# Review on the Role of GNSS Meteorology in Monitoring Water Vapor for Atmospheric Physics
## 6 Conclusions  
GNSS sensing of atmospheric water vapor is already a well established technique that allows for the monitoring of this trace gas with wide (yet land-only) coverage, with high temporal resolution and without weather restrictions. These features make this technique very suitable for testing other techniques, especially when weather is adverse. In comparison with other products, GNSS water vapor data are high quality, being considered a reference for the validation of other ground-based and satellite-based water vapor measurements and model estimates. However, we consider that special care must be taken with some issues. First, IWV is dependent on height and therefore when comparing data related to different heights (i.e., ground-based stations and different heights, satellite or NWP pixels at a mean height of the area, different from the real height of the GNSS station) it is important to apply the corresponding correction to account for this height difference. Second, the homogeneity of the series must be checked (especially if there are changes in antenna or processing strategy). Third, it is considered a rule of thumb that GNSS retrievewater vapor information of about 100 km radius, which can be used to set a condition on distance to GNSS station when performing validations. Nowadays, time-series of GNSS data products are starting to be long enough to study water vapor in long term studies, such as its trends and variability across different time scales. Additionally, the increasing amplitude of the ground-based GNSS networks allows for a detailed analysis of its spatial distribution. It is observed that, despite the average behavior based mainly on temperature (according to the Clausius-Clapeyron equation), local circulation is more important. Although there are clear cycles in daily and annual regimes, situations with relevant local circulation can disrupt these. Therefore, it is fundamental to use long-term data to study daily and annual cycles, especially in regions where strong circulation patterns can modify air moisture. GNSS water vapor has been widely used for several meteorological and climate applications such as assimilation, circulation of the atmosphere and radiative effects. It is shown that assimilation can be improved with GNSS data, especially in regions where there may be a lack of moisture information. It must be noted that GNSS are relatively cheap and easy to maintain, and provide continuous measurements for all-weather conditions. This means they can be used in regions with fewer resources or those where human access is difficult. Despite being a well established technique, it must be noted that many other GNSS-based techniques, such as tomography or data fusion approaches, can make the best out of the information that GNSS meteorology has to offer. The use of multi-GNSS techniques, especially, have the potential to improve the quality of these approaches. The use of low-cost devices (i.e., single frequency receivers) can be interesting in terms of retrieving moisture information cost-effectively. In summary, the GNSS technique allows the retrieval of accurate atmospheric water vapor data. This method exhibits great potential to be extraordinarily useful in the monitoring of the weather and climate. Conceptualization, M.A. ; Data curation, J.V.-M.; writing--original draft preparation, J.V.-M.; writing--review and editing, M.A. ; supervision, M.A. All authors have read and agreed to the published version of the manuscript. This work was partly supported by the Ministerio de Economia y Competitividad of the Spanish Government (CGL2017-87917-P), and by Junta de Extremadura and European Regional Development Fund (IB18092). [PERSON] was supported by a predoctoral fellowship (PD18029) from Junta de Extremadura and European Social Fund. The authors declare no conflict of interest.