## 3 Spaceborne GNSS-R Raw IF Data and Processing
### Raw IF Data Processing
#### 3.2.3 Remarks for Raw IF Processing  
The following aspects are remarkable for the processing of the raw IF data:  
* The navigation signals transmitted by modernized GNSS satellites, e.g., Galileo E1 B/C and BDS-3 BIC, consist of both data and pilot components. The complex waveform for each signal component can be generated independently by cross-correlating the reflected signal to its PRN code. However, instead of the complex waveform for each signal component, the combined complex waveform are generated by cross-correlating the reflected signal to the composite PRN codes of these signal components.  
For Galileo E1 signal, both E1B and E1C components are in-phase modulated and the composite code \(c_{\rm E1}(t)\) are generated by [67]  
\[c_{\rm E1}(t)=c_{\rm E1B}(t)D_{\rm E1B}(t)-c_{\rm E1C}(t)D_{\rm E1C}(t) \tag{4}\]  
in which \(c_{\rm E1B}(t)\) and \(c_{\rm E1C}(t)\) are the PRN codes for the E1B and E1C components, and \(D_{\rm E1B}(t)\) and \(D_{\rm E1C}(t)\) are the navigation data bit or the secondary code modulated to the E1B and E1C components. The BDS-3 B1C data and pilot components are modulated in phase and quadrature, and the composite code \(c_{\rm B1C}(t)\) are generated by [68]  
\[c_{\rm B1C}(t)=c_{\rm B1C}(t)D_{\rm B1C}(t)-jc_{\rm B1C}(t)D_{\rm B1C}(t) \tag{5}\]  
where \(c_{\rm B1C}(t)\) and \(c_{\rm B1C}(t)\) are the PRN codes for the B1 CD and B1 CP components, and \(D_{\rm B1C}(t)\) and \(D_{\rm B1C}(t)\) are the navigation databit or the secondary code modulated to the B1 CD and B1 CP components. Note that the navigation databits and the secondary codes are extracted from the direct signal processing channel. Depending on the power ratio of the data component to the pilot component, the coherent combination can improve the SNR of the reflected signal by 1.25-3 dB with respected to only using the pilot component.
* Due to the limitation of the processing time and storage capability, only the complex waveform corresponding to the zero Doppler bin are included in the data products. Nevertheless, the software receiver is also capable of generating the complex DDM with configurable delay and Doppler ranges and resolutions. Moreover, the complex waveforms are generated only with the forward scattering configuration, i.e., corresponding to the signal reflected around the specular point. However, the waveform or DDM can be also generated for the signal scattered from other directions (e.g., backscattering in [48]) or staring at fixed surface regions (e.g., in [34]) by only tuning the open-loop tracking model.
* The complex waveform is generated with a fixed coherent integration time of \(t_{\rm c}=1\) ms. As the complex waveform includes both phase and amplitude information, it is possible to further integrate them coherently with longer coherent integration time. Such configuration can facilitate to characterize the coherence of the reflected signal at different surface conditions and different bistatic geometry (e.g., elevation and azimuth).
* The above-mentioned processing scheme is only for the processing of the reflected signal at one single frequency band. For the direct and reflected signal from the second frequency band, e.g., GPS L2 signal collected by SPIRE RO satellites and BDS-3 B1I signal collected by the BuFeng-1 satellites, the same processing scheme is applied by only changing the PRN code and carrier frequency parameters. For the processing of multi-frequency GNSS-R data, the complex waveforms from different frequencies are synchronized by using the same start time in (1) for the cross-correlations.