Comparison of IGS station position time series with loading model: what can we learn?
T. van Dam1 , X. Collilieux2 , P. Rebischung2 , J. Ray3 , Z. Altamimi2
1 University
of Luxembourg, Luxembourg, Luxembourg.
2 IGN, LAREG, 6 avenue Blaise Pascal 77420 Champs-sur-Marne, France ; GRGS.
3 NOAA National Geodetic Survey, Silver Spring, MD, United States.
Abstract
A
We derived a loading model using a combination of NCEP atmosphere,
ECCO non-tidal ocean, and (G)LDAS surface water load models. Before
comparing such station displacement model to GPS results, we removed
apparent geocenter motion from GPS position time series by taking care
to minimize aliasing of local load signals into the frame parameters. Although the loading correction significantly decreases the series WRMS,
we show that significant residual signals still exist. Some authors have
suggested that near-annual signal at the draconitic period ( 350 days) may
affect the GPS position time series. We explore the relevance of such hypothesis here.
Simultaneous estimation of draconitic and annual signal
AOGS 2012
C
Annual and draconitic harmonic signals can be estimated simultaneously
[4, 2]. We estimated three different models in the GPS z-translation time
series of mit and gfz AC solutions.
B
Fig.2 Raw GPS geocenter motion time series along Z (translation) from gfz and mit
time series and fitted models. A butterworth low pass filter has been applied for
clarity.
Non-tidal loading model. Three loading models have been used
to derive modeled position time series. Summary of the model:
These figures indicate that it is not straight forward to discriminate between a time varying annual signal and an annual + draconitic signal.
Data
• Green’s function approach. Earth model : Gutenberg-Bullen
• Reference Frame: Center of Figure (CF) of the Earth [1]
Annual & draconitic signal in GPS station position time series
E
Loading models may show time-variable annual signal. Estimated draconitic signals are not affected by the loading correction.
Fig.5 Amplitude of the draconitic signal in load-corrected GPS series w.r.t. raw
GPS series.
Is the residual signal annual or draconitic?
F
D
The annual signal (365,25 d) in GPS height time series has been fitted
alone or simultaneously with the draconitic signal (351,5 d) .
Fig.1 Description of the loading model (1998.0-2010.0).
Fig. 6 Histogrammes of the estimated period signals in GPS position series.
GPS position time series. GPS Analysis Center solutions submitted for repro1 (igs05 framework) have been recombined homogeneously with CATREF software (solutions called "igb"). They have
been completed with operational IGS combined weekly solutions.
The following process has been used to derive residual position
time series from these solutions:
1. Segmentation of position time series and empirical correction of the discontinuities.
Is the draconitic estimated reliably?
Conclusions
Fig.3 Amplitude (size of the circles) and phase (color) of the annual signal fitted in
raw GPS and load-corrected GPS position time series (Height).
2. Computation of the GPS apparent geocenter motion information using a well distributed network of stations.
Estimating annual and draconitic signals simultaneously in GPS
positions time series does not improve significantly the agreement
between GPS and loading models at the annual frequency.
We were not able to show that the signal we estimated at the period 351.5 days is artificial and not related to time-variable annual
signals. Further analyzes are necessary.
References
3. Correction of positions for average position, offsets, velocities and geocenter motion.
G
H
[1] Blewitt G. (2003) Self-consistency in reference frames, geocenter definition,
and surface loading of the solid Earth J. Geophys. Res..
[2] Haines B. et al. (2011) A GPS-Based Terrestrial Reference Frame from a Combination of Terrestrial and Orbiter Data, AGU abstract..
[3] Schmid R. et al. (2007) Generation of a consistent absolute phase-center correction model for GPS receiver and satellite antennas J. Geod..
[4] Watson C. et al. (2011) Draconitic Biases in GPS Versus GRACE Estimates of
Hydrology IUGG presentation.
Background: draconitic period. According to [3], the GPS draconitic period can be compute as follows:
2π
2π
TR =
·365.25 =
·365.25
≈
351.5
days
2
J 2 Re
2π − Ω̇GP S · 1
2π + 3π T a2 cos i · 1
Fig.4 Amplitude of the draconitic signal (351,5 days) fitted in GPS load-corrected
position time series (Height). Draconitic estimated simultaneously with annual.
Acknowledgment
Thanks to Daphné Lercier who supplied the LATEX template for this poster.
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