# Review on the Role of GNSS Meteorology in Monitoring Water Vapor for Atmospheric Physics
## 3 Inter-Comparisons with Other Techniques
### Validations of GNSS Water Vapor with Other Techniques as Reference
Therefore, in this work, GNSS IWV was successfully used as a reference to find deficiencies in models. However, one of the best tests for GNSS IWV products is to compare them against radiosonde measurements, a direct method usually considered a reference despite known bias issues, see [61]. [PERSON] et al. [55] compared GNSS IWV from eight stations in \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Work** & **Year** & **N** & **Instr.** & **Start** & **End** & **Region** & **Bias** & **DEV** & **Corr** \\ \hline
[50] & 1996 & 4 & WVR & 1993-05 & 1993-05 & Interior US (US) & 0.05 & 1.28 & - \\ \hline
[51] & 2000 & 556 & WWP & 1996-09 & 1996-09 & Japan & 2-4 & 3.8 & - \\ \hline
[52] & 2001 & 1 & radiosonde, WVR & 1999-01 & 1999-12 & Cagliari (Italy) & 0.4 & 1.9 & - \\ \hline
[53] & 2003 & & WVR & 2000-05 & 2000-06 & Oklahoma & - & 1.3 & 0.99 \\ \hline
[54] & 2005 & 5 &
\begin{tabular}{c} MWR, NWP \\ (M4S), \\ radiosonde \\ \end{tabular} & 2002-08 & 2003-09 & Europe (Italy) & 0.01 & 0.286 & 0.939 \\ \hline
[55] & 2006 & 8 & \
\begin{tabular}{c} NWP(GASP; \\ ECMWF), \\ radiosonde \\ \end{tabular} & 2000-01 & 2000-1 & Australia &
\begin{tabular}{c} \(-1.4\)\(+\)\(1.1\) (GASP), \\ \(-1.8\)\(+\)\(1.3\) (ECMWF) \\ \end{tabular} & 3.1–5.3 (GASP), 2.7–3.5 (ECMWF) & - \\ \hline
[56] & 2010 & 3 & \begin{tabular}{c} MWR, \\ radiosonde \\ \end{tabular} & 2009-08 & 2009-09 & \
\begin{tabular}{c} Ulteungdo (South \\ Korea) \\ \end{tabular} & 3.0 (Radiosonde), 5.0 (MWR) &
\begin{tabular}{c} 12.5 \\ (Radiosonde), 9.3 \\ (MWR) \\ \end{tabular} & - \\ \hline
[57] & 2011 & 4 & radiosonde, AIRS, 12 &
\begin{tabular}{c} 2008-01 \\ MODIS \\ \end{tabular} & 2004-01 & 2004-12 & Antarctica & \(-0.48\) (Radiosonde) & - & - \\ \hline
[59] & 2019 & 4 & radiosonde & 2006-05 & 2018-01 & global & \(-0.87\)–(\(-0.49\)) & 0.61–1.10 & 0.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of studies on validation of GNSS measurements. Column Year is the publication year. N is the number of stations. Instr. is the instruments (besides ground-based GNSS) used in the study. Start and End mark the period considered in the study, in year-month format. Bias is the bias in mm, and DEV is the RMSE or SD also in mm. Corr is the correlation coefficient (R). Australia against collocated or nearby radiosonde stations, obtaining standard deviations of 8.8%, except for Antarctic stations, which increased to 18-21%. It must be noticed that GNSS is considered less sensitive to small amounts of water vapor and, therefore, in regions with small IWV, GNSS technique can perform worse. This was assessed by a cross-validation including, FTIR, sun-photometer, multifilter rotating shadowband radiometer (MFSR), radiosonde, and GNSS in the Izana Observatory (Canary Islands, Spain) during years 2005-2009, showed similar biases with respect to GNSS (\(-\)3.58 to +0.42 mm), and standard deviations (between 0.73 and 2.29 mm), which is consistent with other works. GNSS accuracy was found to be better than 10% for values over 3.5 mm. However, for values under this limit, the accuracy is worse (\(\sim\)20%) and a systematic dry bias appears [62]. Other comparisons against radiosondes, like the one set in four stations of the Malaysian Peninsula by [PERSON] et al. [57] during the year 2008, or the one with four collocated reference GRUAN radiosonde stations [59], assessed the high quality of GNSS to monitor water vapor. RMSE values were in the range 3.5-4.3 mm in the former, while standard deviations were 0.61-1.10 mm in the latter, and correlations were of 0.80-0.88, reaching 0.99 in the second work. Both works presented dry biases, varying between 0.0 and \(-\)1.5 mm in the first cases, and 0.49-0.87 mm in the second. The different biases are mainly associated to dry bias in the radiosonde measurements, while the better results in the second work could be due to the use of GRUAN stations, with high quality standards, and also to the fact that both radiosondes and GNSS stations were exactly collocated.