| Summary: | The incorporation of multi-frequency signals into global navigation satellite systems (GNSS) has presented new possibilities for precise positioning and rapid ambiguity resolution. Inter-frequency clock bias (<i>IFCB</i>) pertains to the time-varying biases among distinct frequencies within carrier phase observations in GNSS signals. The appropriate handling of <i>IFCB</i> is critical in enhancing the accuracy and convergence time of precise point positioning (PPP) solutions. The focus of this study is on the proper modeling of phase <i>IFCB</i> in multi-GNSS multi-frequency PPP. In this paper, the optimal <i>IFCB</i> power spectral density value of 0.6 m/sqrt(s) is first determined. To obtain the optimal stochastic model for <i>IFCB</i>, a thorough comparison and analysis of the product correction and parameter estimation methods is conducted. Additionally, experiments are conducted on the effect of <i>IFCB</i> modeling on the performance of undifferenced and uncombined PPP using data from 130 multi-GNSS experiment stations across the globe over a period of one week in January 2022. The study reveals that the optimal power spectral density for <i>IFCB</i> is within [60, 0.006] m/sqrt(s), modeling <i>IFCB</i> as a random walk is feasible, and the PPP is comparable for the three <i>IFCB</i> processing schemes of product correction, random walk, and white noise. Meanwhile, it is not reasonable to treat <i>IFCB</i> as a random constant or neglect it in the multi-GNSS multi-frequency PPP. In the absence of product correction or for users who require immediate and continuous positioning solutions, modeling <i>IFCB</i>s as random walks can lead to more reliable positioning results and improved convergence performance.
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