GNSS precise point positioning (PPP) has become the research hotspot in the most recent years due to its capability of obtaining precise position with single receiver. Usually, three models are used in PPP, that is,uncombined model, Uofc model and ionosphere-free combination model. The relationships between the these models are described in detail in this paper. On the one hand, it is clarified that the uncombined model is equivalent to the Uofc model and both of them are better than the ionosphere-free model in the sense of ambiguity resolution. On the other hand, in comparison with the Uofc model that eliminates the ionosphere delay by using the equivalence principle, the uncombined model that takes the ionosphere delay as the parameter has its advantage that it can provide the users with the prior constrained conditions for the ionosphere. As a result, the model can be converted to the ionosphere-weighted model easily. In the circumstance of fixing the wide-lane, when comparing with the ionosphere-free model by using ambiguity dilution of precision (ADOP) from the aspect of fixing ambiguity, Uofc model has many advantages such as small noise, origin information preserving etc.,while the ionosphere-weighted model can improve the ambiguity resolution significantly when the high precise prior ionosphere information is available.
