## 2 Current Status of Spaceborne GNSS-R Microsatellites
In 2019, Spire launched two new satellites to perform GNSS-R measurements [47]. In addition to business data, some low-level raw IF data are also collected by GNSS-R satellites and applied in fields such as sea ice height retrieval [48] and ionospheric sounding [49]. FY-3E: FY-3E is a near-polar sun-synchronous orbit satellite, successfully launched in 2021, equipped with a GNSS occultation detector (GNOS-II) [34]. It can obtain the raw IF data of a reflected signal within a specific area, but there is no channel to record the raw direct signal. [PERSON] et al. [50] proposed a novel method for tracking individual DDMs without direct or auxiliary data based on GNOS-II to maximize the spatiotemporal resolution of level 1 DDMs. Among these numerous satellites, most of them only disclose L1 data. Although CYGNSS and TDS-1 provide a small amount of L0-level data, this data has limitations in its distribution range. For example, there is relatively rich data on some land areas, but very little data on the ocean and polar regions, which makes it difficult to meet the research demand for marine and polar scientific research [21]. In addition to CYGNSS and TDS-1 providing a small amount of L0-level data, BF-1 A/B occasionally collects raw IF data in the GPS L1 and BDS B1I frequency bands. It is worth noting that Spire is currently the only satellite that provides dual-frequency L0 data, but the L0 data provided by these two satellites is not publicly available. The GNSS-R receiver carried by China's Tianmu-1 satellites also collect raw IF signals and can simultaneously receive reflection signals from four navigation systems (including BeiDou, GPS, GLONASS, and Galileo) and the Quasi Zenith Satellite System (QZSS). However, the satellites are commercial satellites and these raw data cannot be directly obtained. Although there are currently significant limitations in data acquisition, raw IF signal observation data from satellite missions such as Spire, BF-1 A/B, and Tianmu-1 can be distributed and shared among participating members. Spire and BF-1 A/B have already played a significant role in research in many fields, such as sea surface winds, land surface soil moisture, sea surface height measurements, ionosphere, ice sheet height, and so on [46; 51; 52; 53; 54]. In the future, with the rapid development of multi-frequency and multi-system GNSS technology and the launch of more spaceborne GNSS-R satellites, there will be more accumulation of raw data, which will be applied in different fields. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mission**} & **Type of GNSS-R** & \multirow{2}{*}{**Frequency**} & **GNSS** & **Is L0-Level Data** & **Is L1-Level Data** \\  & **(Receiver Technique)** & & **Band/Polarization** & **System** & **Publicly** & **Publicly** \\  & & & & & **Available?** & **Available?** \\ \hline \multirow{2}{*}{UK-DMC [6]} & \multirow{2}{*}{cGNSS-R} & L1/ & \multirow{2}{*}{GPS} & \multirow{2}{*}{\(\mathbf{\kappa}\)} & \multirow{2}{*}{\(\mathbf{\kappa}\)} \\  & & LHDCP & & & \\ \multirow{2}{*}{UK-TDS-1 [31]} & \multirow{2}{*}{cGNSS-R} & L1/ & \multirow{2}{*}{GPS} & \multirow{2}{*}{\(\mathbf{\psi}\)} & \multirow{2}{*}{\(\mathbf{\psi}\)} \\  & & LHCP & & & \\ \multirow{2}{*}{CYGNSS [32]} & \multirow{2}{*}{cGNSS-R} & L1/ & \multirow{2}{*}{GPS} & \multirow{2}{*}{\(\mathbf{\psi}\)} & \multirow{2}{*}{\(\mathbf{\psi}\)} \\  & & LHCP & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: History of spaceborne GNSS-R microsatellites. \(\mathbf{\kappa}\) represents L0/L1 level data not publicly available, \(\mathbf{\psi}\) represents L0/L1 level data publicly available, and \(\mathbf{\mathbf{\psi}}\) represents L0/L1 level data with an uncertain status regarding public availability.