## 4 Retrieval of Sea Surface Height
### Retrieval of Sea Surface Height Using Carrier-Phase Measurement
Table 4 summarizes the key contents of the two methods for sea surface height retrieval (i.e., code delay and carrier phase) in the relevant literature mentioned above. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Retrieval Method** & **Reference** & \begin{tabular}{c} **Satellite (Airborne)** \\ **Data Used** \\ \end{tabular} & \
\begin{tabular}{c} **Reference** \\ **Surface/Data** \\ \end{tabular} &
\begin{tabular}{c} **Result (Accuracy)** \\ \end{tabular} \\ \hline Carrier phase & [43] & CYGNSS & DTU18 MSS & The altimetric results show that the measurement system precision is 3/4.1 cm (median/mean) at 20 Hz sampling, cm level at 1 Hz, comparable to dedicated radar altimeters. The combined precision, including systematic errors, is 16/20 cm (median/mean) precision at 50 ms integration (a few cm level at 1 Hz). & Among the three retracing methods, the VZ18 DDM method is the most superior. When averaging for 1 s, the VZ18 DDM method is based on single-sample and Gaussian smoothing with standard deviations of 5.8 m and 1.9 m, respectively. & Compared to the mean sea surface topography, the surface height residuals are found to be 6.4 m, 1\(\sigma\) with a 1 s integration time. \\ \hline Code delay & [26] & TDS-1 & DTU15 MSS & The results show a good consistency with MSS model. \\ \hline \multirow{2}{*}{[103]} & \multirow{2}{*}{SPIR} & \multirow{2}{*}{SPIR, TDS-1} & With airborne data, two different delay estimators, based on the DER method and the FIT method, are assessed. By taking the information of the whole leading edge instead of that at the SP, the altimetry precision obtained with the FIT method is proven to be a factor 1.3–1.5 times better than that of DER. \\ \hline \multirow{2}{*}{[22]} & \multirow{2}{*}{CYGNSS} & \multirow{2}{*}{DTU18 MSS} & The two-way ranging precision can reach up to 3.9 and 2.5 m with 1 s GPS and Galileo group delay measurement (a factor of \(\sim\)2 better for altimetry solution), and its evolution with the signal-to-noise ratio shows good consistency with the theoretical model. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary and introduction of the key contents of two methods for retrieving sea surface height in the relevant literature. Figure 7: Flow chart of coherent GNSS-R signal carrier-phase-based altimetry retrieval [108]. By utilizing the raw IF data of spaceborne GNSS-R for sea surface carrier-phase height measurement, we can find that carrier-phase measurement is of great help in retrieving high-precision sea surface height. However, there is currently little research on using this technology to retrieve sea surface height. Research on using carrier-phase measurement to retrieve sea surface height still has a long way to go, because the available data is limited and the coherence of reflected signals is affected by the ocean environment. Even under calm sea surface conditions, the availability of data is limited. In the future, the development of spaceborne GNSS-R missions specifically designed to carry out this work is highly likely to study some submesoscale ocean phenomena in different regions of the Earth [43].