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Monitoring UT1 from astro-geodetic techniques. State of the art and prospective
D. Gambis, C. Bizouard Earth Orientation Center of IERS Paris Observatory

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Overview
Why monitoring UT1? ╖ Reference frames metrology
н Satellite precise orbit determination н Gravity fields determination н Space navigation

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Geophysics: Connection of EOP with fluid layers (LOD), constraints for geophysical models

I - UT1 determination, monitoring, state of the art, realization and accuracy II н UT1 Predictions ╖ Short-term real time applications ╖ Long-term: Leap seconds announcements

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Combinations

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Various techniques (VLBI, GPS, SLR, DORIS, LLR) allow the determination of a part or all EOP Techniques have their specific strengths and weaknesses. They are complementarity and redundant, this justifies the process of their combination

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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Contribution of the various techniques to IERS
The number of stars matches the relative contribution of techniques

PRODUCTS Extragalactic ref. Frame Tie to solar system Tie to Earth Precession-nutation Universal Time Earth Rotation High-frequency UT Polar Motion

LLR

VLBI

SLR

GPS

DORIS

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Terrestrial Reference Frame Network coverage Long-term geocenter * Tectonic plate motion Densification
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Techniques contributing to UT

Technique
ASTROMETRY LLR SLR VLBI GPS

since
1899 1969 1976 1981 1993

EOP
UT1 UT0 LOD UT1 24h UT1 Intensive LOD

Time Res.
5 days 1 day 3 days 3-4 days 1 day 1 day

Accuracy
UT1: UT0: LOD: UT1: UT1: LOD: 1 ms 100 s 60 s 5 s 20 s 7 s

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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accuracy statistics: VLBI standard, UT1

Joint Discussion JD6, Rio, 6 н 7 August 2009

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VLBI intensive, UT1

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Comparisons INT1, INT2 , R1 and R4 to C04

INT1

INT2

Intensive

Standard

Joint Discussion JD6, Rio, 6 н 7 August 2009

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USE of LOD(GPS) in UT1 combination
Data ╖ UT1 standard VLBI sessions н Series from IVS ╖ UT1 intensive sessions ╖ Daily LOD from IGS (igs00p03), 12h epochs, Problem of systematic biaises due to GPS orbit mis-modeling ╖ Integration of LOD can be used UT1 Densification Correct when VLBI intensive are erroneous (50-100 s possible) Fill gaps when UT1 intensive are missing (sometimes 4-5 days) Quasi-real time estimates (last VLBI intensive epoch to now)
Joint Discussion JD6, Rio, 6 н 7 August 2009 10

Method of Combined Smoothing (Vondrak approach)
╖ ╖ ╖ UT1 is observed by VLBI with a time resolution of 3-4 days, 5-8 s UT1 intensive 15-20 s LOD is observed by satellite methods with a short-term accuracy (7 s) and high resolution (1 day) LOD first derivative of UT1 LOD=-d (UT1-TAI)/dt

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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Method of Combined smoothing

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Two relatively smooth curves н a) One fitting well to VLBI UT1 estimates н b) Second one fitting well to GPS LOD estimates н c) Both curves are tied by constraints: the latter is the first derivative of the former

╖ Combined smoothing is a generalization of Vondrak smoothing (Vondrak and Gambis, 1999; Vondrak and Cepek, 2000) ╖ Compromise between 3 conditions:

Joint Discussion JD6, Rio, 6 н 7 August 2009

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GAPS

5 days missing

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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Comparison of various UT1 series to BULLETIN A

Std + Intensive Only

UT(GPS)

NO intensive

Joint Discussion JD6, Rio, 6 н 7 August 2009

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GPS contribution to UT1: Conclusions

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UT1: Current accuracy in the range of 8 s for Standard, 15-20 s for Intensive However, in case of erroneous data or gaps, UT(GPS) can be valuable to densify and homogenize UT1 estimates Gain of only 3 s (45 as) when using intensive sessions!!
н Intensive sessions results need to be improved

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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UT1: Predictions

Short-term for real time applications Use of Atmospheric Angular Momentum forecasts Leap second announcement

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Use of Atmospheric Angular momentum forecasts for real-time UT1 predictions
Atmospheric Angular Momentum (AAM) fluctuations are generated by dynamical interactions between the atmosphere and the underlying planet. These fluctuations are compensated by opposite fluctuations in the earth rotation when assuming the conservation of the angular momentum of the whole system atmosph╒re + solid earth 3 the axial Atmospheric Angular Momentum (AAM) functions can be expressed as the sum of two terms: - A pressure-term related to the redistribution of the air masses. - A wind-term related to the relative angular momentum of the atmosphere
r2 r 3 = -0.70 PS cos 2 dS - 1.00 Cg Cg

u cos dPdS

The Length-of-day (LOD) variation can be directly expressed as:
LOD = LOD
3

UT1 is obtained by the integration of LOD
Joint Discussion JD6, Rio, 6 н 7 August 2009 18

Procedure

1 - AAM forecasts are transformed into a LODR series 2 нLODR is then integrated into a 7-day UT1R prediction 3 - The method is adaptative, i.e. the bias error on LODR (linear drift on UT1) computed on the previous 7-day interval is used for the real time forecast 4 н Absolute mean errors of differences are given from 1 to 7 days. This mean error give the quality of the forecasts performances.

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Prediction performance over one week

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Short-term UT1 predictions: Conclusions
╖ The procedure developped for IERS Prediction Comparison Campaign is now adapted in a daily routine, However it has to be "tuned" to the AAM forecasts reporting time which seem now to be stabilized
╗ NCEP every day at 8h22 and 14h03 UTC, JMA at 17h35

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The use of AAM leads to an improvement of about 40% compared to usual statistical predictions (i.e. of 450 s for a 7-day prediction). Historically, the atmospheric forecasts have been improving over many years due to improvements in the major weather centers. We can expect better forecasts in the future.

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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Long-term UT1 prediction for Leap second announcement

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Coordinated Universal Time UTC
╖Atomic Time since 1955 ╖Continuous atomic time scale of BIH, called TAI in 1971 ╖UTC was introduced on 1972 January 1 (CCIR 1970) ╖UTC is maintained close to UT1 |UTC-UT1| < 0.9 s

╖This requires to monitor UT1-UTC, and to predict it for leap second announcements ╖The UTC system is a good compromise to keep UTC close to UT1
Joint Discussion JD6, Rio, 6 н 7 August 2009 23

Monitoring UT1 - UTC

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The Earth Orientation Center of the IERS at Paris Observatory is in charge of the leap second announcement The relevant bulletins are: н Bulletin C: Announcement of the leap seconds in UTC н Bulletin D: Announcement of the value of DUT1 truncated at 0.1s for transmission with time signals.

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Joint Discussion JD6, Rio, 6 н 7 August 2009

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Future of the UTC timescale Question ITU-R 236/7 (2001)
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?

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Communities of users of Bulletins C and D
н н н н н н н н н н н н н Astronomy, astrophysics Time Service laboratories Computer centers Radio signal laboratories Radio-astronomy activities Geodesy Navigation, civil and military Geophysics Radio stations Post and telecommunication Hydrographic and oceanographic labs Surveying and mapping institutes Civil engineering Space research Etc ......
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Rotation of the earth and leap second occurence

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General arguments against the present situation with the leap second insertion
╖ A discontinuous time scale is not convenient for various applications
н Navigation н Synchronisation of computers

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Leap seconds are cumbersome, manual action needed to enter them Ignoring leap seconds will not be a significant problem for civil purposes

Joint Discussion JD6, Rio, 6 н 7 August 2009

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General arguments for statu-quo excerpts from different surveys and polls
╖ ╖ ╖ ╖ ╖ ╖ ╖ ╖ ╖ ╖ ╖ Good compromise between accurate time scale and solar time : practical identification of UT1 with UTC In many country legal time is based on solar time No strong argument to change. The current system works " If it ain't bust, don't fix it ! " After the implementation of the leap second on 31 December 2005, problems reported were minor Alternative time scales exist for scientific applications (TAI, UTC(GPS). General public are not aware of astronomy and identify UT1 and UTC Any changes in these areas will likely cause substantial confusion and disasters (principle of security) In particular, risk of confusion and problems in the case of the increase of the tolerance UT1-UTC (One hour?) In a few decades who will remember the origin of the procedure? Important costs for software modifications ....
Joint Discussion JD6, Rio, 6 н 7 August 2009 29

Discussions progress
╖ ╖ To date no final recommendation of the SRG to ITU-R November 2005 at the ITU-R WP-7A meeting, the US representatives submitted a proposal :

1 - Maintenance of UTC time scale 2 - Suppression of leap seconds adjustements allowing UTC to be close to UT1, time scale based on the earth rotation (UT1-UTC < .9 s) 3 - Replace leap seconds by leap hours, the first one not before the year 3000 4 - Change to be done starting on 21 December 2007.00 UTC 5 н There were no consensus at ITU. The proposal will be submitted
Joint Discussion JD6, Rio, 6 н 7 August 2009 30

Our Conclusions
╖ ╖ ╖ ╖ No real justification for the proposed change Requirement to have a civil time close to the mean solar time No real benefit to change More time is needed to investigate the consequences of a change, difficult to go back if problems arise.

Joint Discussion JD6, Rio, 6 н 7 August 2009

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Do we really want to loose synchronisation between Earth Rotation and UTC ?
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