Twin jets as a potential distance detector
  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. 
Yi-Ping Qin, Guang-Zhong Xie