# Presentation: 154711

## ITU-R WORKING PARTY 7A
SPECIAL RAPPORTEUR GROUP 
ON UTC

The Future of the UTC Timescale

Civil GPS Service Interface Committee
Timing Subcommittee
11 September 2001

**SPECIAL RAPPORTEUR GROUP **
**ON UTC**
**The Future of the UTC Timescale**
**Civil GPS Service Interface Committee**
**Timing Subcommittee**
**11 September 2001**

## UTC ISSUES RAISED

**Problems raised on discontinuous nature of UTC, i.e. leap seconds **
**Awareness of upcoming leap second (Availability of Notice)**
**User System design requires manual or overly complex inclusion**
**Discontinuous operation simply not possible**
**Leap second basis re-examined by IAU and URSI**
**Transition to UTC integrated Atomic Time with Solar Time **
**Tolerance of |UT1 - UTC| < 0.9 s considered essential for Celestial Navigation Users **

## SRG PURPOSE

**Use of UTC and leap second procedure is established by ITU-R Recommendation TF.460-5 **
**UTC concerns raised to the ITU-R and undertaken by adopting**
**Question ITU-R 236/7 (2001), T****he Future of The UTC Time Scale**
**SRG formed to:**
**Focus studies into this highly significant question and leap second issues**
**Address additional concerns raised concerning Satellite Systems**
**Independent System Time (e.g. GPS Time)**
**Use of TAI **
**Investigate Proposed Changes to UTC in coordination with ITU-R Sector Members and CCTF**
**Report Results and Suggested Recommendations to ITU-R**

## Question ITU-R 236/7 (2001)FUTURE OF THE UTC TIMESCALE

**The following questions are to be studied**:
**1. What are the requirements for globally-accepted time scales for use both in navigation and telecommunications systems, and for civil time-keeping?**
**2. What are the present and future requirements for the tolerance limit between UTC and UT1?**
**3. Does the current leap second procedure satisfy user needs, or should an alternative procedure be developed?**

## ITU-R RECOMMENDATIONSon UTC

**Recommendation ITU-R TF.460-5  (1970-1974-1982-1986-1997)**
**All standard-frequency and time signal emissions should conform as closely as possible to UTC.  **
**The procedure for the insertion of leap seconds into UTC is described**
**Ensure UTC does not differ by more than 0.9 seconds from UT1**
**Further application of UTC :**
**Recommendation ITU-R TF.486-2, "Use of UTC Frequency as Reference in Standard Frequency and Time Signals Emissions", (1974-1978-1998); **
**Recommendation ITU-R TF.535-2, "Use of the Term UTC", (1978-1982-1998); **
**Recommendation ITU-R TF.458-3 "International Comparisons of Atomic time Scales" (1970-1978-1990-1998); **
**Recommendation ITU-R TF.536, "Time-Scale Notations", (1978).**

## POTENTIAL SYSTEM IMPACT

**Satellite Navigation Systems (U.S.)**
**GPS - UTC(USNO) **
**WAAS Synchronized to GPS and UTC(USNO)**
**LORAN-C **
**Telecommunications Systems **
**Increasing Use of GPS to Obtain Precise Frequency**
**Computer Networking (Internet)**
**NTP Servers disseminating UTC**
**Broadcast Services**
**WWV, WWVB, WWVH**
**Scientific Uses**
**Geophysical Systems Using GPS Extensively **

## UT1 AND EARTH ROTATION*

**Earth deceleration**
**Tidal**
**Change in figure**

******* - Chadsey and McCarthy, Proc PTTI, 2000**

## CHANGE IN LENGTH OF DAY*

**CHANGE IN LENGTH OF DAY*******

******* - Chadsey and McCarthy, Proc PTTI, 2000**

## ELEMENTS OF THE QUESTION

**1.  Requirements of Time & Frequency Reference:**
**Accuracy, Stability, Based on the SI Second**
**Uniformity and Accessibility**
**Reliability**
**Relation to Legal Time**
**Coverage Needed - Global?**
**Relation to Civil / National  Timekeeping ?**
**2.  Tolerance **
**Could a Greater Tolerance be Accommodated?**
**3.  User Needs and Alternatives**
**Availability of Leap Second Information for Users**
**Amplify Possible Alternatives**
**Relationship of System Internal Time to Time Scales**

## RANGE OF OPTIONS

**1.Maintain the Status Quo - ****Use UTC as currently defined**
**Additional Actions:**
**Clarify Time Scales Available and Considerations for Use **
**Provide more advanced Notice and Availability**
**Examine utility of a Navigation Time Scale (Celestial Users)**
**2. Modify Leap Second Tolerance and Occurrence**
**Increase Tolerance of |UTC – UT1| **
**Establish Longer Prediction Interval and Frequency of Occurrence**
**Establish Fixed Interval Adjustment with Multiple Leap Seconds**
**Correction at Predicted Intervals Based on Deceleration Model, Re-evaluated at Fixed Intervals**
**3. Transition to Another Timescale**
**International Atomic Time (TAI) - (Navigation Time Scale for Celestial Users may be needed)**
**New Time Scale Based on Re-Definition of SI Second**

## STATUS QUO*

******* - Chadsey and McCarthy, Proc PTTI, 2000**

## DISCONTINUE LEAP SECONDS*

******* - Chadsey and McCarthy, Proc PTTI, 2000**

## REDEFINE THE SECOND*

******* - Chadsey and McCarthy, Proc PTTI, 2000**

## STUDY PLAN ACTIVITIES

**Interaction with Technical Societies and Agencies Needed**
**Notices in Journals and Agencies Newsletters**
**Coordination With ITU-T**
**Telecommunications and Navigation Users **
**SRG Meetings Scheduled Concurrent with PTTI 2001 and**
**EFTF 2002 **
**Anticipate Final Report for ITU-R WP7A Meeting October 2002**

## SRG ACTIVITIES 

**The SRG working by e-mail coordinated by the Chairman**
**ronald.beard@ties.itu.int**
**Contributions invited for submission to the SRG**
**To the Chairman by electronic mail**
**Attendance at SRG Meetings**
**Participation invited**
**To gain the widest consensus possible, it is requested that participants represent a technical area agency or society participation**