Knowledge of differential inter-system biases(DISBs)is critical to integrate observations from mixed GNSS.If the corresponding DISB could be calibrated in advance,only one pivot satellite is sufficient for ambiguity resolution on overlapping frequencies,which is the so-called tight combining(TC)strategy.Considering that GPS and Galileo transmit signals on two identical frequencies(e.g.L1/E1 and L5/E5a),a tightly combined GPS/Galileo RTK positioning model is proposed in this paper.Traditional DD model has been slightly adjusted to avoid the hand-over problem of reference satellites.The estimation of code and fractional part of phase DISB is archived through zero and ultra-short baselines.