The realization and evaluation of mixed GPS/BDS PPP ambiguity resolution

Aiming at shortening convergence time and improving positioning accuracy, multi-GNSS precise point positioning (PPP) ambiguity resolution (PPP-AR) has been an important issue in the past decade. In this paper, a mixed (or inter-system) GPS and BDS PPP-AR model with inter-system biases considered is proposed. Datasets from the IGS MGEX network are utilized in the study to evaluate the proposed model. As a critical correction in multi-GNSS PPP-AR, the inter-system bias (ISB) can be treated as a fixed constant or unknown estimate. The effects of various ISB processing methods on other key corrections for PPP-AR, such as fractional cycle bias (FCB) and inter-system phase bias (ISPB), are analyzed. Experimental results indicate that fixing or estimating ISB approaches will not affect GPS FCB estimations. However, various ISB dealing methods will have a significant influence on some BDS FCB and ISPB estimations at some stations because of the limited BDS tracking satellites over long periods of observation. Regardless of the presence of unstable FCB products on some BDS satellites, narrow-lane FCBs on other satellites are time-continuous, and their daily changes are within the range of 0.3 cycle. And in aspect of the time to first fix (TTFF), fixing ISB is superior to estimating it. The performance of the mixed GPS and BDS PPP-AR is evaluated. Experimental results indicate that compared with the intra-system PPP-AR, the mixed method has no superiority when ISB is estimated. While it has a slight improvement in TTFF, i.e., from 969.64 to 897.96 s, however, the total fixed rate decreases from 86.5 to 85.56% when ISB is fixed as a constant. In addition, the mixed PPP-AR shows significant improvement over the intra-system PPP-AR under circumstances with limited satellite visibility.