Top: BH (filled black circle) that launches a jet in the horizontal direction. The different core positions at each frequency are shown in ellipses. The K-band core’s distance from the BH is shown with the red circle ‘1’. The distance between the adjacent frequencies K and U bands is shown with the red circle ‘2’. Bottom: schematic diagram describing the phase-referencing positions (d₁, d₂) and the core shifts of the target (CS^t) and a calibrator (CS^c). All coordinates are specified for each source.