Gps time


GPS, UTC, and TAI Clocks

GPS, UTC, and TAI Clocks The following are based on your PC clock:
  • Local time is the date/time reported by your PC (as seen by your web browser). If your PC clock is accurate to a second then the other time scales displayed above will also be accurate to within one second.
  • UTC, Coordinated Universal Time, popularly known as GMT (Greenwich Mean Time), or Zulu time. Local time differs from UTC by the number of hours of your timezone.
  • GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not perturbed by leap seconds GPS is now ahead of UTC by
  • Loran-C, Long Range Navigation time, is an atomic time scale implemented by the atomic clocks in Loran-C chain transmitter sites. Loran time was zero at 0h 1-Jan-1958 and since it is not perturbed by leap seconds it is now ahead of UTC by
  • TAI, Temps Atomique International, is the international atomic time scale based on a continuous counting of the SI second. TAI is currently ahead of UTC by TAI is always ahead of GPS by 19 seconds.

See also: Nixie Tube Leap Second Countdown Clock.

For more information about time scales and leap seconds see:

  • Systems of Time Time Service Department, U.S. Naval Observatory, Washington, DC
  • A brief history of time scales Steve Allen, UCO/Lick Observatory
  • Le temps UTC/TAI Bureau International des Poids et Mesures, BIPM UTC/TAI Time Server
  • Leap Seconds Time Service Department, USNO
  • Future of Leap Seconds Steve Allen, UCO/Lick Observatory
  • Modified Julian Date Frequently Asked Questions, Time Service Department, USNO
  • Astronomical Time Keeping Astronomical texts for the layman
  • Astronomical Calendars MAA Scholar Project
  • Some basic information about the different time scales by Paul Schlyter
  • Time Scales, UTC, and Leap Seconds From the Time and Frequency Users Manual
  • Astronomical Times Richard Fisher, National Radio Astronomy Observatory
  • Loran-C Timing Operations U.S. Naval Observatory, Washington, DC
  • Loran-C Times of Coincidence (TOC) calculator
  • International Earth Rotation Service (IERS) General Information
  • A Few Facts Concerning GMT, UT, and the RGO by Richard B. Langley
  • Time and Frequency FAQ Time & Frequency Division, NIST
  • Leap Second and UT1-UTC Information Time & Frequency Division, NIST
  • Time Scales Time Metrology, National Physics Laboratory (NPL)
  • The Leap Second Time Metrology, National Physics Laboratory (NPL)
  • Leap seconds: why and how Physikalisch-Technische Bundesanstalt (PTB)
  • The Time of Internet Italian Standard Time, Guided Tour to Time Measurement
  • The Australian National Time System The Second and International Atomic Time
  • What are all those different kinds of time Astronomy Frequently Asked Questions
  • Chandra, A Time Tutorial Definitions of various time systems and formulae
  • The Times of your Life Solar Time, Julian Days, Sidereal Time
  • Tidbits about TIME Archives of timezone.com
  • Radio Time Checks Origins and Meaning of GMT/UTC
  • 1972 Troubled Times Leap Seconds herold[sic] world-wide cataclysm (huh?)
  • Leap Seconds Urban Legend (huh?)
  • The Greenwich Meridian in the Space Age Time, Atomic Time, Longitude, etc.
  • The world time system New Scientist
  • History of the Prime Meridian - Past and Present by Jeremy Paul
  • Time Physical fundamentals
  • UTC New Years Y2K No leap second here!
  • Earth rotation Difference in seconds between a UT1 and a TAI clock
Do you have comments/questions? Return to LeapSecond.com home page

Page created with vi -- last updated Mon Jan 07 18:45:35 UTC 2002

www.leapsecond.com

GPS Time

To learn about CNS Systems' commercial products see our other Web site at www.cnssys.com. There you will find information on:

Do you remember Nixie tube displays.  Click here to see your computer's time displayed on Nixie Tubes.

Timing for VLBI (updated 2015) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on May 4-7, 2015 at MIT Haystack Observatory by Katie Pazamickas, EXELIS; Dr. Tom Clark, NVI; and Richard M. Hambly, CNS Systems, Inc. Click here to download the viewgraph presentation here.Click here to download the PDF version of the presentation.

Timing for VLBI (updated 2013) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on May 6-9, 2013 at MIT Haystack Observatory by Dr. Tom Clark NVI/NASA GSFC and Richard M. Hambly, CNS Systems, Inc. Click here to download the viewgraph presentation here.Click here to download the PDF version of the presentation.

Low cost GPS-based time and frequency products, an update presented at the 44th Annual PTTI Meeting on November 26 - 29, 2012 in Reston, Virginia, USA.

ABSTRACT: The performance of low cost GPS-based time and frequency products continues to improve. Our initial use of these products was to provide a cost effective solutions to the problem of epoch synchronization and performance monitoring of the hydrogen masers at isolated stations in the global Very Long Baseline Interferometry (VLBI) network. Now these products are also used in a wide variety of applications such as providing network time for CDMA cell sites and secure isolated data networks, providing general purpose high accuracy timing at isolated locations, synchronization of RF transmitters for shared spectrum applications etc. Motorola and iLotus GPS timing modules (the VP, UT+, M12+ and M12M series) have been the basis for many timing products over the years. The latest of these, the M12M, is now a mature product although an end of life has not been announced. We report here on a new OEM module that has been developed to replace these aging units maintenance of existing and current production products and in new applications. This module is designed as a drop in replacement that is functionally, mechanically and electrically equivalent to the M12M. To insure compatibility, it emulates the Motorola commands and messages. These new modules are based on the uBlox 6T GPS engine. In addition to compatibility with the M12+ and M12M, the receiver offers programmable xPPS signals, smaller quantization errors due to higher clock speed, quantization corrections reported with picoseconds resolution, and a 50-channel engine. These new u-blox based M12 compatible OEM modules are available now and are priced competitively with the original module. This paper reviews the improvements this family of products has achieved over the years and looks at the performance of the new generation of low cost timing engines as they compare with the most recent previous generation.

Using Low-Cost COTS Software Defined Radios for Phase Cal and RFI Monitoring presented at the International VLBI Technology Workshop on October 22-24, 2012 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center (Retired), Greenbelt MD.

ABSTRACT: Many years of VLBI experience have shown that station problems can often best be diagnosed by using a spectrum analyzer or communications receiver to "see" or "listen to" the IF (baseband) RF signal. In the Mark-3 and -4, we often offset one LO by 10 kHz and monitored the 10 kHz phase cal tone with a a simple LC filter and scope. We could use an HF radio to listen to the phase cal tone or to an RFI signal. With the Mark-5 and their Digital Back Ends, we lost access to these basic analog monitoring points. With invaluable inputs from Russ McWhirter, the latest RDBE code includes the ability to observe the sign bit from any of the 32 PolyPhase Filter Bank (PFB) channels. The one-bit sample gives us the ability to monitor any frequency in the 32 MHz wide PFB IF. In the past few years, experimenters in the Radio Amateur community have developed a number of SDRs that can be used spectrum analyzers with resolution bandwidths (RBW) in the sub Hz-range. In this paper I will describe and demonstrate several commercial, off-the-shelf SDRs suitable for VLBI use that are available in the $500-$2500 range.

Next Generation GPS Timing for the Mark 5 Era presented at the International VLBI Technology Workshop on October 22-24, 2012 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center (Retired), Greenbelt MD and Richard M. Hambly, CNS Systems,, Inc.

ABSTRACT: Abstract: The performance of low cost board-level GPS-based time and frequency products continues to evolve. Our initial use of these products was to provide a cost effective solutions to the problem of epoch synchronization and monitoring the performance of the hydrogen masers at isolated stations in the global Very Long Baseline Interferometry (VLBI) network. Now these products are also used in a wide variety of applications such as providing network time for CDMA cell sites and secure isolated data networks, providing general purpose high accuracy timing at isolated locations, synchronization of RF transmitters for shared spectrum applications etc. Motorola and iLotus GPS timing modules (the VP, UT+, M12+ and M12M series) have been the basis for many timing products over the years. The latest of these, the M12M, is now a mature product approaching end of life. We report here on the development of a new OEM GPS module to replace these aging units for maintenance of existing and new applications. The new module is designed as a drop in replacement that is functionally, mechanically and electrically equivalent to the M12M. To insure full compatibility, it emulates the Motorola commands and messages. The new modules are based on the replacement module, which is Swiss uBlox 6T GPS engine. In addition to compatibility with the M12+ and M12M, the receiver offers programmable xPPS signals, smaller quantization errors due to higher clock speed, and higher resolution "sawtooth" quantization corrections. The new uBlox based, M12 compatible, OEM modules are available now and are priced competitively with the original Motorola/iLotus modules. For nearly two decades, the HP/Agilent 53131A and 53132A time-interval counter, in conjunction with CNS Systems TAC32 support software, has been the low-cost "standard" for user in VLBI. Agilent has declared the 131/132 to be a "dead" unsupported product. We will report on our evaluation of several new, low-cost, compatible counters that are now supported in Tac32Plus and are suitable for use in future Mark-5 VLBI systems

Low-cost, High Accuracy GPS Timing presented at the Institute of Navigation's GPS 2000 Conference, September 20, 2000 by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD, Richard M. Hambly, CNS Systems Inc., Severna Park MD, and Reza Abtahi, CNST, San Jose CA. Get a copy of the paper here. Right click to download the viewgraph presentation here.

Timing for VLBI presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on March 12, 2001 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD. Right click to download the viewgraph presentation here.

Critical Evaluation of the Motorola M12+ GPS Timing Receiver vs. the Master Clock at the United States Naval Observatory, Washington DC. presented at the 34th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting on December 3, 2002 at the Hyatt Regency, Reston VA by Richard M. Hambly, CNS Systems, Inc. Get a copy of the paper here. Right click to download the viewgraph presentation here.

Timing for VLBI (updated 2003) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on September 21-24 2003 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD and Richard M. Hambly, CNS Systems, Inc. Right click to download the viewgraph presentation here.

Timing for VLBI (updated 2005) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on May 9-12 2005 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD and Richard M. Hambly, CNS Systems, Inc. Right click to download the viewgraph presentation here.

Timing for VLBI (updated 2007) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on April 30 - May 3, 2007 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD. Right click to download the viewgraph presentation here.

Timing for VLBI (updated 2009) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on April 27-30, 2009 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD and Richard M. Hambly, CNS Systems, Inc. Right click to download the viewgraph presentation here. Right click to download the PDF version of the presentation here.

Timing for VLBI (updated 2011) presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on May 9-12, 2011 at Haystack Observatory by Richard M. Hambly, CNS Systems, Inc. Right click to download the viewgraph presentation here.Click here to download the PDF version of the presentation.

Modern & New Spectrum Analyzers for the Mk-5/VLBI2010 WorldRevised and presented by Richard Hambly in the absense of author Tom Clark. Click here to download the viewgraph presentation here. Click here to download the PDF version of the presentation.

Improving the Performance of Low Cost GPS Timing Receivers presented at the 38th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting on December 7, 2006 at the Hyatt Regency, Reston VA by Dr. Thomas A. Clark, NASA Goddard Space Flight Center (retired), and Richard M. Hambly, CNS Systems, Inc. Right click to download the viewgraph presentation here. Click here to download the PDF version of the presentation..

Classic HP Application Notes GPS and Precision Timing Applications, Application Note 1272. The Science of Timekeeping, Application Note 1289.

What's all this VLBI stuff, anyway? presented at the International VLBI Service for Geodesy and Astrometry (IVS) Technical Operations Workshop (TOW) meeting on April 30 - May 3, 2007 at Haystack Observatory by Dr. Thomas A. Clark, NASA Goddard Space Flight Center, Greenbelt MD. Right click to download the viewgraph presentation here.

Quartz Crystal Resonators and Oscillators For Frequency Control and Timing Applications - A Tutorial by John R. Vig, January 2007.

AMSAT OSCAR-E, A New LEO Satellite from AMSAT-NA by Richard M. Hambly, W2GPS as published in the AMSAT Journal, Volume 25, No. 3, May/June 2002 and in CQ/VHF Magazine, Summer 2002. Get a copy of the paper here. Right click to download the viewgraph presentation here. This is the presentation given at the Maryland-DC area AMSAT Meeting and Space Seminar, Sunday, May 5, 2002, at NASA Goddard Space Flight Center, Greenbelt MD. Get a copy of the viewgraph presentation here. This is the presentation given at the AMSAT Forum. Hamvention 2002, Saturday, May 18, 2002, HARA Arena Complex, Room 1, Dayton OH.

AMSAT OSCAR-E Project Status Update, A New LEO Satellite from AMSAT-NA by Richard M. Hambly, W2GPS as published in the AMSAT Journal, Volume 25, No. 7, Nov/Dec 2002 and in CQ/VHF Magazine, Winter 2002. Get a copy of the paper here. Get a copy of the viewgraph presentation here. This is the presentation given at the 20th Space Symposium and AMSAT-NA Annual Meeting, Saturday, November 9, 2002, at the Lockheed Martin Recreation Area (LMRA), Fort Worth TX.

Microsat Design, What Do People Want? by Richard M. Hambly, W2GPS as published in the AMSAT Journal, Volume 25, No. 7, Nov/Dec 2002. Get a copy of the paper here (requires Adobe Acrobat Version 5 or newer). Right click to download the viewgraph presentation here. This is the presentation given at the 20th Space Symposium and AMSAT-NA Annual Meeting, Saturday, November 9, 2002, at the Lockheed Martin Recreation Area (LMRA), Fort Worth TX.

AMSAT OSCAR-E Project, Summer 2003 Status Update by Richard M. Hambly, W2GPS as published in the AMSAT Journal, Volume 26, No. 4, Jul/Aug 2003 and in CQ/VHF Magazine, Summer 2003. Get a copy of the paper here (requires Adobe Acrobat Version 5 or newer). Right click to download the viewgraph presentation here. This is the presentation given at the AMSAT Forum, Dayton Hamvention 2003 on Saturday, May 17, 2003, at HARA Arena Complex, Room 1, Dayton, Ohio.

AMSAT OSCAR-E Project, Fall 2003 Status Update by Richard M. Hambly, W2GPS as published in the Proceedings of the AMSAT-NA 21st Space Symposium, November 2003, Toronto, Ontario, Canada. Get a copy of the paper here. Right click to download the viewgraph presentation here.

C-C RIDER, A New Concept for Amateur Satellites by Tom Clark, W3IWI as published in the Proceedings of the AMSAT-NA 21st Space Symposium, November 2003, Toronto, Ontario, Canada. Get a copy of the paper here. Right click to download the viewgraph presentation here.

C-C Rider Revisited by Tom Clark (W3IWI), Bob McGwier (N4HY), Phil Karn (KA9Q) and Rick Hambly (W2GPS) as published in the Proceedings of the AMSAT-NA 22nd Space Symposium, October 2004, Arlington, Virhginia. Get a copy of the paper here. Right click to download the viewgraph presentation here.

Eagle Software Defined Transponder Milestone On Tuesday, August 16, 2005 Tom Clark W3IWI, Rick Hambly W2GPS, and Bob McGwier N4HY (left to right in this picture ) made the first ever live contacts via the prototype Eagle satellite Software Defined Transponder (SDT). The SDT software was written by Frank Brickle AB2KT, and Bob McGwier N4HY. The transponder was set up in Rick's lab (shown) and the contacts were made from Rick's Ham shack, completely independent of the lab. Hear the historic contact.

Software Defined Transponders, Future AMSAT Missions: Phase 3E and Eagle by Tom Clark (K3IO, ex W3IWI) and Bob McGwier (N4HY) as presented at the 10th Annual SVHFS Technical Conference in Greenville SC, April 28 & 29, 2006 Get a copy of the paper here.

AMSAT North America, A Status Report by Richard M. Hambly W2GPS, AMSAT President, as presented at various conferences in the Spring of 2006, most recently at AMSAT-DC Meeting and Space Seminar, April 8, 2006, Historical Electronics Museum, 1745 West Nursery Road, Linthicum MD 21090 Get a copy of the PowerPoint presentation here.  

www.gpstime.com

GPS.gov: Timing Applications

In addition to longitude, latitude, and altitude, the Global Positioning System (GPS) provides a critical fourth dimension – time. Each GPS satellite contains multiple atomic clocks that contribute very precise time data to the GPS signals. GPS receivers decode these signals, effectively synchronizing each receiver to the atomic clocks. This enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks.

Precise time is crucial to a variety of economic activities around the world. Communication systems, electrical power grids, and financial networks all rely on precision timing for synchronization and operational efficiency. The free availability of GPS time has enabled cost savings for companies that depend on precise time and has led to significant advances in capability.

For example, wireless telephone and data networks use GPS time to keep all of their base stations in perfect synchronization. This allows mobile handsets to share limited radio spectrum more efficiently. Similarly, digital broadcast radio services use GPS time to ensure that the bits from all radio stations arrive at receivers in lockstep. This allows listeners to tune between stations with a minimum of delay.

Companies worldwide use GPS to time-stamp business transactions, providing a consistent and accurate way to maintain records and ensure their traceability. Major financial institutions use GPS to obtain precise time for setting internal clocks used to create financial transaction timestamps. Large and small businesses are turning to automated systems that can track, update, and manage multiple transactions made by a global network of customers, and these require accurate timing information available through GPS.

The U.S. Federal Aviation Administration (FAA) uses GPS to synchronize reporting of hazardous weather from its 45 Terminal Doppler Weather Radars located throughout the United States.

Instrumentation is another application that requires precise timing. Distributed networks of instruments that must work together to precisely measure common events require timing sources that can guarantee accuracy at several points. GPS-based timing works exceptionally well for any application in which precise timing is required by devices that are dispersed over wide geographic areas. For example, integration of GPS time into seismic monitoring networks enables researchers to quickly locate the epicenters of earthquakes and other seismic events.

Power companies and utilities have fundamental requirements for time and frequency to enable efficient power transmission and distribution. Repeated power blackouts have demonstrated to power companies the need for improved time synchronization throughout the power grid. Analyses of these blackouts have led many companies to place GPS-based time synchronization devices in power plants and substations. By analyzing the precise timing of an electrical anomaly as it propagates through a grid, engineers can trace back the exact location of a power line break.

Some users, such as national laboratories, require the time at a higher level of precision than GPS provides. These users routinely use GPS satellites not for direct time acquisition, but for communication of high-precision time over long distances. By simultaneously receiving the same GPS signal in two places and comparing the results, the atomic clock time at one location can be communicated to the other. National laboratories around the world use this "common view" technique to compare their time scales and establish Coordinated Universal Time (UTC). They use the same technique to disseminate their time scales to their own nations.

New applications of GPS timing technology appear every day. Hollywood studios are incorporating GPS in their movie slates, allowing for unparalleled control of audio and video data, as well as multi-camera sequencing. The ultimate applications for GPS, like the time it measures, are limitless.

As GPS becomes modernized, further benefits await users. The addition of the second and third civilian GPS signals will increase the accuracy and reliability of GPS time, which will remain free and available to the entire world.

www.gps.gov

How accurate is the TIME DISPLAY on my GPS?

How accurate is the TIME DISPLAY on my GPS? Does my GPS display GPS time,  UTC time, Local time or WHAT? How does the GPS system keep time synchronzied with UTC time? Marc Brett gives us the answer.

See also the  Garmin Engineering and Joe Mehaffey's comments at end

> Which is it?  "within one microsecond" or "ahead by several seconds" or  "ahead by 12 seconds"?< >Also:  Why does my "locked on"  GPS sometimes read "right on" and sometimes differ from standard time by 11 or 12 seconds?<

GPS has become the world's principal supplier of accurate time. It is used extensively both as a source of time and as a means of transferring time from one location to another. There are three kinds of time available from GPS: GPS time, UTC as estimated and produced by the United States Naval Observatory, and the times from each free-running GPS satellite's atomic clock.  The Master Control Station (MCS) at Falcon Air Force Base near Colorado Springs, Colorado gathers the GPS satellites' data from five monitor stations around the globe. A Kalman filter software program estimates the time error, frequency error, frequency drift and Keplerian orbit parameters for each of the satellites and its operating clock.   This information is uploaded to each satellite so that it can be broadcasted in real time. This process provides GPS time consistency across the constellation to within a small number of nanoseconds and accurate position determination of the satellites to within a few meters.

Because of this process, GPS cannot tolerate the introduction of leap seconds. Hence, in 1980, when the Department of Defense started keeping time on the GPS satellites, its system time and frequency were set to agree with UTC(USNO MC). At that time, TAI minus UTC was 19 seconds.  Since then, UTC has been delayed many leap seconds and GPS time has not. Hence, GPS time is still very close to TAI minus 19 seconds. The specification on GPS time is that it is to be kept within one microsecond of UTC(USNO MC) modulo one second.  In other words, as a leap second is introduced into UTC(USNO MC) time, no such step occurs in GPS time. But GPS time is still steered to agree as well as possible with UTC(USNO MC), as if no leap seconds had occurred since 1980. In practice, the steering performance is much better than the one-microsecond specification; typically, it is well within 40 nanoseconds.

In order to provide an estimate of UTC time derivable from a GPS signal, a set of UTC corrections is also provided as part of the broadcast signal. This broadcast message includes the time difference in whole seconds between GPS time and UTC. During 1996 GPS time minus UTC time was 11 seconds. Also included in this message is the rate and time difference estimate between GPS time and UTC(USNO MC) modulo one second. This allows a receiver, in principle, to calculate an accurate estimate of UTC(USNO MC). The mission goal is 28 ns (1 sigma). Outside of the purposeful current degradation of the GPS signal (called Selective Availability, SA) by the DoD for security purposes, this calculation may have an accuracy of about 10 nanoseconds (ns) on an rms basis. Since USNO has been successful in predicting UTC to within about 10 ns, combining these two independent error sources yields a real-time potential uncertainty for UTC available from GPS at about the 14-ns level. In practice, SA prohibits achieving this accuracy level unless special clock systems and filtering techniques are employed.  The SA degradation can be filtered away.

(Quoted from Hewlett Packard Application Note 1289: The Science of Timekeeping, by David W. Allan, Neil Ashby and Clifford C. Hodge.  See the entire article at:    http://www.allanstime.com/Publications/DWA/Science_Timekeeping/index.html

-- Marc Brett  +44 181 560 3160            Western Geophysical [email protected]                     455 London Road, Isleworth FAX: +44 181 847 5711                   Middlesex TW7 5AB    UK

Some have wondered how accurate the time display is on Garmin GPS receivers such as G-12XL, G-II+, and the  G-III.   Here is an answer provided by Garmin Engineering.   This also explains why the GPS can be locked for awhile and still differ from UTC by 11 or 12 seconds.  (This answer applies to other brands of GPS receivers as well.)

Start of Garmin quote>

Provided the unit has collected current leap second count from the navigation message, (current leap second difference from GPS time is only broadcast once in a 12.5 minute Nav. message), or current leap second has not changed since the last time the unit collected this variable, the time displayed on the front of the unit should be accurate to within 1 second of UTC.

>end of Garmin Quote

Joe Mehaffey comments: This means that IF your GPS does not have (or does not save) the leap second offset from last time it was operated,  your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS.  Jack and I have observed that "typically" Garmin GPS receivers display time which is delayed from about 1/2 to 1 second behind UTC.  Lowrance GPS receivers are usually between 1 and 2 seconds delayed behind UTC.  In both cases,  this is a result of the display driver subroutine having low priority as the "GPS internal clock" is within a few nanoseconds of correct.

Similarly,  the NMEA time output on the serial link is typically delayed a second or two depending on various factors.

Joe Mehaffey

gpsinformation.net

Time Systems (GPS)

Three time systems are used in satellite surveying. They are sidereal time, dynamic time and atomic time (cf., e.g., Hofman-Wellenhof et al. 1997; Leick 1995; McCarthy 1996; King et al. 1987).

Sidereal time is a measure of the Earth’s rotation and is defined as the hour angle of the vernal equinox. If the measure is counted from the Greenwich meridian, the sidereal time is called Greenwich Sidereal Time. Universal Time (UT) is the Greenwich hour angle of the apparent Sun, which is orbiting uniformly in the equatorial plane. Because the angular velocity of the Earth’s rotation is not a constant, sidereal time is not a uniformly-scaled time. The oscillation of UT is also partly caused by the polar motion of the Earth. The universal time corrected for the polar motion is denoted by UT1.

Dynamical time is a uniformly-scaled time used to describe the motion of bodies in a gravitational field. Barycentric Dynamic Time (TDB) is applied in an inertial coordinate system (its origin is located at the centre-of-mass (Barycentre)). Terrestrial Dynamic Time (TDT) is used in a quasi-inertial coordinate system (such as ECI). Because of the motion of the Earth around the Sun (or say, in the Sun’s gravitational field), TDT will have a variation with respect to TDB. However, both the satellite and the Earth are subject to almost the same gravitational perturbations. TDT may be used for describing the satellite motion without taking into account the influence of the gravitational field of the Sun. TDT is also called Terrestrial Time (TT).

Atomic Time is a time system kept by atomic clocks such as International Atomic Time (TAI). It is a uniformly-scaled time used in the ECEF coordinate system. TDT is realised by TAI in practice with a constant offset (32.184 sec). Because of the slowing down of the Earth’s rotation with respect to the Sun, Coordinated Universal Time (UTC) is introduced to keep the synchronisation of TAI to the solar day (by inserting the leap seconds). GPS Time (GPST) is also an atomic time.

The relationships between different time systems are given as follows:

where dUT1 can be obtained by IERS, (dUT1 < 0.7 sec, cf., Zhu et al. 1996), (dUT1 is also broadcasted with the navigation data), n is the number of leap seconds of date and is inserted into UTC on the 1st of January and 1st of July of the years. The actual n can be found in the IERS report.

Time argument T (Julian centuries) is used in the formulas given in Sect. 2.4. For convenience, T is denoted by TJD, and TJD can be computed from the civil date (Year, Month, Day, and Hour) as follows:

where

where JD is the Julian Date, Hour is the time of UT and INT denotes the integer part of a real number. The Julian Date counted from JD2000.0 is then JD2000 = JD – JD2000.0,where JD2000.0 is the Julian Date of 2000 January 1st 12h and has the value of 2 451545.0 days. One Julian century is 36 525 days.

Inversely, the civil date (Year, Month, Day and Hour) can be computed from the Julian Date (JD) as follows:

where b, c, d, and e are auxiliary numbers.

Because the GPS standard epoch is defined as JD = 2 444244.5 (1980 January 6, 0h), GPS week and the day of week (denoted by Week and N) can be computed by

where N is the day of week (N = 0 for Monday, N = 1 for Tuesday, and so on).

For saving digits and counting the date from midnight instead of noon, the Modified Julian Date (MJD) is defined as

GLONASS time (GLOT) is defined by Moscow timewhich equals UTC plus three hours (corresponding to the offset of Moscow time to Greenwich time), theoretically. GLOT is permanently monitored and adjusted by the GLONASS Central Synchroniser (cf. Rofibach 2000). UTC and GLOT then has a simple relation

whereis the system time correction with respect towhich is broadcasted by the GLONASS ephemerides and is less than one microsecond. Therefore there is approximately

where m is called a number of "leap seconds" between GPS and GLONASS (UTC) time and is given in the GLONASS ephemerides. m is indeed the leap seconds since GPS standard epoch (1980 January 6, 0h).

Galileo system time (GST) will be maintained by a number of UTC laboratory clocks. GST and GPST are time systems of various UTC laboratories. After the offset of GST and GPST is made available to the user, the interoperability will be ensured.

what-when-how.com

GPS FAQ

Frequently Asked Questions about GPS

BackgroundGPS (Global Positioning System) is a satellite navigation system to determine accurate location and time.  It was developed by the U.S. Department of Defense, and is widely used today for both navigation and time synchronization.  The satellites are owned by the Department of Defense, paid for by U.S. tax dollars, and reception of satellite signals is available for public use. More on GPS technology...

ClockWatch Star Sync GPS time synchronization works by using a GPS receiver that is connected to your computer.   Beagle Software has designed a software interface to turn your computer into a network time source.

Here are some frequently asked questions about GPS.  Feel free to   with other questions you have.

How does Star Sync get the time from GPS ?What type of GPS receiver works best?How accurate is GPS time synchronization?How does GPS work? Where does GPS work? Understanding GPS technology...What type of hardware do I need?Do I need an Internet connection to use GPS?I already have a GPS receiver; will ClockWatch Star Sync work with it?Why would I want to use GPS? Can other computers and devices get their time from Star Sync?

How does Star Sync get the time from the GPS?

The GPS receiver outputs UTC (Coordinated Universal Time) date and time of day in the transmitted data. After the initial position fix, the date and time of day are calculated using GPS satellite information and are synchronized with the one-pulse-per-second output.

The highly accurate one-pulse-per second output is provided to Star Sync from the Garmin GPS35. The signal is generated after the initial position fix has been calculated and continues until power down. This rising edge of the signal is synchronized to the the start of each GPS second. The information transmitted to Star Sync is referenced to the pulse immediately preceding the NMEA 0183 RMC sentence. Beagle Software specially configures the Garmin GPS35 so that this pulse is usable by the Star Sync software. 

What type of GPS receiver works best?Some GPS receivers work better for receiving time signals.  This is due to the default port used for time, which is considered by navigational receivers to be a secondary priority.  You will have more accurate time if you use a receiver that has PPS (pulse per second) output available, and that this output is the priority. 

Beagle Software offers the Garmin GPS35 receiver, which is specifically designed for time output.

How Does GPS work? GPS receivers integrate a radio and a navigation computer and can receive the faint, twenty-watt signals coming from the satellites. The computer uses these signals to calculate the distance between the satellites and the receiver. With this information, the computer can further calculate the position and velocity of the receiver. The number of satellites visible to a receiver constantly varies between four and eleven according to time and location. Each satellite broadcasts a number of unique spread-spectrum codes, but only one, the Coarse Acquisition (C/A) code, is easily accessible for civilian use. The C/A in orbit 11,000 miles above earth, GPS satellites transmit at twenty watts a number of unique spread-spectrum code. The number of satellites visible to a GPS receiver constantly varies between four and eleven according to time and location. Code is effectively a timing signal synchronized to an international time standard-Universal Coordinated Time (UCT). UCT is kept by a world-wide ensemble of cesium and hydrogen maser frequency standard atomic docks. The highest-quality GPS receivers measure the C/A code to better-than- nanosecond precision. More... 

How accurate is GPS time synchronization?GPS time synchronization is highly accurate, but your results will vary depending on your receiver and computer.  The Garmin GPS35 receiver is accurate to within a millisecond of UTC; however, the time that is ultimately synchronized to your computer will not be as accurate.  Why?  Because computer systems are not fast enough nor consistent enough for better accuracy.  Our tests show an accuracy of +/- 0.1 second using the Garmin GPS35. 

Where can GPS work?GPS reception is available around the globe.  You will need to have a clear view of the skies so the receiver can triangulate at least three satellites.  An office window works well, but you may have problems receiving signals in the inner area of buildings. In general, metal and masonry block GPS signals, while glass, wood, or plastic does not.

Under specific conditions, GPS will not provide the time. For instance, the 1,542 MHz GPS signal does not penetrate buildings, which makes it difficult to receive signals indoors away from windows. Also the signal can be critically weakened by heavy foliage and interfered with by other sources such as poorly maintained television broadcasting equipment.  

What type of hardware do I need?You need a GPS receiver, antenna, power supply, data cable and serial port connector.  These come standard with the Garmin GPS35.  You also need access to a power source.  You may also purchase accessories, including mounting units from Beagle Software.

The only other requirement is a functioning Windows PC with an available serial (COM) port. 

Do I need an Internet connection to use GPS?No.  GPS time synchronization does not require an Internet connection.

I already have a GPS receiver; will ClockWatch Star Sync work with it?ClockWatch Star Sync will work with most GPS receivers with NMEA 0183 data output connected to an available serial port on the computer running Star Sync software.

Why would I want to use GPS for time synchronization?Time syncing using GPS is a good alternative for remote or highly secured computers.  ClockWatch Star Sync does not require an Internet or modem connection.

GPS time synchronization may potentially be more accurate than syncing over the Internet or by modem.  The true accuracy depends on the receiver as well as the computer.  Beagle Software's experience is that the Star Sync when used with the supplied Garmin GPS35 receiver is consistently accurate within 0.1 second.

Can other computers and devices get their time from Star Sync?

Yes, computers, routers, printers, PBXs and other devices on your LAN can use Star Sync as a central timeserver. All they need to do run an NTP or SNTP client and point to the computer running Star Sync. Many computers (including Windows) and networked devices have this capability built in.

 

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For more information:  Advanced Features or Star Sync software  Star Sync GPS Receivers:      Installation Information      Antenna Mounting Options  Frequently Asked Questions about GPS  GPS Glossary

Star Sync Main PageStar Sync Site Index

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UT1, UTC, TAI, ET, TT, GPS time

Time Scales: UT1, UTC, TAI, ET, TT, GPS time By Paul Schlyter, Stockholm, Swedenemail: [email protected] WWW: http://stjarnhimlen.se/ 1993? First version as an ASCII document.1995 and on: Posted on Usenet several times and became part of the sci.astro FAQ part 32004-09-?? First HTML version, for an article in the Swedish ham radio magazine QTC2005-04-19 Replaced obsolete links with working ones2005-06-22 Major overhaul: corrected several small errors, updated tables, added new links.Thanks to John Stockton (http://www.merlyn.demon.co.uk/) for a number of suggestions to this page.2005-07-10 Added a link to Steve Allen's detailed history of the time scales. 2017-09-30 Updated or removed obsolete links.

The different time scales

TAI = International Atomic Time (Temps Atomique International = TAI) is defined as the weighted average of the time kept by about 200 atomic clocks in over 50 national laboratories worldwide. TAI-UT1 was approximately 0 on 1958 Jan 1. UTC = Coordinated Universal Time. Differs from TAI by an integral number of seconds. When needed, leap seconds are introduced in UTC to keep the difference between UTC and UT less than 0.9 s. UTC was introduced in 1972. UT = Universal time. Defined by the Earth's rotation, formerly determined by astronomical observations but today GPS satellites are used instead. This time scale is slightly irregular. There are several different definitions of UT, but the difference between them is always less than about 0.03 s. UT0 = "raw", uncorrected UT as derived from meridian circle observations or from more modern methods involving GPS satellites. UT1 = UT0 corrected for polar wandering - usually one means UT1 when saying UT. UT2 = UT1 corrected for seasonal variations in the Earth's rotational speed, by adding + 0.022 * sin(2*pi*t) - 0.017 * cos(2*pi*t) - 0.007 * sin(4*pi*t) + 0.006 * cos(4*pi*t) seconds to UT1, where t is the fraction of the year (zero at 1 Jan). UT2 is nowadays considered obsolete. ET = Ephemeris Time. Was used 1960-1983, and was replaced by TDT and TDB in 1984. For most purposes, ET up to 1983 Dec 31 and TDT from 1984 Jan 1 can be regarded as a continuous time-scale. TDT = Terrestial Dynamical Time. Was used 1984-2000 as a time-scale of ephemerides from the Earth's surface. TDT = TAI + 32.184. Replaced ET (Ephemeris Time) in 1984, was replaced by TT (Terrestial Time) in 2001. TDB = Barycentric Dynamical Time. Used as a time-scale of ephemerides referred to the barycentre of the solar system. Differs from TDT by at most a few milliseconds. TDB = TT + 0.001 658s * sin(g) + 0.000 014s * sin(2*g) g = 357.53_d + 0.985 600 28_d * ( JD - 245 1545.0 ) (higher order terms neglected; g = Earth's mean anomaly) TT = Terrestial Time. Originally used instead of TDT or TDB when the difference between them didn't matter. Was defined in 1991 to be consistent with the SI second and the General Theory of Relativity. Replaced TDT in the ephemerides from 2001 and on. TCG = Geocentric Coordinate Time. Defined in 1991 along with TT TCB = Barycentric Coordinate Time. Defined in 1991 along with TT delta-T = ET - UT prior to 1984 TDT - UT 1984 - 2000 TT - UT from 2001 and on delta-UT = UT - UTC DUT = predicted value of delta-UT, rounded to 0.1s, given in some radio time signals. GPS time = TAI - 19 seconds. GPS time matched UTC from 1980-01-01 to 1981-07-01. No leap seconds are inserted into GPS time, thus GPS time is 13 seconds ahead of UTC on 2000-01-01. The GPS epoch is 00:00 (midnight) UTC on 1980-01-06. The differences between GPS Time and International Atomic Time (TAI) and Terrestrial Time (TT), also know as Terrestrial Dynamical Time (TDT), are constant at the level of some tens of nanoseconds while the difference between GPS Time and UTC changes in increments of seconds each time a leap second is added to UTC time scale. GPS week = a numbering of weeks starting at the GPS epoch 1980-01-06 00:00 GPS time (which back then was equal to UTC). Weeks are numbered from 0 and up until 1023, then it "rolls back" to 0 and are again numbered from 0 and up, etc. One GPS week rollover cycle is therefore 1024 weeks = 7168 days = ca 19.62 years. So far there's been one such GPS week number roll-over, on 1999-08-22 00:00 GPS time - a few older GPS receivers then ceased to show the correct date. ET 1960-1983 TDT 1984-2000 UTC 1972- GPS 1980- TAI 1958- TT 2001- ----+---------+-------------+-------------------------+----- | | | | |<------ TAI-UTC ------>|<----- TT-TAI ----->| | | | 32.184s fixed | |<GPS-UTC>|<- TAI-GPS ->| | | | 19s fixed | | | | <> delta-UT = UT1-UTC | | (max 0.9 sec) | -----+------------------------------------------------+----- |<-------------- delta-T = TT-UT1 -------------->| UT1 (UT) TT/TDT/ET Older time scales: GMT = Greenwich Mean Time. It's ambiguous, and is now used (although not in astronomy) in the sense of UTC in addition to the earlier sense of UT (in astronomical navigation, GMT still means UT). Prior to 1925, GMT was reckoned for astronomical purposes from Greenwich mean noon (12h UT) to avoid a date change in the middle of the night in Europe - a new GMT date then started 12 hours after the start of the corresponding civil date. (Prior to 1805 the Royal Navy Day started 12 hour before local mean solar time, thus the Royal Navy Day was then approx. 24 hours ahead of GMT). GCT = Greenwich Civil Time. Used in the US from 1925 to mean the "new" GMT starting at Greenwich mean midnight, to distinguish it from the "old" GMT. When UT was adopted, GCT fell out of use. LMT = Local Mean Time. The mean solar time at the local meridian. LCT = Local Civil Time, the same as LMT. Used in the US together with GCT.

Delta-T

delta-T varies continuously, depending on the Earth's rotation.

UT1 is variable with respect to UTC. Leap seconds were introduced in UTC to keep delta-UT within +-0.9s.

TAI-UTC is always an integral number of seconds, and is varied when leap seconds are added (or removed, but that hasn't happened yet) at the end of every year, or every half-year, or every third month, in that order of priority.

Table of time scales 1972-present, and some predictions

TT = TAI+32.184s ==> UT1-UTC = TAI-UTC - (TT-UT1) + 32.184s Starting at TAI-UTC GPS-UTC TT-UT1 UT1-UTC 1972-01-01 +10 - +42.23 -0.05 1972-07-01 +11 - +42.80 +0.38 1973-01-01 +12 - +43.37 +0.81 1973-07-01 " - +43.93 +0.25 1974-01-01 +13 - +44.49 +0.69 1974-07-01 " - +44.99 +0.19 1975-01-01 +14 - +45.48 +0.70 1975-07-01 " - +45.97 +0.21 1976-01-01 +15 - +46.46 +0.72 1976-07-01 " - +46.99 +0.19 1977-01-01 +16 - +47.52 +0.66 1977-07-01 " - +48.03 +0.15 1978-01-01 +17 - +48.53 +0.65 1978-07-01 " - +49.06 +0.12 1979-01-01 +18 - +49.59 +0.59 1979-07-01 " - +50.07 +0.11 1980-01-01 +19 0 +50.54 +0.64 1980-07-01 " " +50.96 +0.22 1981-01-01 " " +51.38 -0.20 1981-07-01 +20 1 +51.78 +0.40 1982-01-01 " " +52.17 +0.01 1982-07-01 +21 2 +52.57 +0.61 1983-01-01 " " +52.96 +0.22 1983-07-01 +22 3 +53.38 +0.80 1984-01-01 " " +53.79 +0.39 1984-07-01 " " +54.07 +0.11 1985-01-01 " " +54.34 -0.16 1985-07-01 +23 4 +54.61 +0.57 1986-01-01 " " +54.87 +0.31 1986-07-01 " " +55.10 +0.08 1987-01-01 " " +55.32 -0.14 1987-07-01 " " +55.57 -0.39 1988-01-01 +24 5 +55.82 +0.36 1988-07-01 " " +56.06 +0.12 1989-01-01 " " +56.30 -0.12 1989-07-01 " " +56.58 -0.40 1990-01-01 +25 6 +56.86 +0.32 1990-07-01 " " +57.22 -0.04 1991-01-01 +26 7 +57.57 +0.61 1991-07-01 " " +57.94 +0.24 1992-01-01 " " +58.31 -0.13 1992-07-01 +27 8 +58.72 +0.46 1993-01-01 " " +59.12 +0.06 1993-07-01 +28 9 +59.55 +0.63 1994-01-01 " " +59.98 +0.20 1994-07-01 +29 10 +60.38 +0.80 1995-01-01 " " +60.78 +0.40 1995-07-01 " " +61.20 -0.02 1996-01-01 +30 11 +61.63 +0.55 1996-07-01 " " +61.96 +0.22 1997-01-01 " " +62.29 -0.11 1997-07-01 +31 12 +62.63 +0.55 1998-01-01 " " +62.97 +0.21 1998-07-01 " " +63.22 -0.04 1999-01-01 +32 13 +63.47 +0.71 1999-07-01 " " +63.66 +0.52 2000-01-01 " " +63.82 +0.36 2000-07-01 " " +63.98 +0.20 2001-01-01 " " +64.09 +0.09 2001-07-01 " " +64.20 -0.02 2002-01-01 " " +64.30 -0.12 2002-07-01 " " +64.41 -0.23 2003-01-01 " " +64.47 -0.29 2003-07-01 " " +64.55 -0.37 2004-01-01 " " +64.57 -0.39 2004-07-01 " " +64.65 -0.47 2005-01-01 " " +64.68 -0.50 2005-07-01 " " +64.80 -0.62 2006-01-01 +33 14 +64.85 +0.33 2006-07-01 " " +64.99 +0.19 2007-01-01 " " +65.15 +0.03 2007-07-01 " " +65.34 -0.16 2008-01-01 " " +65.45 -0.27 2008-07-01 " " +65.63 -0.45 2009-01-01 +34 15 +65.78 +0.40 2009-07-01 " " +65.95 +0.23 2010-01-01 " " +66.07 +0.11 2010-07-01 " " +66.24 -0.06 2011-01-01 " " +66.32 -0.14 2011-07-01 " " +66.47 -0.29 2012-01-01 " " +66.60 -0.42 2012-07-01 +35 16 +66.77 +0.41 2013-01-01 " " +66.91 +0.27 2013-07-01 " " +67.13 +0.05 2014-01-01 " " +67.28 -0.10 2014-07-01 " " +67.49 -0.31 2015-01-01 " " +67.64 -0.46 2015-07-01 +36 17 +67.86 +0.32 2016-01-01 " " +68.10 +0.08 2016-07-01 " " +68.40 -0.22 2017-01-01 +37 18 +68.59 +0.59 2017-07-01 " " +68.81 +0.37 2018-01-01 " " +69.0 0.2 (pred) 2019-01-01 " " +69.5 -0.3 (pred) 2020-01-01 " " +69.9 -0.7 (pred) 2021-01-01 ? ? +70 (pred) 2022-01-01 ? ? +70 (pred) 2023-01-01 ? ? +71 (pred) 2024-01-01 ? ? +71 (pred) 2025-01-01 ? ? +71 (pred) 2026-01-01 ? ? +72 (pred) 2027-01-01 ? ? +72 (pred) (last updated 2017-09-30)

Delta-T 1620-1972

delta-T = ET - UT for the years 1620 - 1972 ET-UT, s Year +0.0 +1.0 +2.0 +3.0 +4.0 ---------------------------------------------------- 1620 +124 +119 +115 +110 +106 1625 +102 +98 +95 +91 +88 1630 +85 +82 +79 +77 +74 1635 +72 +70 +67 +65 +63 1640 +62 +60 +58 +57 +55 1645 +54 +53 +51 +50 +49 1650 +48 +47 +46 +45 +44 1655 +43 +42 +41 +40 +38 1660 +37 +36 +35 +34 +33 1665 +32 +31 +30 +28 +27 1670 +26 +25 +24 +23 +22 1675 +21 +20 +19 +18 +17 1680 +16 +15 +14 +14 +13 1685 +12 +12 +11 +11 +10 1690 +10 +9 +9 +9 +9 1695 +9 +9 +9 +9 +9 1700 +10 +9 +9 +9 +9 1705 +9 +9 +9 +10 +10 1710 +10 +10 +10 +10 +10 1715 +10 +10 +11 +11 +11 1720 +11 +11 +11 +11 +11 1725 +11 +11 +11 +11 +11 1730 +11 +11 +11 +11 +12 1735 +12 +12 +12 +12 +12 1740 +12 +12 +12 +12 +13 1745 +13 +13 +13 +13 +13 1750 +13 +14 +14 +14 +14 1755 +14 +14 +14 +15 +15 1760 +15 +15 +15 +15 +15 1765 +16 +16 +16 +16 +16 1770 +16 +16 +16 +16 +16 1775 +17 +17 +17 +17 +17 1780 +17 +17 +17 +17 +17 1785 +17 +17 +17 +17 +17 1790 +17 +17 +16 +16 +16 1795 +16 +15 +15 +14 +14 1800 +13.7 +13.4 +13.1 +12.9 +12.7 1805 +12.6 +12.5 +12.5 +12.5 +12.5 1810 +12.5 +12.5 +12.5 +12.5 +12.5 1815 +12.5 +12.5 +12.4 +12.3 +12.2 1820 +12.0 +11.7 +11.4 +11.1 +10.6 1825 +10.2 +9.6 +9.1 +8.6 +8.0 1830 +7.5 +7.0 +6.6 +6.3 +6.0 1835 +5.8 +5.7 +5.6 +5.6 +5.6 1840 +5.7 +5.8 +5.9 +6.1 +6.2 1845 +6.3 +6.5 +6.6 +6.8 +6.9 1850 +7.1 +7.2 +7.3 +7.4 +7.5 1855 +7.6 +7.7 +7.7 +7.8 +7.8 1860 +7.88 +7.82 +7.54 +6.97 +6.40 1865 +6.02 +5.41 +4.10 +2.92 +1.81 1870 +1.61 +0.10 -1.02 -1.28 -2.69 1875 -3.24 -3.64 -4.54 -4.71 -5.11 1880 -5.40 -5.42 -5.20 -5.46 -5.46 1885 -5.79 -5.63 -5.64 -5.80 -5.66 1890 -5.87 -6.01 -6.19 -6.64 -6.44 1895 -6.47 -6.09 -5.76 -4.66 -3.74 1900 -2.72 -1.54 -0.02 +1.24 +2.64 1905 +3.86 +5.37 +6.14 +7.75 +9.13 1910 +10.46 +11.53 +13.36 +14.65 +16.01 1915 +17.20 +18.24 +19.06 +20.25 +20.95 1920 +21.16 +22.25 +22.41 +23.03 +23.49 1925 +23.62 +23.86 +24.49 +24.34 +24.08 1930 +24.02 +24.00 +23.87 +23.95 +23.86 1935 +23.93 +23.73 +23.92 +23.96 +24.02 1940 +24.33 +24.83 +25.30 +25.70 +26.24 1945 +26.77 +27.28 +27.78 +28.25 +28.71 1950 +29.15 +29.57 +29.97 +30.36 +30.72 1955 +31.07 +31.35 +31.68 +32.18 +32.68 1960 +33.15 +33.59 +34.00 +34.47 +35.03 1965 +35.73 +36.54 +37.43 +38.29 +39.20 1970 +40.18 +41.17 +42.23

Links to more information

More information can be obtained at:

Wikipedia: http://en.wikipedia.org/wiki/International_Atomic_Time

Steve Allen's detailed history of the different time scales: http://www.ucolick.org/~sla/leapsecs/timescales.html

BIPM - Bureau International des Poids et Mesures: http://www.bipm.fr/en/scientific/tai/

USNO Time Service Department: http://tycho.usno.navy.mil/

IERS - International Earth Rotation and Reference Systems Service: http://www.iers.org/

Past leap seconds info:ftp://maia.usno.navy.mil/ser7/tai-utc.dat

Delta-T data: ftp://maia.usno.navy.mil/ser7/deltat.data TT/ET - UT1 UT1-UTC data: ftp://maia.usno.navy.mil/ser7/Daily data since 1972, and predictions -- large! ftp://maia.usno.navy.mil/ser7/finals.allExplanation to the previous file: ftp://maia.usno.navy.mil/ser7/readme.finalsPredictions of TT-UT and UT1-UTC: http://maia.usno.navy.mil/ser7/deltat.preds Rapid service and prediction of Earth orientation parameters: http://maia.usno.navy.mil/

IERS - International Earth Rotation Service: http://hpiers.obspm.fr/IERS Leap Seconds Bulletins: ftp://hpiers.obspm.fr/iers/bul/bulc/ ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat Info on GPS time:http://tycho.usno.navy.mil/gpstt.htmlhttp://tycho.usno.navy.mil/gps_datafiles.html

Time signals by shortwave radio

Europe: RWM (Moscow) CW 4996, 9996, 14996 kHz North America: WWV (USA) AM 2500, 5000, 10000, 15000, 20000 kHz CHU (Canada) AM 3330, 7850, 14670 kHz Worldwide list - Another list - Time signals (Wikipedia)

World standard time zones

In October 1884, an International Time Conference adopted the Greenwich Meridian as the prime meridian or zero degree point and divided the world into 24 equal divisions of 15 degrees each.

Here is a table and a map of the current International Time Zones: http://en.wikipedia.org/wiki/Time_zoneMap of time zones

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