# Presentation: 249799

## FUTURE OF THE UTC TIME SCALE

45th CGSIC Meeting
Timing Subcommittee
September 2005

Ron Beard, Chairman 
Special Rapporteur Group
U.S. Naval Research Laboratory
Washington, D.C.

**45****th**** CGSIC Meeting**
**Timing Subcommittee**
**September 2005**
**Ron Beard, Chairman **
**Special Rapporteur Group**
**U.S. Naval Research Laboratory**
**Washington, D.C.**

## TOPICS

**Time Scales. Origins and Uses**
**Question on the Future of UTC**
**SRG Study Activities**
**Progress**
**Summary Conclusions**
**Proposed Modification**

## ORIGINS OF UTC TIME SCALE

**1834 -**** Mean Solar Time ****introduced at Greenwich with publication of Nautical Almanac and Ephemerides**
**1870 – U.S. introduced time zones for Railroads time table**
**1884 - *****International Meridian Conference*****  recognized Prime Meridian at Greenwich, ****Universal Day**** (not to interfere with local or other standard time) to be ****Mean Solar Day ****to begin at Midnight on the initial meridian) **
**1912 – Bureau International de l’Heure (BIH) created by international conference in France to form single approximation of theoretical Universal Time known as Definitive Time**
**1919 – BIH recognized by the International Astronomical Union (IAU)**
**1925 – Ephemerides introduced day beginning at midnight, termed ****Greenwich Mean Civil Time**** ****(GMCT)****, or**** Greenwich Mean Time (GMT)**
**1948 – ****Universal Time (UT)**** adopted by IAU, NBS Atomic clock in operation **
**1952 – ****Ephemeris Time (ET)**** based on Sidereal Year defined by IAU**
**1956 – Ephemeris second defined by CIPM as 1/13 556 925.9747 of tropic year for Jan 0 at 12 h Ephemeris Time**
**1955 – NPL Cesium Atomic clock, Commercial Cesium Standards following year**
**1960 – Atomic Time introduced and coordination began by BIH**
**1967 – ****SI second**** defined by CGPM,**** ****BIH ****Atomic time**** recommended by IAU **
**1970 - ****International Atomic Time (TAI)**** defined by CIPM, ratified by CGPM in 1971, Coordinated Universal Time (UTC) defined by CCIR**
**1971 - ****Coordinated Universal Time (UTC)**** recognized by CGPM**
**1988 – UTC responsibility assumed by BIPM, astronomical and geodetic aspects assumed by IERS**

## TIME SCALE USES

**TRADITIONAL USES**
**Astronomical – Days based on rotation of the earth and relationship of celestial bodies with location.**
**Nautical – Time used for determination of longitude and observation of stars for positioning by celestial observation**
**Civil – Day used for daily activities and agriculture (seasons)**
**PRESENT USES**
**Astronomical – Days based on rotation of the earth and relationship of celestial bodies with location.**
**Civil – Day used for daily activities and agriculture (seasons)**
**Coordination of Radio Broadcasts and Frequency Generation**
**Coordination of Telecommunications **
**Time tagging electronic data and messages in Computer Networks**

## DEFINITION OF UTC ITU-R TF.460-6 STANDARD-FREQUENCY AND TIME-SIGNAL EMISSIONS(1970-1974-1978-1982-1986-1997-2002)

**To maintain worldwide coordination of standard frequency and time signals**
**Disseminate standard frequency and time signals in conformity with the SI second**
**Continuing need for UT immediate availability to an uncertainty of 0.1 second**
**TAI**** - International reference timescale of atomic time based on SI second as realized on a rotating geoid.  Continuous scale from origin 1 Jan 1958**
**UTC ****- Basis of coordinated dissemination of standard frequency and time signals.  Corresponds exactly in rate with TAI but differs by integral number of seconds.**
**	****UTC scale adjusted by insertion or deletion of seconds to ensure agreement with UT1**
**DUT1**** - Dissemination to include***** predicted***** *****difference *****UT1 – UTC **
**	****	****   ****(values given by IERS in integral multiples of 0.1 s)**

**Leaps Seconds may be introduced as the last second of a UTC month**
**December and June Preferred, March and September second choice**

##  Letter from Director BIPM to Secretary General, ITU (1999)

**The CCTF requested He communicate with the ITU**
**The CCTF was discussing issues concerning discontinuities in UTC  creating problems in coordinating telecommunications systems **
**Time as used in navigation satellite and telecommunications systems could possibly lead to multiple independent timekeeping systems (e.g. GPS Time) vice UTC.**
**Difficulties in computer systems and networks to adjust for time steps or leap seconds **
**Suggested a continuous timescale was available in TAI and systems with continuous needs should use it**

**ITU-R issued new ****Question ITU-R 236/7, T****he Future of The UTC Time Scale**

## The Future of The UTC Time Scale Question ITU-R 236/7 

**1. What are the requirements for globally-accepted time scales for use both in navigation and telecommunications systems, and for civil time-keeping?**
**Accuracy, Stability, Based on the SI Second**
**Uniformity, Accessibility**
**Reliability**
**Availability**
**Civil / National Timekeeping **
**2. What are the present and future requirements for the tolerance limit between UTC and UT1?**
**|UT1 – UTC| Tolerance of 0.9 seconds**
**Could a Greater Tolerance be Accommodated?**
**3. Does the current leap second procedure satisfy user needs, or should an alternative procedure be developed?**
**Availability of Leap Second Information for Users**
**Alternatives Used (Establishing System Independent Time)**
**Relationship of Telecom & NAVSAT System Internal Time to Time Scales**

## SPECIAL RAPPORTEUR GROUP ON UTC

**Focus studies on applications and uses of UTC **
**Address additional issues concerning proliferation of independent time systems (e.g. GPS Time)**
**Multiple systems with different time, interfaces, protocols and representations**
**Examine possible general transition to TAI **
**Investigate proposed changes to UTC **
**Coordinate with ITU-R Sector Members and CCTF**
**Report results**

## SRG ACTIVITIES

**MEETINGS:  Special Sessions at PTTI, FCS, CGSIC, EFTF, CCTF, U.S. Data Call **
** **
**SURVEYS:  **
**URSI  **
**IAU/IERS **
**NIST Web Site Survey**
**CRL Survey**
**Notices and Call for Participation **
**ITU-R Letter to Sector Members (IAU, URSI, CCTF, IAUGG, and Nations**
**IAU Working Group**
**URSI Commission J Working Group**
**Special Colloquium Hosted at IEN, Torino**
**Status Reports provided to CCTF**
** **

## PROGRESS OF ACTIVITIES

**Overall SRG efforts were generally ignored**
**Surveys were inconclusive and data calls were less than fruitful**
**Astronomical community at large has moved to dynamic relativistic time scales based on TAI (1980)**
**Definition of Spatial Reference Systems are benefiting from new capabilities such as GPS**
**Assessment of developments in radio- and tele-commnication are indicative:**
**Ad  hoc time in systems are driven by need for “Real-time” accuracy and precision**
**“****Local Time” determined by statistical process of many standards/clocks are being employed in new systems - UTC(xyz)**
**Telecommunications capabilities increasing by distributed syntonized operation (CDMA Network)**
** **

## COLLOQUIUM ON THE UTC TIMESCALE28-30 May 2003Istituto Elettrotecnico Nazionale “Galileo Ferraris” Torino, (Italy)

**UTC as currently defined was recognized as having long term deficiencies**
**Linked to UT1 to disseminate Earth rotation parameters **
**Deceleration of the Earth will increase number of leap seconds needed**
**Traditional timekeeping for astronomical and geodetic applications were:**
**Independent of internal system operation**
**Utilized independent techniques to observe celestial phenomena**
**Civil timekeeping is assumed to continue with low accuracy **
**approximately 0.1 s accuracy linked to the length of the Solar Day **
**Modern telecommunication systems require worldwide accurate standard frequency and timekeeping approaching nanosecond levels.  **
**Individual Systems are establishing worldwide independent systems to maintain continuous synchronization**
**New name for time scale suggested if modified**
**Introduction of a new timescale (or a major shift to an existing one) is perceived to be very disruptive and confusing in implementation**
**Members of Astronomical Community very concerned about current software for pointing telescopes and orbit determination program**

## Significant Issues from Colloquium

**Consequences of Decoupling of solar time and precise timekeeping **
**Change name to reduce confusion? (e.g. International Time vice Universal Time)**
**Studies from IAU and IERS indicate Rate of Rotation decreasing**
**Without Leap seconds UT1 and UTC will diverge (Magnitude and rate?) **
**Establish non-precise UT1 dissemination for Civil time uses**
**Computer operating systems and protocols establishing their own time due too  discontinuous reference services**
**Cost of reprogramming telescope pointing software unknown**
**Impact on orbit determination software not full examined**

## SUMMARY CONCLUSIONS 

**Creation of New Time Scale to be known as ‘International Time’ is not recommended**
**New name and scale would create more confusion and complications**
**Transition to TAI for existing systems could have major consequences (Note UTC and TAI coordinated scales post processed from local realizations UTC(XYZ) and TAI(XYZ)**
**Necessity of broadcasting DUT1 was largely unsupported **
**Most users apparently using UTC directly as an approximation to UT1 **
**Broadcast of DUT1 should be discontinued**
**IERS maintains responsibility for determining UT1 and its dissemination**
**Redefinition of a new “UTC” is not necessary**** **
**Adjust current definition to continue continuous broadcast time scale on transition date**
**This approach capitalizes on the current organizational, systems support structure, and timing centers coordinating their real-time realizations**** **
**A long-term continuous time scale would be achieved**
**Gradual divergence from solar time may be an issue in low precision “civil” timekeeping **
**The increasing error was estimated as a few seconds over three years**** growing to one hour in the year 2600 **

## U.S. PROPOSAL

**Modify UTC **
**Increase UT1 – UTC tolerance to Leap Hours**
**Effectively Freeze UTC in Parallel with TAI**
**Discontinue Leap Seconds for UTC**
**Impact on currently defined broadcast values of DUT1**
**Broadcast service limitation for growing offset**
**Discontinue DUT1 Broadcast**
**Availability of UT1 Information **
**Alternate Publication/Dissemination by IERS **
**Establish transition date/event/time **

## ITU-R Working Party 7A Special Rapporteur Working Group on UTCCurrent Membership

**Ronald Beard (USA), Chairman**
**William Klepczynski (USA - IAU)**
**Felicitas Arias (BIPM)**
**Yury Domnin (Russia)**
**Donald Hanson (USA)**
**Thomas Bartholomew (USA)**
**Francoise Baumont (France)**
_**Representatives from other Organizations**_** **
**Daniel Gambis (IERS)**
**S. Leschiutta (CCTF)**
**D. McCarthy (IAU)**
**D. Matsakas (URSI)**