## 7. Tropsphere Monitoring  
The troposphere is the layer of the atmosphere closest to the Earth's surface, with a thickness of 8-18 km, containing a large amount of water vapor, clouds, and aerosols. Water vapor in the troposphere plays a crucial role in global atmospheric radiation, energy balance, and the water cycle, and is an important factor in weather formation and evolution. The wind speed and direction in the troposphere have significant impacts on aircraft. Conducting tropospheric monitoring is of great significance for weather forecasting, disaster prevention and mitigation, climate change research, and ensuring aviation safety. Therefore, conducting tropospheric monitoring and data analysis can deepen our understanding of the Earth's ecosystem and has important scientific significance and practical value in improving climate and weather forecasting. Total columnar water vapor (TCWV), also known as integrated water vapor (IWV) or total precipitable water vapor, refers to the total amount of water vapor contained in the vertical column of the atmosphere and is a key parameter for studying climate change and meteorological systems. In the past few decades, ground observation, in situ observation, and remote sensing technologies have been commonly used to obtain TCWV, such as satellite observation and reanalysis, radiosondes, microwave radiation measurements, LiDAR, ground Global Navigation Satellite System (GNSS) receiver networks, Very Long Baseline Interferometry (VLBI), and GNSS Radio occultation (GNSS-RO) [117]. However, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Satellite Data Used** & **Type of Data** & **Result (Accuracy)** \\ \hline
[112] & TDS-1 & L1 & In low latitudes, open oceans, and low wind speeds, there are coherent scattering components. Under high wind speeds, it is mostly irrelevant. \\ \hline
[113] & TDS-1 CYGNSS & L1 & The results show stronger fluctuations in the peak of the DDM associated with ionospheric scintillation events, possibly related to tropical storms. \\ \hline
[44] & CYGNSS & L1 & All earthquakes with magnitudes above 4 exhibit a small but detectable positive correlation with ionospheric amplitude scintillation, and the results will improve with increasing magnitudes. \\ \hline
[38] & TDS-1 & L1 & The improvement in RMS error during high solar activity is 5.3\%, and during low solar activity it is 23.5\% (compared to GNSS TEC). \\ \hline
[49] & Spire & L0 & The slant global ionospheric TEC maps (TEC) retrieved from GNSS-R measurements and GIM follow similar trends, and the TEC time series based on GNSS-R provides an almost “frozen in time” observation of ionospheric structure. \\ \hline
[114] & Spire & L0 & The results during the low solar = activity period show that the total tilted electron content fluctuates up to about 300 TECU, the relative ionospheric delay of GPS L1 frequency is 19 m, the Doppler frequency shift is 2 Hz, and the range of peak electron density height variation is from 215 to 330 km. \\ \hline \hline \end{tabular}
\end{table} 
Table 6. Summary of the methods and accuracy of using spaceborne GNSS-R L1 data and raw IF data for ionospheric total electron content and disturbance observations in the current literature. due to the various limitations of these technologies (accuracy, spatial density, coverage range, spatial and temporal resolution, cloud coverage, etc. ), there is still an urgent need for accurate IWV observations, especially in inland water bodies, oceans, and polar ice scenes. Tropospheric wet delay (ZWD) is a key parameter for spatial geodesy and meteorological applications, and modeling ZWD is crucial for IWV retrieval and climate monitoring [118]. Currently, ray tracing is one of the most accurate methods for determining ZWD. It is based on a rigorous theoretical foundation and is, therefore, commonly used as a benchmark for ZWD modeling and validation.