## 3 Spaceborne GNSS-R Raw IF Data and Processing
### Raw IF Data Processing  
The raw IF data processing is based on a software-defined radio GNSS receiver extended for remote sensing purposes. The earlier versions of this receiver have been used in processing GNSS-R and GNSS RO raw IF data sets collected in different ground-based and airborne experiments, e.g., [61; 62; 63]. With the accumulation of the spaceborne GNSS-R raw IF datasets, the software receiver is redesigned to the spaceborne scenarios by updating the computations open-loop tracking parameters. It is worth mentioning that the software receiver is selected as the official Level-0 raw IF data processor for the ESA's Scout-2 HydroGNSS mission, which will be adapted to process the raw IF data collected at both L1/E1 and L5/E5A bands. The main outputs of the processing are the complex waveforms of the reflected signals, which can be generated following either the conventional GNSS-R processing or the interferometric processing [10]. However, in this paper as well as in the provided data products, only the conventional GNSS-R processing is introduced. For each GNSS satellite, the complex waveforms are computed by cross-correlating the reflected signal with a set of local generated clean replicas as  
\[ \begin{split} z^{\mathrm{r}}(t_{0},\Delta\tau_{i})=\int_{0}^{t_{ \mathrm{c}}}& s^{\mathrm{r}}(t_{0}+t_{1})c[\Phi_{\mathrm{code}}^{ \mathrm{r}}(t_{0}+t_{1})+\Delta\tau_{i}]\\ &\times e^{-2\pi[f_{\mathrm{carr}}^{\mathrm{r}}(t_{0})t_{1}+ \phi_{\mathrm{carr}}^{\mathrm{r}}(t_{0})]}dt_{1}\end{split}  \tag{1}\]  
in which \(t_{0}\) is the start time of the cross-correlation indicating the time label of each complex waveform, \(\tau_{i}\) is the delay of each waveform lag, \(t_{\mathrm{c}}\) is the coherent integration time, \(c(\cdot)\) is the local generated PRN code series after the navigation bit correction, \(s^{\mathrm{r}}\) is the reflected signal, \(\Phi_{\mathrm{code}}^{\mathrm{r}}\) is the code phase of the local generated PRN code, \(f_{\mathrm{carr}}^{\mathrm{r}}\) and \(\phi_{\mathrm{carr}}^{\mathrm{r}}\) are the frequency and phase of the local generated carrier replica, respectively. These local generated code and carrier replicas parameters, known as open-loop (OL) tracking model, are computed corresponding to the reflected signal through the specular point, which are estimated from the direct signal carrier and code parameters together with the bistatic geometry. In addition to the raw IF samples of the direct and the reflected signals, the inputs of the processing also include the meta data of the mission science products (e.g., the Level 1 products), the GNSS orbit products from the International GNSS Service (IGS), and the earth surface elevation data derived by combining the GMTED2010 digital elevation model (DEM) [64] and the DTU18 mean sea surface (MSS) model [65]. The processing scheme of the reflected signal is presented in Figure 2, in which the main processing procedures, i.e., the direct signal processing and the open-loop model computation are described in detail as follows. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**Mission**} & \multirow{2}{*}{**Freq Band**} & \multirow{2}{*}{\(f_{\mathrm{IF}}\)} & \multirow{2}{*}{\(\mathrm{BW}\)} & \multirow{2}{*}{\(f_{\mathrm{s}}\)} & \multirow{2}{*}{**QP**} & \multirow{2}{*}{**Length**} & \multirow{2}{*}{**\# RF CHs**} & \multirow{2}{*}{**Geog. Dist [deg] in Lat**} \\  & & & [MHz] & & [MHz] & & [Second] & & \\ \hline TDS-1 & L1 & 4.188 & \(\sim\)2.5–4.1 & \(\sim\)16 & 2 bits, I & 60–90 & 1/1 & \([-90,90]\) \\ \hline CYGNSS & L1 & 3.8724 & \(\sim\)2.5 & \(\sim\)16 & 2 bits, I & 30–60 & 1/2 & \(\sim\)\([-45,45]\) \\ \hline BF-1 & L1 & 0 & \(\sim\)2.0 & \(\sim\)4 & & & & \\ \cline{2-8}  & B1I & 0 & \(\sim\)4.0 & \(\sim\)8 & & & & \\ \hline SPIRE RO & L1 & \(\sim\)1.6 & \(\sim\)2.0 & \(\sim\)6 & & & & \\ \cline{2-8}  & L2 & \(\sim\)\(-1.6\) & \(\sim\)2.0 & \(\sim\)6 & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Configurations of Raw IF Data Collection From Different GNSS-R Missions.