short-term tidal variations in ut1: compliance between modeling … · 2010. 8. 5. · englich et...
TRANSCRIPT
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
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Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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
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Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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
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Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Major tidal terms w.r.t. IERS 2003 model 6
Model has still deficiencies in both tidal bands
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Major tidal terms w.r.t. IERS 2003 model 7
Model has still deficiencies in both tidal bands
Confirmed by GNSS observations
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Major tidal terms w.r.t. IERS 2003 model 8
Account for the effect of the lunisolar torque on the
triaxial Earth (Brzezinski and Capitaine, 2002)
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Quality/noise level check - zero terms 9
3-fold formal error seems reasonable
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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.
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Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
“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
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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)
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Residual amplitudes of all terms 14
IERS
COMBmajor terms
IERS
COMBside terms
Mqm Msqm Mtm Mstm Mf’Mf Msf Mm Msm
6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81
Tidal wave
Period(d)
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Residual amplitudes of all+zero terms 15
IERS
COMBmajor terms
IERS
COMBside terms
IERS
COMBzero term
3-fold
formal
error
Mqm Msqm Mtm Mstm Mf’Mf Msf Mm Msm
6.86 7.10 9.13 9.56 13.63 13.66 14.77 27.56 31.81
Tidal wave
Period(d)
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Terms which differ by 1 cycle in 18.6 y constrained 16
IERS
COMBmajor terms
IERS
COMBside terms
IERS
COMBzero term
Mqm Msqm Mtm Mstm Mf’Mf Msf Mm Msm
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
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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.
Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
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).
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Englich et al., IAU General Assembly 2009, Rio de Janeiro, 3-14 Aug. 2009
Thank you!