Determination of Total Precipitable Water from GNSS Data in Thailand

The relationship between the total precipitable water (TPW) in the atmosphere and Global Navigation Satellite System (GNSS) Zenith Total Delay (ZTD) has been investigated. The study was performed for ZTD observations derived from GPS measurements at 5 sites across Thailand, over one-year period, July 2017 to August 2018. The ZTD was then collocated in space and time with the TPW data available from Global Data Assimilation System (GDAS) Numerical Weather Prediction (NWP) model. The result showed that the ZTD observations are highly correlated with the TPW but there are some biases between the ground base station locations largely due to the altitude differences. An empirical model was then developed to remove the altitude dependence and provide a unique relationship between the ZTD and the TPW. This model can be useful in the further TPW measurements from widely available GNSS receivers.