Precise point positioning (PPP) has been demonstrated to be a powerful sensor to monitor tropospheric zenith delay (TZD). In the PPP, some unmodelled biases still remain in the GPS observables and will inevitably degrade the precision of tropospheric zenith delay estimation. The researches show that the precision of PPP-derived TZD can be obviously improved through a suitable stochastic model for GPS measurements. Previous stochastic models assume that the accuracy of high-elevation GPS measurements is higher than that of low-elevation GPS measurements. A new stochastic model based on satellite elevation angel cosine is used in the presented paper. It examines the quality of the PPP-derived TZD calculated at shanghai GPS stations through comparison with those of nearby radiosonde soundings. It is found that the PPP-derived TZD have good agreement with results of radiosonde soundings. It is feasible to monitor tropospheric zenith delay by the singular GPS station in PPP.