## 7. Tropsphere Monitoring
In addition to ray tracing methods, GNSS has been widely proven to be a unique and powerful tool for estimating ZWD, with advantages such as a high accuracy and high temporal resolution. However, measuring ZWD in harsh environments or locations that are not conducive to setting up stations is challenging, especially in the ocean (due to the uneven distribution of stations, modeling accuracy is inevitably affected), inland water bodies, and polar ice. In addition, various models have been developed to estimate ZWD, including the Hopfield model, the Saatamoinen model, the UNB3 series model, the GPT series model, and the VMF3 model [119; 120]. However, these models have some limitations, as it is difficult to accurately estimate the ZWD of low-elevation observations due to the significant spatial and temporal variations in low-level atmospheric humidity. Therefore, ZWD modeling has always been one of the main limiting factors for spatial geodetic observation analysis and application. Tropospheric delay is an important source of error in GG-R phase height measurement. However, low elevation angles have the opposite effect on the retrieval of surface height and tropospheric delay in GNSS-R. According to the error analysis in reference [28], it is shown that, at an elevation angle of 5\({}^{\circ}\), a total zenith delay (ZTD) of 1 cm in the troposphere, amplified by the tropospheric delay mapping function and phase height retrieval, results in an altitude retrieval error of approximately 65.8 cm. On the contrary, sub-meter-level surface height retrieval deviations (greater than most sea-level model errors) can lead to minor errors in tropospheric delay retrieval at a 5\({}^{\circ}\) elevation angle [121]. Due to the uncertainty of humidity delay, the amplification of tropospheric errors in low-elevation GNSS-R provides a valuable opportunity for retrieving atmospheric water vapor content. [PERSON] et al. [28] proposed a ZTD calibration method, which further corrects ZTD errors by fitting altimetry retrieval with MSS and ocean tide models, assuming that the overall trend of the model is accurate over relatively long orbits (such as 300 km). This calibration method significantly improves the root mean square error of height measurement and also proves, for the first time, that the ZTD in the troposphere can be estimated from GG-R. Unfortunately, there is no specific method for ZTD estimation reported in the literature. [PERSON] and [PERSON] [122] first proposed using GNSS reflection signals to solve the rank defect problem in water vapor tomography reconstruction. By optimizing the spatial resolution of the model and implementing an appropriate number of GNSS-R stations, the rank defects of the design matrix were reduced and the reconstruction efficiency was improved. The research results showed that using reflection signal averaging can improve the rank defect of the design matrix by more than 90%, and the average deviation and RMSE of the reconstructed image were 0.2593 and 1.847 ppm, respectively, effectively improving the reconstruction accuracy of the model. [PERSON] et al. [123] introduced the potential and challenges of spaceborne GNSS-R technology in high-precision height measurement applications. The article pointed out that, in order to achieve a centimeter-level ranging accuracy, it is necessary to ensure that the reflected signal can be consistently tracked, which often occurs in low-incident rays. However, at low elevations, tropospheric delay significantly increases, becoming the main source of error in GNSS-R carrier-phase height measurement. A tropospheric error estimation method for low approach angles was proposed to address this issue. Using the Global Pressure and Temperature-3 model, a comparison was conducted for 328 days at different elevation angles, taking into account the effects under different climate conditions. Research has found that tropospheric errors have significant regional, seasonal, and elevation dependencies, with lower elevations leading to larger error ranges.