GPS signals experience range delays as they propagate through the neutral atmosphere. These range errors are dependent on pressure, temperature and humidity along the signal path. By removing all other sources of ranging error, the effects due to atmospheric moisture may be isolated and estimates of zenith wet delay (ZWD) derived from GPS observations. Typical methods for ZWD estimation are based on post-mission software operating in near real-time (e.g. Bernese, GAMIT). The real-time availability of precise GPS satellite orbit and clock products has enabled the development of a novel positioning methodology, however, known as precise point positioning (PPP). Based on the processing of undifferenced pseudorange and carrier phase observations from a single GPS receiver, PPP provides a new way to perform real-time
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