Geoscience Reference
In-Depth Information
3 Study of APL Effects on Space Geodetic Measurements
Studies of APL effects on space geodetic measurements include the detection of the
loading signal in the measurements (van Dam and Herring
1994
;vanDametal.
1994
;
Petrov and Boy
2004
), the application of APL corrections at the observation level
versus the post-processing level (Tregoning and van Dam
2005
; Böhm et al.
2007
;
Dach et al.
2011
), the impact of APL modeling on the precision of the measurements
and other parameters (van Dam and Herring
1994
;vanDametal.
1994
; Petrov and
Boy
2004
; Dach et al.
2011
). A recent study was carried out by van Dam et al.
(
2010
) who investigated the effects of unmodeled topographic variability on surface
pressure estimates and subsequent estimates of vertical surface displacements.
van Dam and Herring (
1994
) used 1085 VLBI baseline length measurements
(1984-1992) from 74 stations to detect the presence of APL signals in the measure-
ments and to investigate the impact of applying APL corrections on the measurement
precision. Their analysis indicates that 62%of APL signal is found in the VLBI base-
line residuals. For very accurate measurements, this signal has to be removed in order
to avoid misinterpretation of the results. Applying APL effects on the observation
level significantly reduces the weighted root-mean-square (WRMS) scatter of the
baseline length residuals on 11 of the 22 baselines investigated.
Petrov and Boy (
2004
) carried out further studies on the presence of APL signals
in the VLBI baseline measurements and coordinates. They stated that their approach
can estimate the APL displacements with errors less than 15% of the effect itself.
Their analysis of VLBI measurements of 40 stations for the time period from 1980 to
2002 demonstrates that approximately 95%of the power of modeled vertical pressure
loading signal and 97% of the signal in the baseline lengths is found in VLBI data.
They found also that approximately 84% of the horizontal signal is contained in
the VLBI measurements. Neglecting this signal adds noise to the horizontal position
with an RMS of 0.6mm and to the estimates of the EOP with an RMS of 20
as.
van Dam et al. (
1994
) assessed the influence of APL effects on GNSS station
heights by analyzing daily positions of 20-40 GNSS sites for the time period of
approximately 300 days. The application of APL corrections reduces the variance
of the station heights by up to 24% and the WRMS scatter of the baseline length
residuals. Approximately 62% of the investigated GNSS baselines show a reduction
in their WRMS scatter. Fifty seven percent of APL signal is evident in the GNSS
baseline length measurements. Furthermore, the use of regression coefficients of
local pressure measurements appears to be valid at many GNSS sites. However,
there are sites where the coefficients are unreliable.
Similar studies were done by Dach et al. (
2011
) who evaluated the impact of
different methods of APL corrections in GNSS data analysis. They applied the cor-
rections from a geophysical model at observation level, on weekly mean estimates
of station coordinates at the post processing level, and they also solved for regression
coefficients between the station displacements and the local pressure. Analysis of
GNSS measurements from IGS stations in the period from 1994 to 2008 showed that
the repeatability of station coordinates improves by 20% when applying the cor-
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