Table 4: VLBA rms astrometric errors (in $\mu $as) for a relative source separation $\delta -\delta _0= 1\hbox {$^\circ $ }$. Individual astrometric error contributions from calibrator position, Earth orientation parameter, station coordinate, and dry and wet troposphere uncertainties are given separately, while the last two lines indicate the total astrometric errors when all model parameters are perturbed together.
    Declination of source  
    $-25\hbox {$^\circ $ }$   $0\hbox{$^\circ$ }$   $25\hbox{$^\circ$ }$   $50\hbox {$^\circ $ }$   $75\hbox{$^\circ$ }$   $85\hbox {$^\circ $ }$
Error component   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $   $\Delta\alpha \cos\delta \hfil$ $\Delta \delta $
Calibrator position (1 mas error)     7   8    1   7    8 11   21  2   59  2   199  1
Calibrator position (0.25 mas error)     2   7    1   3    2  4    5  2   20  1    43  1
Earth orientation     5   7    5   3    5  4    4  3    3  1     3  1
Antenna position     2   9    2   6    2  5    3  3    2  2     2  2
Dry troposphere    17  54    6  19    2 12    5  9   11  9    12 13
Wet troposphere (mean)    80 272   32  98   11 41   12 32   43 41    60 62
Wet troposphere (max)   112 358   43 114   19 61   17 46   59 55    74 71
Total (mean wtrp)    84 284   30  99   16 42   25 33   81 36   189 67
Total (max wtrp)   121 481   44 134   20 56   26 46   92 65   212 74


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