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We present a detailed analysis of 30 very long baseline interferometricobservations of the BL Lac object S5 1803+784 (z=0.679), obtained between meanobservational time 1994.67 and 2012.91 at observational frequency 15 GHz. Thelong-term behaviour of the jet ridge line reveals the jet experiences anoscillatory motion superposed on its helical jet kinematics on a time-scale ofabout 6 years. The excess variance of the positional variability indicates thejet components being farther from the VLBI core have larger amplitude in theirposition variations. The fractional variability amplitude shows slight changesin 3-year bins of the component's position. The temporal variability in theDoppler boosting of the ridge line results in jet regions behaving as flaring"radio lanterns". We offer a qualitative scenario leading to the oscillation ofthe jet ridge line, that utilizes the orbital motion of the jet emitter blackhole due to a binary black hole companion. A correlation analysis impliescomposite origin of the flux variability of the jet components, emerging due topossibly both the evolving jet-structure and its intrinsic variability.

Flaring radio lanterns along the ridge line: long-term oscillatory motion in the jet of S5 1803+784