Objectives  In order to make full use of the observed value information of each frequency, an undifferenced and uncombined integer ambiguity resolution method between BeiDou satellite navigation system (BDS) long-range reference stations is proposed.

  Methods  First, the error observation equation is established directly by using the observations of different frequencies, and the relative zenith tropospheric wet delay error and ionospheric delay error are estimated by random walk strategy to increase the constraint between epochs. Second, a linear calculation method of undifferenced integer ambiguity real-time is used to obtain the undifferenced integer ambiguity of all satellites in the current epoch of the reference network. It solves the problem that ambiguity needs to be inherited or re-superimposed on normal equations in the reference star transformation. Because the information of each frequency observation is fully utilized, the influence of linear combination amplification noise on integer ambiguity fixing is avoided, and the ambiguity fixing success rate of uncombined method is much higher than that of ionosphere-free combination method.

  Results and Conclusions  The results show that the average fixed speed of ambiguity of each reference station is 20 epochs (sampling interval 1 s). The integer ambiguity resolution of the carrier phase of the reference station can be realized quickly.