## 4 Retrieval of Sea Surface Height  
### Code Delay Sea Surface Altimetry  
Code delay is an important concept in GNSS positioning systems, which refers to the difference between the signal received by the receiver and the expected arrival time. This difference may be caused by various factors, such as atmospheric delays, receiver and satellite clock errors, and changes in signal propagation paths. Code delay sea surface height measurement is achieved by measuring the round-trip time of signals (i.e., code delay), calculating the distance of the signal propagation, and then estimating the sea surface height. In spaceborne GNSS-R applications, sea surface height measurement is not a specialized task, so the code phase height measurement method based on cross-correlation processing is widely used, with an accuracy of about 9 m [101]. However, using the raw IF signal of spaceborne GNSS-R can obtain results with a higher temporal resolution and trajectory direction spatial resolution [102]. In order to study the delayed sea surface altimetry performance of the raw IF signal code of spaceborne GNSS-R, [PERSON] et al. [103] established a marine altimetry accuracy evaluation model in 2018 by analyzing the statistical characteristics of GNSS-R measurement waveforms. The study explored the relationship between height measurement accuracy and waveform statistical characteristics and derived a statistical characteristics analysis model for noncoherent average waveforms. Using the GPS L5 signals obtained on board, a comparison was made between two delay estimators, based on leading derivative (DER) and waveform fitting (FIT), verifying that the height measurement accuracy of the FIT method was 1.3-1.5 times higher than that of DER. The influence of nonlinear processes (such as square sum noncoherent averaging) on the covariance of complex waveforms was derived, and the covariance model was extended to noncoherent averaging waveforms. Slow-time correlation was analyzed, and the statistical model was validated using airborne SPIR and TDS-1 satellite data, demonstrating a reasonable consistency (within \(\sim\)10%) between experimental and modeled waveform covariance. The article pointed out that in the parameter space of waveform covariance, it is necessary to consider the influence of significant wave height and non-Gaussian ocean parameters. In future work, it is also necessary to study the waveform statistics of all or partial coherent reflections on smooth surfaces (such as inland water bodies, sea ice, and wetlands) and their impact on height measurement accuracy. In order to consider the influence of more factors in the measurement of sea surface height due to code delay, [PERSON] et al. [37] evaluated the performance of spaceborne GNSS-R in ocean altimetry by analyzing the GNSS-R data of TDS-1. The study considered various influencing factors such as orbit, ionospheric, and tropospheric delays and compared them with the average sea surface topography. The results indicated that although the measurement accuracy of 1 s of integration time was limited, the TDS-1 dataset experimentally demonstrated the feasibility of spaceborne GNSS-R height measurement, with a residual surface height of 6.4 m. The study discussed performance-limiting factors and provided recommendations for future GNSS-R altimetry missions for observing mesoscale ocean circulation. Hardware and software performance can be improved to accurately determine and calibrate trajectories. GNSS-R altimetry missions are expected to provide more accurate oceanographic observations in the future. Through hardware and software improvements and precise orbit determination and calibration, future spaceborne GNSS-R missions are expected to provide more accurate oceanographic observations. In 2019, [PERSON] et al. [22] conducted a study using raw data collected from the CYGNSS constellation to conduct an in-depth analysis of the performance of spaceborne GNSS-R technology in ocean altimetry.