# Preliminary Validation of Surface Reflections from Fengyun-3C Radio Occultation Data
## 3 Methods and Results
### FY-3C Radio Holographic Analysis  
The reflected signals of GNSS RO events always present different frequency spectra than direct signals [11; 13; 18; 22]. Radio holographic analysis is still the main method to identify reflections in RO events [26]. After identification of the portion of reflected signals, a detailed analysis can help determine the atmospheric and surface properties [13]. The FY-3C RO wave field was expressed as follows [13]:  
\[E(t)=A(t)e^{i\varphi(t)} \tag{3}\]  
where \(A(t)\) is SNR on L1 channel, measured amplitude, \(\varphi(t)\) is carrier phase measurements and \(t\) is time instant. Besides the observed wave field \(E(t)\), we require the reference wave field [13]:  
\[E_{m}(t)=e^{i\varphi_{m}(t)} \tag{4}\]  
where \(\varphi_{m}(t)\) is the smoothed fit to \(\varphi(t)\). Then, the sliding wave field is given as the following function [13]:  
\[\Delta E(t)=A(t)e^{i(\varphi(t)-\varphi_{m}(t))} \tag{5}\]  
After that, Fourier Transform (FT) analysis is applied to the \(\Delta E(t)\). Since the direct signal in the observed wave field of FY-3C is large enough to dominate the sliding wave field, the spectrum of the direct single is almost at zero frequency, and the spectrum of reflected signal can be identified in the time-frequency domain. Figures 4-6 show the SNR on L1 channel, radio holographic image and geographical location of three individual FY-3C RO events over the ocean at different latitudes. The examples show two events with reflections at the beginning and, lastly, show an event without a reflection. Under radio holographic analysis, the frequency shift of the reflected signal is identifiable from the radio holographic image. These three events are distributed in different locations, and the reflections of FY-3C also show varying behaviors depending on the different SVM flag value of COSMIC events. In Figure 4, the FY-3C RO event was distributed at the ocean's surface. The SNR is above noise level during the first 10 s of this rising event. The radio holographic image shows the main spectra power near 0 Hz and a secondary branch is varied from the main spectra; after about 12 s, the power is down to the normal floor. The SVM flag value of the COSMIC RO-reflected event from the matched pair is about 1.57, which presents substantial reflection. In Figure 5, reflection happens during the last time of the setting event and the power of the SNR is evidently above the noise level for about 60-70 s. From the holographic image, there are visible negative frequency shifts between 60-70 s, which are considered as the reflected portion of the signal. This FY-3C event also presents reflection and the SVM flag value of COSMIC RO event from the matched pair is about 1.12. Figure 6 shows an unclear reflection at the end of the setting event and the SVM flag value of matched COSMIC RO event is about 1.02. It is worth noting that the reflected spectrum in Figure 6 is less significant than that in Figure 5. The FY-3C event in Figure 6 with this spectrum should be classified as no clear reflection, although the matched COSMIC event did present a clear reflection. Indeed, the minimum capture altitude of FY-3C signal in Figure 6 is below 2 km, but this does not guarantee a reflection event. Therefore, it seems to relate to instrumental problems rather than the ocean surface's roughness. Both FY-3C and COSMIC receivers have open-loop tracking capability in the lower part of the troposphere [9,32]. With the open-loop tracking technique, the tracking ability for rising RO event is much improved [37]. FY-3C and COSMIC receivers adopt a 100 Hz open-loop sampling rate and a 50 Hz open-loop sampling rate, respectively [38, 39, 40, 32]. In other words, the FY-3C receiver is as capable as the COSMIC received for capturing detailed atmospheric information at a lower height. Furthermore, under the similar objective condition of the matched pairs, FY-3C receivers can capture reflected signals on the surface, similarly to COSMIC receivers.