2D-Gaussian components at low frequencies. (a) Radio spectrum (5 GHz to 43 GHz) of core and jet features in 3C 454.3 in 2005 July 14. The flux densities of the upstream components are labelled ‘Ca’ and ‘Xa’ and the downstream components ‘Cb’ and ‘Xb’. (b) Core-shift vectors of each frequency pair and average vector direction in a polar grid. The dotted lines are given at intervals of 30°. In the bottom right corner is a zoomed-in image of the core-shift vector pairs for the high-frequency pairs. All vectors consistently point in the large-scale jet direction towards the west (see Fig. 1). The true cores are features Cb and Xb at the C and X bands, respectively, since they produce core-shift vectors whose direction is consistent with the jet direction. Comparisons of the core-shift vectors produced by the other C and X component choices are displayed in Appendix B.