Abstract: We compare several stochastic models used in GPS PPP mode.The results show that the precision of PPP-derived ZPD can be obviously improved through a suitable stochastic model for GPS measurements.Low-elevation observations cover more information which can improve the estimation of ZPD.The new stochastic model based on satellite low elevation angle cosine square is proposed.The results show that the stochastic model using satellite elevation angle-based cosine square function is better than previous stochastic models,for example,sine square stochastic model and sinestep model.