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These studies have shown (e.g., Petrachenko et al. 2009, Pany et al. 2011; various IVS Memoranda) that deficiences in tropospheric delay modeling are the major error source for VLBI2010 among the three sources mentioned above. [It is important to note here, that we did not consider source structure effects for the simulations, nor did we simulate systematic effects.]
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Figure 1 (from Pany et al. 2011): a) median (over all stations) 3D position rms in mm versus structure constant Cn and b) versus effective height H. These two parameters, Cn and H, are the key quantities for the simulation of tropospheric delays according to Nilsson et al. (2007). c) median 3D position rms in mm versus Allan Standard Deviation (ASD) of the clocks and d) versus white noise added per observation. The simulations have been carried out for a 16-station network with the following default values: Cn = 1.0 x 10-7 m-1/3 , H = 2 km, wind 10 m/s towards East, ASD = 1 x 10-14 at 50 minutes, white noise = 4/sqrt(2) ps per baseline observation.
The simulation of tropospheric delays is based on the turbulence model and the strategy described by Nilsson et al. (2007). In Vienna, the source code by Böhm et al. (2007) is applied. The key parameters for this model are the Cn parameter (structure constant), the effective scale height H, and - to a lesser extent - the wind velocity. A recent list of station-wise parameters as determined by Tobias Nilsson from GPS measurements is provided below. In this table, the scale height H is 2000 meters for all sites.
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