Closure quantities measured by very long baseline interferometry (VLBI)
observations are independent of instrumental and propagation instabilities and
antenna gain factors, but are sensitive to source structure. A new method is
proposed to calculate a structure index based on the median values of closure
quantities rather than the brightness distribution of a source. The results are
comparable to structure indices based on imaging observations at other epochs
and demonstrate the flexibility of deriving structure indices from exactly the
same observations as used for geodetic analysis and without imaging analysis. A
three-component model for the structure of source 3C371 is developed by
model-fitting closure phases. It provides a real case of tracing how the
structure effect identified by closure phases in the same observations as the
delay observables affects the geodetic analysis, and investigating which
geodetic parameters are corrupted to what extent by the structure effect. Using
the resulting structure correction based on the three-component model of source
3C371, two solutions, with and without correcting the structure effect, are
made. With corrections, the overall rms of this source is reduced by 1 ps, and
the impacts of the structure effect introduced by this single source are up to
1.4 mm on station positions and up to 4.4 microarcseconds on Earth orientation
parameters. This study is considered as a starting point for handling the
source structure effect on geodetic VLBI from geodetic sessions themselves.Comment: 5 figures, 15 pages, accepted by Journal of Geodesy at 19 Dec., 201
