## 5 Conclusions  
This paper constructs a coherent integration time optimization model from the perspective of signal processing to improve iGNSS-R ocean altimetric precision. The research mainly involves three aspects: the processing of IF data and the extraction of precision information; the derivation and verification of the coherent integration time optimization model; and the application of the validated model to airborne and spaceborne altimetry mission scenarios to predict the optimal solution for precision.  
(1) To estimate the SP delay for airborne data processing, we use the DER method. The altimetric precision is evaluated based on the random characteristics of the SSH. (2) For the purpose of optimizing the precision performance, we consider the influence of the correlation between waveforms on the covariance of the power waveform, and derive the coherent integration time optimization model from the statistical properties of the waveform. The model is verified by airborne measurements. Results indicate that the average deviation between model and measurement precisions is 0.16 m, which is acceptable. (3) Based on the optimal coherent integration time of the model solution, we processed the experimental data, and the precision is improved by about 0.1 m relative to that obtained using the empirical parameters. The positive validation of the coherent integration time optimization model provides a tool for evaluating the optimal coherent integration time and its precision under different orbital altitudes, sea surface roughness, elevation angles, and antenna gains in spaceborne iGNSS-R altimetry scenarios. The results show that the optimal coherent integration time for the spaceborne scene is shorter than that for the airborne scene. In the case of low SNR, the model can improve the precision to the decimeter level. Due to the combined effect of the SNR and the Doppler bandwidth, the optimal coherent integration time increases with the orbit altitude and the sea surface roughness.  
The coherent integration time optimization model developed in this paper is capable of optimizing the waveform processing parameters and improving the altimetric precision. It can be applied to the optimization of data processing and high-precision retrieval for future spaceborne iGNSS-R altimetry missions. In future research, we will also analyze and evaluate the influence of the calibration errors of the instrument, the receiver orbit, and the atmosphere on the precision.  
Writing--original draft, X.S.; Writing--review and editing, [PERSON], F.W. and [PERSON]; [PERSON], [PERSON] and [PERSON] contributed equally to this paper. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China under Grant (41774014, 41574014), the Liaoning Revitalization Talents Program under Grant (XLYC2002082), the Frontier Science and Technology Innovation Project and the Innovation Workstation Project of Science and Technology Commission of the Central Military Commission under Grant (085015), the Independent Research and Development Start-up Fund of Qian Xuesen Laboratory of Space Technology (Y-KC-WY-99-ZY-000-025), and the Outstanding Youth Fund of the China Academy of Space Technology.  
Not applicable.  
We would like to thank the Institute of Space Sciences (ICE, CSIC) and the Institute for Space Studies of Catalonia (IEEC) for providing the raw data processing results of the airborne experiment. We would also like to thank IGS for providing SP3 precise orbit documents.  
The authors declare no conflict of interest.