Navigation Time Scales and the

Future of UTC

P. Kenneth Seidelmann University of Virginia

Time Scales

• Mean Solar Time • Universal Time (UT1) • Ephemeris Time (ET) • International Atomic Time • Coordinated Universal Time (UTC) • Dynamical Time Scales • Specialty Time Scales, e.g. internal GNSS Times

Navigation Time Scales-History • Mean Solar Time • GMT • UT = GMT • UT1 = GMT • UTC = GMT ? • UT1−UTC < 0.9s • UTC ≈ UT1 ≈ GMT for celestial navigation • GMT civil time in UK

• Greenwich time legal basis of other nations • GMT not for scientific uses

Coordinated Universal Time

• Broadcast time services • UT1 and TAI • Frequency offsets unacceptable • 1972 definition by CCIR • UTC based on TAI

• <0.9 s from UT1 • DUT1 • BIH • BIPM • IERS

• Guinot (2011)* • UT1 preserved as a representation of GMT w/ “a

departure which may reach one or two seconds…” • Imprecise solar-transits relative to stars: ~0.1s

• Algorithm changes to BIH “mean observatory”: 10’s of ms

• Non-uniformity from polar motion: ~30 ms at Greenwich • Plate tectonics: centimeters (μs) per annum. • Longitude origin perturbed w/ terrestrial system

changes (0.356s)

• Expected discrepancy: ~(TT-UT) / 365¼ • R.A. of Newcomb’s mean sun related to ET

*Guinot, B. (2011) “Solar time, legal time, time in use.” Metrologia, Vol. 48, pp. S181-S185.

How Well Has UT Kept Up w/ GMT

(TT – UT1) 365¼

Simulated Transits (Newcomb’s Mean Sun)

*Seago & Seidelmann (2013) “The Mean-Solar-Time Origin of Universal Time and UTC.” Paper AAS 13-486

“Modern” Mean Sun

*Seago & Seidelmann (2013) “The Mean-Solar-Time Origin of Universal Time and UTC.” Paper AAS 13-486

Redefinition of UTC 2000 - 2012

• ITU WP 7A Proposal • Surveys, Discussions, Studies

• Surveys favor the status quo • Lack of cost estimates • No consensus

• Opposition at all levels of ITU-R • Working Party 7A • Study Group 7

• Forwarded to 2012 Radiocommunication Assembly

Status of Redefinition of UTC

• 2012 Radiocommunication Assembly (RA12) • 190 national administrations

• some favored redefinition, some opposed, some unsure

• ISO determination concerning UTC name • Decision delayed until 2015 • RA12 recommended continued studies

• continuous reference time-scale • other technical options • broader implications

Study Group 7 Considerations

• Are time transmissions only for telecom? • Question not limited to technical and regulatory issues • Societal implications of modification of civil time scale • Organizational issues

• Use of other technical options • new time scale with fixed offset from TAI? • other ‘leap’ adjustment schedules?

• Timetable required for bringing new time scale into use

Some Options

• Retain status-quo UTC as is • Redefine UTC:

• Cease leap seconds (not adopted in 2012) • Cease leap seconds but also change name • Replace leap seconds with other mechanism

• Recognize alternative adjustment methods • Computer clock slewing around leap second

• Officially supplement UTC with another scale: • TAI? • GNSS time? • Other?

Impact of Change on Navigation

• UTC would differ from UT1 without limit • Correction needed to transmitted time scale

• In emergency, correction method must be known • Near real-time UT1 values/corrections needed • Almanac changes

• Name change to UTC? • Explanation and training needed • Fielded software must change accordingly

• UTC would have two different operational definitions (pre- and post-leap second)

Implementation –Almanac Time

• UTC as time scale in almanac, not UT1? • UT1-UTC assumption, one value for year,

multiple values per year, an algorithm? • Correction Estimate for year on cover? • Navigator needs true value of UT1-UTC and

difference from assumed value • Navigator must correct assumed value, if

necessary

Issues Concerning UTC Redefinition

• Significance w/ respect to Radiocommunication • Technical, legal, and public issues

• Narrow study focus under ITU-R • “Continuous” time scale

• Involvement of International Organizations • ITU-R non consensus • IAU, IERS, URSI, AAS, ISO

• Terminology • Change definition -> change name

• Alternative Time Scales • GPS time already in use • TAI or equivalent, as an supplementary time scale

• User Preferences • Surveys favor status quo • “Minor anomalies” with leap seconds reported • Limited documentation supporting redefinition

• Software and Hardware Modifications • GNSS • Consult computer scientists and S/W developers

Issues Concerning UTC Redefinition

• Distribution of UT1 • Availability and distribution of UT1 by computer

network and to navigators • Legal Considerations

• Status of UTC as official time in countries • Mean Solar Time as official time in countries

• Re-education • Revision of literature and textbooks

• Change of knowledge base affects large number of users otherwise unaffected

• Potentially confusing to non-experts

Issues Concerning UTC Redefinition

• Celestial Navigation and Almanacs • Changes in almanacs and explanations of use • Problem of navigation in an emergency

• Rate of Earth Rotation • Long and short term trends in Earth-rotation rate

• Long-Term Societal Effects • Long term adjustment procedure?

Issues Concerning UTC Redefinition

Personal Recommendation

• Officially acknowledge current practices • Retain UTC as is • Official endorsement of distinct uniform atomic

time scale for those needing such… • Real-time TAI(k) • GNSS-based time scale • Some other offset from TAI

• Consider other operational methods of introducing leap second in time scales

Personal Recommendation

• Options with no cost increases or organizational obstacles • CIPM oversight of time scales, TAI, UTC, GNSS • ITU distribution of UTC • GNSS distribution of GNSS time scales

• Basically recognizing current practices

What’s Next?

• Expanded studies per WRC resolution • Studies should consider

• user requirements • options

• pros and cons of each

• Broader implications considered • International Colloquium on Requirements for UTC

and Civil Timekeeping on Earth, May 29-31, 2013 • http://futureofutc.org

• Educate all nations concerning the issues • Decision in 2015?