## 5 Discussion on Potential Applications of the Data Products
### Demonstration of GNSS-R Altimetry  
Altimetry using reflected GNSS signal is one of the main potential applications of GNSS-R concept. However, there is still no dedicated GNSS-R altimetry mission in orbit. The publicly available Level 1 datasets from TDS-1 and CYGNSS are generated mainly for ocean scatterometry applications, and there are several limitations to using them for altimetry purpose. For example, the main variable for reconstructing the onboard OL delay in CYGNSS Level 1 data product, i.e., _\"add_range_to_sp\"_, is truncated to 0.1 chip numeric  
Figure 7: A demonstration of surface characterization using the coherence of the reflected signal. The raw IF data was collected by CYGNSS FM02 on 13 September 2018 over Florida, US. The GNSS transmitter is Galileo PRN 31. Each pixel along the track corresponds to a ground distance of \(\sim\)120 m. The background image is extract from Google Earth. precision (\(\sim\)30 m), which can significant increase the total altimetry error and mask the truly achievable altimetry precision. In our processing of the raw IF data sets, the OL delay model, i.e., \(\delta\rho^{\rm dr}_{\rm OL}\) in (2), is provided in full precision in the netCDF files for each complex waveform, which makes it possible to demonstrate GNSS-R altimetry. In fact, such demonstrations have been attempted by using either group delay or carrier phase delay over different types of surface, e.g., over sea ice in [24; 32], ice sheet in [43], open ocean in [26; 31; 74], and lakes in [25]. To facilitate the users' further explorations on GNSS-R altimetry using the complex waveform data products, here we explain briefly how to use the data products to generate the altimetry profiles along the specular point tracks:  
* The first step is to compute the delay of the reflected signal from the group delay or carrier phase observables. The group delay can be estimated from the incoherently averaged power waveform (i.e., \(Z^{\rm r}_{\rm inc}\) in (7)) through waveform retracking, which is to determine the position of the specular point in the waveform window. For the incoherent reflections from rough ocean, the determination of the specular point relies on proper modeling of the reflected waveform as introduced in [26]. On the other hand, for the GNSS signals reflected coherently from smooth surfaces (e.g., lakes or sea ice), the position of the specular point can be assumed to be the peak of the power waveform. Once the position of the specular point is determined in the waveform window, the group delay residual \(\Delta\rho^{\rm dr}_{\rm obs,sp}\) relative to the OL delay can be computed from the variable \"_delay_of_bin_\". The observed bistatic delay of the reflected signal \(\rho^{\rm dr}_{\rm obs,sp}\) can be computed through  
\[\rho^{\rm dr}_{\rm obs,sp}=\delta\rho^{\rm dr}_{\rm OL}+\Delta\rho^{\rm dr}_{ \rm obs,sp} \tag{10}\]  
For the coherent reflections, the carrier phase residual \(\Delta\phi^{\rm r}_{\rm obs}\) can be computed from the peak of the coherently integrated complex waveform by  
\[\Delta\phi^{\rm r}_{\rm obs}=\rm angle\Big{(}z^{\rm k}_{\rm peak}\Big{)} \tag{11}\]  
The bistatic carrier phase delay \(\rho^{\rm dr}_{\rm obs,cp}\) can be computed directly from the unwrapped carrier phase residual \(\Delta\Phi^{\rm r}_{\rm obs}\) by  
\[\rho^{\rm dr}_{\rm obs,cp}=\delta\rho^{\rm dr}_{\rm OL}+(N_{\rm L}+\Delta \Phi^{\rm r}_{\rm obs})\cdot\lambda_{L} \tag{12}\]  
in which \(N_{\rm L}\) is an unknown integer ambiguity and \(\lambda_{L}\) is the wavelength of the carrier. It is noted that the carrier phase residual \(\Delta\phi^{\rm r}_{\rm obs}\) can be noisy and with cycle slips due to multipath or the other fading effects, and some advanced filtering and processing algorithms (e.g., [75]) can be applied to improve the robustness of the phase unwrapping.