## 7. Tropsphere Monitoring
There is no research in the literature on the retrieval of ZWD and IWV for inland water bodies or (and) Antarctic scenes, nor is there any report on the specific methods and experimental results of using single-frequency Multi GNSS (GPS, Galileo, and BeiDou-3) raw IF signal data from the TDS-1 and CYGNSS satellites to retrieve ZWD and/or IWV, let alone the results of using Galileo and BeiDou-3 reflection signals to retrieve ZWD and IWV. The analysis in [22] indicated that the waveform front of Galileo E1 B/C and BeiDou-3 BIC signals was much steeper than that of GPS L1 C/A signals, which means that the ranging sensitivity is better and is expected to play an important role in high-precision ranging applications of GNSS reflected coherent signals. There are limitations in the use of dual-frequency GPS raw IF reflection signal observation data in the current Spire constellation, which seriously hinder researchers from further innovating and developing the cutting-edge topic of GG-R ZWD and IWV retrieval. Spire CubeSats is a small satellite network deployed by Spire, which collects and analyzes Earth observation data in real time through advanced sensors and communication equipment, covering fields such as meteorology, oceanography, and navigation. It also provides global positioning and tracking services, providing data support and solutions for industries such as aviation and navigation. Through case studies, it has been proven that this method can achieve the high-precision detection of tropospheric delay and water vapor in oceans and ice. Then, Spire grazing angle GNSS-R data can be utilized. It was  
Figure 8: Flow chart of tropospheric monitoring (for relatively smooth ocean and ice surfaces) [121]. found that under appropriate conditions, both the ocean surface and sea ice can reflect coherent GNSS signals. Compared with other satellite observation technologies, grazing angle GNSS-R can fill some gaps in atmospheric water vapor observation data. However, research has also pointed out the relative oblique tropospheric delay and elevation-related errors in GNSS-R carrier-phase measurement, and called for future work to strengthen error quantification and improve measurement methods. In addition, research suggests that this method complements GNSS RO, but also suggests the possibility of further utilizing low-elevation reflection signals. Therefore, future research should explore this field more comprehensively to promote the further development and application of this method. These studies used L0-level data, so we can conclude that applications based on space-borne GNSS-R raw IF signals have shown unique advantages in monitoring tropospheric water vapor. By capturing the IF signal of GNSS, the propagation characteristics of the signal and its changes as it passes through the troposphere can be analyzed in detail. These signals exhibit different delay and attenuation characteristics during reflection and direct reflection due to different water vapor contents in the troposphere. By accurately measuring and analyzing these characteristics, researchers can estimate the water vapor content in the troposphere, providing a new source of data for meteorological forecasting and climate research. However, although this technology has made some progress, it still faces some challenges. Problems such as modeling errors in the troposphere and elevation-related errors still need to be addressed. Future research should strengthen error quantification and improve measurement methods, while exploring the possibility of complementing GNSS RO and further utilizing low-elevation reflection signals. The TDS-1 and CYGNSS satellites launched by the UK and the US in July 2014 and December 2016, respectively, collected single-frequency Multi GNSS (GPS, Galileo, and BeiDou-3) raw IF signal data reflected from some land, inland water bodies, oceans, and polar sea ice on a global scale.