Short-term tidal variations in UT1: compliance between modeling … · 2010. 8. 5. · Englich et...

Short-term tidal variations in UT1:

compliance between modeling

and observation

S. Englich, H. Schuh, R. Weber

IAU 2009

JD 6 – Time & Astronomy

Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009

Short-term (tidal) variations in UT1

Diurnal and sub-diurnal variations

Ocean tides – effect up to 80 μs

Atmospheric tides – effect ca. 1 μs

Lunisolar torque on triaxial Earth – effect ca. 4 μs

Variations up to 35 days

Solid Earth tides – effect up to 2 ms

Ocean tides – effect ca. 0.2 ms

AAM, OAM

2


Outline

VLBI/GNSS solutions for UT1/LOD determination

Diurnal and sub-diurnal variations

Estimation of ocean tidal terms

Comparison to the IERS 2003 model + model for semi-diurnal

spin libration

Variations up to 35 days

Estimation of “zonal” tidal terms

Comparison to the IERS 2003 model and to a combined model

suggested by R. Gross

3


VLBI/GNSS solutions for UT1/LOD determination

VLBI GNSS

Observation

data

all sessions suitable for EOP

determination

GPS and GLONASS of 120 IGS

network sites

Time span 1984-2008 (25 years) 8 months of 2008

Reference

framesITRF2005, ICRF Ext. 2 IGS05, CODE orbits

EOP

nutation fixed to IAU2000A +

dX, dY from IERS C04

1h and 5h UT1

nutation fixed to IAU2000A

1h LOD

Software OCCAM Gauss-Markov model Bernese GPS Software

4


Ocean tidal terms

Amplitude estimation: least squares adjustment

Removal of low-frequency signal with IERS 05 C04 series

UT1/LOD variations as pseudo-observations

• 70 terms (+ 6 zero terms with no signal expected)

• ratio of inseparable terms constrained with ratio of tidal potential

VLBI δUT1:

GPS δLOD: → δUT1

5

)cos()sin(11

i

n

iiii ξBξAUT ∑

=

+=δ

)(sin')cos('1

i

n

iiii BALOD ξξδ ∑

=

+=

[ ] °= 90 & 2

)()1( se by retard pha

dperiodLODAmpUTAmp

π


Major tidal terms w.r.t. IERS 2003 model 6

Model has still deficiencies in both tidal bands



Model has still deficiencies in both tidal bands

Confirmed by GNSS observations



Account for the effect of the lunisolar torque on the

triaxial Earth (Brzezinski and Capitaine, 2002)


Quality/noise level check - zero terms 9

3-fold formal error seems reasonable


Conclusions part I

The IERS 2003 model for the diurnal and sub-diurnal

UT1 variations does not completely remove all signal

in these bands.

Significant residual amplitudes remain, which should

be considered when studying variations with longer

periods in order to avoid aliasing effects (especially if

the sampling interval exceeds 12h).

It would be beneficial to include a model for the

effect of the triaxiality of the Earth on UT1 in the

next IERS Conventions.

10


“Zonal” tidal terms (< 35 d)

Amplitude estimation: least squares adjustment

VLBI δUT1:

• 41 terms (+ 1 zero term)

Removal of largest known effects / low frequencies:

Atmospheric influence:

• from Atmospheric Angular Momentum Data (NCEP, 6 h)

Diurnal and sub-diurnal ocean tides:

• reduced with the before estimated VLBI-model

Filtering of signal >35 d

11

)cos()sin(11

i

n

iiii ξDξCUT ∑

=

+=δ


Terms are calculated with respect to ...

Model of IERS Conventions 2003

not quite clear how it is composed

Combination of models suggested by R. Gross

Yoder et al. (1981) elastic body tide model

Wahr and Bergen (1986) inelastic body tide model

Kantha et al. (1998) ocean tide model

12

IERS

COMB


Residual amplitudes of major terms 13

Mqm Msqm Mtm Mstm Mf’Mf Msf Mm Msm

IERS

COMBmajor terms

6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81

Tidal wave

Period(d)


Residual amplitudes of all terms 14

IERS

COMBmajor terms

IERS

COMBside terms


6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81

Tidal wave

Period(d)


Residual amplitudes of all+zero terms 15

IERS

COMBmajor terms

IERS

COMBside terms

IERS

COMBzero term

3-fold

formal

error


6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81

Tidal wave

Period(d)


Terms which differ by 1 cycle in 18.6 y constrained 16

IERS

COMBmajor terms

IERS

COMBside terms

IERS

COMBzero term


6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81

Tidal wave

Period(d)

3-fold

formal

error


Phasor plot 17

The main part of

the residual signal

in the fortnightly

tidal band is due

to the cosine or

the out-of-phase

part respectively.


Conclusions part II

The comparison of observed UT1 variations <35 d with the

corresponding terms of the IERS 2003 model for the zonal

tides revealed rather large discrepancies (>40 μs) in the

fortnightly tidal band, in particular in the out-of-phase

parts.

The combination of models proposed by R. Gross accounts

for most of the fortnightly tidal signal and reduces the

residual amplitudes almost below the 3-sigma level.

The IERS model could be updated, especially concerning

the ocean tidal contribution and represented more clearly,

preferably in two parts (solid Earth tides, ocean tides).

18


Thank you!

[email protected]

Short-term tidal variations in UT1: compliance between modeling … · 2010. 8. 5. · Englich et...

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