The continuous evolution of global navigation\nsatellite systems (GNSS) meteorology has led to an\nincreased use of associated observations for operational\nmodern low-latency numerical weather prediction (NWP)\nmodels, which assimilate GNSS-derived zenith total delay\n(ZTD) estimates. The development of NWP models with\nfaster assimilation cycles, e.g., 1-h assimilation cycle in the\nrapid update cycle NWP model, has increased the interest\nof the meteorological community toward sub-hour ZTD\nestimates. The suitability of real-time ZTD estimates\nobtained from three different precise point positioning\nsoftware packages has been assessed by comparing them\nwith the state-of-the-art IGS final troposphere product as\nwell as collocated radiosonde (RS) observations. The ZTD\nestimates obtained by BNC2.7 show a mean bias of\n0.21 cm, and those obtained by the G-Nut/Tefnut software\nlibrary show a mean bias of 1.09 cm to the IGS final troposphere\nproduct. In comparison with the RS-based ZTD,\nthe BNC2.7 solutions show mean biases between 1 and\n2 cm, whereas the G-Nut/Tefnut solutions show mean\nbiases between 2 and 3 cm with the RS-based ZTD, and the\nambiguity float and ambiguity fixed solutions obtained by\nPPP-Wizard have mean biases between 6 and 7 cm with\nthe references. The large biases in the time series from\nPPP-Wizard are due to the fact that this software has been\ndeveloped for kinematic applications and hence does not\napply receiver antenna eccentricity and phase center offset\n(PCO) corrections on the observations. Application of the\neccentricity and PCO corrections to the a priori coordinates\nhas resulted in a 66 % reduction of bias in the PPP-Wizard\nsolutions. The biases are found to be stable over the whole\nperiod of the comparison, which are criteria (rather than the\nmagnitude of the bias) for the suitability of ZTD estimates\nfor use in NWP nowcasting. A millimeter-level impact on\nthe ZTD estimates has also been observed in relation to\nambiguity resolution. As a result of a comparison with the\nestablished user requirements for NWP nowcasting, it was\nfound that both the G-Nut/Tefnut solutions and one of the\nBNC2.7 solutions meet the threshold requirements,\nwhereas one of the BNC2.7 solution and both the PPPWizard\nsolutions currently exceed this threshold
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