  A preview study of using observational data of proper motions and Doppler
shifted velocities of twin-jets to determine the distance of sources inside and
outside our galaxy is made. We investigate the feasibility of this method by
studying the uncertainty of the distance caused by the uncertainties of the
measured quantities. It shows that, when the motion of components of the jet is
relativistic, then the uncertainty of the distance is within the same order of
the uncertainties of the measured values of proper motions and Doppler shifted
velocities. In particular, when assuming the pattern speed equals the flow
speed in the jet, for 10% uncertainties of the measured quantities, the
uncertainty of the distance caused by them would be well within 13%. With
current technique, this method is realizable. For the convenience of choosing
sources to observe, some sources as potential targets are also listed in this
paper.
