European GNSS

SPF

Service Provision Facility

EURAMET

European Regional Metrological Org.

SV

Satellite Vehicle

EWR

Extended White Rabbit

SVF

Service Validation Facility

FO CONV

Fiber Optic COnverter (for Antenna & Server)

SW

Software

GCC

Galileo Control Centre

TAI

 International Atomic Time

gcc

C language compiler

TA(PL)

 Polish Atomic Time

GDO

GPS Disciplined Oscillator

TDMA

Time Division Multiple Access

GGTO

Galileo GPS Time Offset

TFL

Time and Frequency Laboratory

GMT

Greenwich Mean Time

TLC

Time Local Clock (the same as UT)

GNSS

Global Navigation Satellite System

TMC

Time Master Clock (see  TSG)

GPS

Global Positioning System

TRF

Time Reference Facility (see TMC)

GRx

Galileo Receiver

TSA

Time Stamping Authority

GSA

GNSS Supervising Authority

TSI

Time Service Infrastructures

GST

Galileo System Time

TWSTFT

Two Way Satellite Time & Freq Transfer

HTTP

Hypertext Transfer Protocol

UT

User Terminal (see also TLC)

HTTPS

Hypertext Transfer Protocol Secure

UTC

Universal Time Coordinated

HW

Hardware

WP

Work Package

IPR

Intellectual Property Rights

WR

White Rabbit

IRIG-B

Time Code (AM or DM)

WPL

Work Package Leader

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