NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

92 Commission 3 – Earth Rotation and Geodynamics
3. Combination of different observation techniques
Earth orientation parameters (EOP) based on homogeneous
and continuous VLBI and GPS data were consistently
combined by THALLER et al. (2006b) using techniquespecific
datum-free normal equation systems. Especially
the rigorous combination of UT1-UTC and LOD delivered
by VLBI and GPS revealed that both techniques perfectly
complement each other and the applied combination did
not suffer from systematic effects present in the GPSderived
LOD values. The local geodetic ties between GPS
and VLBI antennas generally play an essential role within
the inter-technique combination. Several studies already
disclosed non-negligible discrepancies between terrestrial
measurements and space-geodetic solutions. THALLER et
al. (2006b) demonstrated to what extent these discrepancies
propagate into the combined EOP solution.
An overview of the combination studies performed by the
Forschungseinrichtung Satellitengeodäsie der TU München
(FESG) and the Deutsches Geodätisches Forschungsinstitut
(DGFI) based on the data of the continuous IVS campaign
CONT02 was given by THALLER et al. (2006a). The cooperation
of the two institutions established the basis for
a detailed adaption of GPS and VLBI software concerning
models and parameterization to avoid systematic differences
between the technique contributions. Regarding sub-daily
earth rotation parameters the study emphasizes that a
combination of the space techniques improves the results
compared to single-technique solutions significantly.
Furthermore, THALLER et al. (2006a) described a combination
scheme for long sub-daily EOP time series from VLBI
and GPS removing the weakness of UT1 estimations of
satellite techniques and, consequently, offering the opportunity
to study sub-daily tidal excitations and the influence
of high-frequency or episodic geophysical effects on earth
rotation.
In a joint effort the Technical Universities of Munich and
Dresden performed a reprocessing of a global GPS network
over the last decade in order to dispose existing inhomogeneities
and inconsistencies of GPS time series of global
geodetic parameters due to changes at the individual
International GNSS Service Analysis Centers hampering
geophysical interpretations of these long time series.
According to STEIGENBERGER et al. (2006), first results of
the reprocessing of 11 years of data showed significant
improvements in the quality and homogeneity of estimated
parameters, and formal errors of sub-daily earth rotation
parameters could be reduced by 30%. In addition, advanced
modelling approaches of second- and third-order ionospheric
corrections and absolute antenna phase center
corrections for receivers and satellites were tested to
achieve further improvements.
4. Analysis and prediction
Results of the analysis of earth rotation data derived from
the continuous VLBI campaign CONT02 were presented
by HAAS and WÜNSCH (2006). Regarding high-frequency
variations, 40-60% of polar motion and about 80% of UT1
could be explained by the ocean tide model of Ray. The
remaining residuals were found to be on the level of several
tens of micro-arcseconds. So far, they cannot be reproduced
completely by models based on non-tidal angular momentum,
atmospheric tides and luni-solar torques acting on the
tri-axial earth. However, the diurnal signal detected in polar
motion residuals could partly be explained by models due
to non-tidal angular momentum and atmospheric tides. In
the residuals of polar motion from CONT02 the authors
identified third-diurnal variations close to the S3 tide constituent
with retro- and pro-grade amplitudes on the order
of 40 mas, what is much larger than predictions based on
atmospheric effects. With respect to the diurnal frequency
band in UT1 the agreement between theoretical models and
observations was also poor and the empirical values were
generally larger than the modeled ones.
GUO et al. (2005b) investigated the double peak of the
Chandler wobble (CW) in the spectrum of polar motion by
comparing the polar motion data series (annual wobble
removed) with a synthetic double-frequency CW time
series. They observed a reasonable agreement between their
peak times, which is an argument for the hypothesis of a
double frequency CW, but is inconclusive with regard to
whether the CW really has two frequencies. For the determination
of the spectral properties of polar motion data a
folding averaging algorithm (FAA) presented by GUO et
al. (2005c) was used.
By means of an adaptive network based fuzzy inference
system (ANFIS), AKYILMAZ and KUTTERER (2003, 2004)
studied the short-term prediction of earth rotation parameters
up to 40 days into the future; applying a similar
approach, AKYILMAZ and KUTTERER (2005) extended the
prediction period to one year. After removing well-known
influences such as solid earth and ocean tide effects as well
as seasonal atmospheric variations from the daily time
series ERP C04 provided by IERS, the residual values were
used for both training, i.e., optimization of parameters, and
validation, i.e., comparison of predicted data with independent
observed data, of the network. A comparison of
predicted LOD and polar motion values with corresponding
results from other methods, e.g., artificial neural networks
(ANN), revealed root-mean-square errors which were equal
or even lower than those from the other considered
methods. The authors emphasized that the advantage of the
applied prediction method lies not only in the high precision,
but also in a comparatively easy handling. However,
despite its significantly reduced complexity, ANFIS
modelling is still more complicated than several other
methods, such as, for example, the one used in the IERS
EOP service.
References
AKYILMAZ O., KUTTERER H. (2003): Prediction of earth orientation
parameters by fuzzy inference systems. DGFI Report,
No. 75, München.
AKYILMAZ O., KUTTERER H. (2004): Prediction of earth rotation
parameters by fuzzy inference systems. Journal of Geodesy,
Vol. 78/1-2, S. 82-93.
AKYILMAZ O., KUTTERER H. (2005): Fuzzy inference systems for
the prediction of earth rotation parameters. In: Sanso, F.