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rections at the observation level and by 10% when applying them as weekly mean
values at the post processing level to the resulting weekly coordinates, both compared
with a solution without applying APL corrections. Furthermore, Dach et al. ( 2011 )
stated that APL corrections via regression coefficients are less beneficial than APL
corrections at the observation level. This is due to the fact that the distribution of the
pressure in the vicinity of the station has a significant impact on APL displacements
and the effects in the horizontal components are completely ignored.
Bock et al. ( 2005 ) showed that an improvement of SLR measurements has been
obtained when APL effects are modeled, but the magnitude of the improvement is
rather small. Furthermore, there appears to be no noticeable effect in the SLR station
time bias after accounting for APL effects.
We have studied the performance of the Vienna APL corrections and the Petrov
and Boy ( 2004 ) models by analyzing 3183 24h VLBI sessions from January 1990
to December 2009. The number of participating stations in each individual ses-
sion was varying from 3 to 8. Figure 9 shows the difference between the variance
of time series of site vertical components with and without applying APL correc-
tions. In general, the application of APL models obviously improves the accuracy
of the estimated coordinates, especially for the vertical components and to a lesser
degree for the horizontal components (not shown here). We found that the variance
of vertical components is deteriorated for only four sites, all of them are located
near the coast, after applying either the Vienna-APL model or the Petrov and Boy
( 2004 ) model. SC-VLBA andMK-VLBA are placed on Bahamas andHawaii islands,
respectively, while SESHAN25 is near the coast. NL-VLBA is an inland site but it is
close to the Great Lakes. Those sites are probably affected by the oceanic response
2.5
2
1.5
1
0.5
0
−0.5
−1
Vienna−APL
Petrov & Boy
−1.5
−2
−2.5
Stations involved in more than 100 sessions
Fig. 9 Variance difference of time series of vertical component. Station name and latitude are
plotted above the bar
 
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