  Accelerating superluminal motions were detected recently by multi-epoch Very
Long Baseline Interferometry (VLBI) observations. Here, a Linear Accelerating
Superluminal Motion (LASM) model is proposed to interpret the observed
phenomena. The model provides a direct and accurate way to estimate the viewing
angle of a relativistic jet. It also predicts that both Doppler boosting and
deboosting effects may take place in an accelerating forward jet. The LASM
model is applied to the data of the quasar 3C 273, and the initial velocity,
acceleration and viewing angle of its three components are derived through
model fits. The variations of the viewing angle suggest that a supermassive
black hole binary system may exist in the center of 3C273. The gap between the
inner and outer jet in some radio loud AGNs my be explained in terms of Doppler
deboosting effects when the components accelerate to ultra-relativistic speed.
