Precise Point Positioning (PPP) is a powerful technique to achieve homogeneous positioning accuracy globally and is becoming the dominating service for precise positioning.Typically,PPP still takes about 30 minutes of initialisation time to obtain a converged solution of better than 10 cm.In order to shorten the initialization time,integer ambiguity fixing approaches for PPP (PPP-AR) have been developed in recent years.In most of the existing ambiguity resolution approaches,ionosphere-free observation models are used and therefore they are restricted by the noise of range observations and the short wavelength of the narrow-lane ambiguities as well.However,with the GNSS modernization,a unified processing strategy without forming combinations and differencing is more appealing.In this work,we apply this new approach in uncalibrated phase delay (UPD) estimation and zero-difference (ZD) AR and compare it with the traditional model.The satellite UPDs associated with ionosphere-free model and raw observation model are first estimated from a global distributed GNSS network.70 stations tracking both GPS and Beidou (BDS) are used in positioning validation.The potential contribution of BDS is also investigated.