## 1 Introduction  
Accurately measured sea surface height (SSH) is one of the critical parameters of marine ecosystem monitoring, which is of paramount significance to applications such as fishery, oil drilling, and commercial navigation. Global navigation satellite systems reflectometry (GNSS-R) or the passive reflection and interference system (PARIS) can perform ocean altimetry as a passive remote sensing technique [1]. It has some unique advantages compared with the traditional tide gauge station and monostatic radar ocean altimetry. On the one hand, the receiver can capture multiple global navigation satellite system (GNSS) signals simultaneously, which increases the spatial coverage. On the other hand, as a novel bistatic passive remote sensing method, it has the characteristics of low cost, low power consumption, all-weather and high time revisit rate, which can make up for the shortcomings of the existing technology to a large extent. Since the technology was proposed, ground-based, air-based, and space-based altimetry experiments have been conducted successively to verify its feasibility [2; 3; 4]. Some signal processing techniques for GNSS-R ocean altimetry have been proposed. Conventionally, GNSS-R altimetry involves cross-correlating the reflected signals with the local replicas of the open navigation signals (GGNSS-R) [5]. The encrypted signal (e.g., P(Y)) has a broader bandwidth, which will bring a sharper autocorrelation function (ACF) and a higher range precision. With it, the reconstructed code GNSS-R (r-GNSS-R) altimetry improves the precision by reproducing the encrypted signals [6]. The interferometric altimetry (iGNSS-R) utilizes the full composite components of the direct signal to correlate with the reflected signal [5]. The partial interferometric GNSS-R (piGNSS-R) altimetry extracts a portion of the encrypted signal from the full composite components to obtain a sharper ACF, which is an extension of iGNSS-R [7]. Nevertheless, in the case of iGNSS-R and piGNSS-R, the loss of signal-to-noise ratio (SNR) caused by dual-channel noise is more severe than that of cGNSS-R, which leads to a two-fold improvement in precision only [8]. The SNR should be improved by increasing the signal processing time, as this will diminish the influence of thermal noise and speckle noise on altimetric precision [9]. However, a longer integration time corresponds to a longer specular point (SP) movement distance. Considering the moving speed of the sub-satellite point (km/s) and the size of the spatial footprint (\(\sim\)10 km), the actual inverted single height represents the average over the larger sea area which further reduces the along-track resolution [10]. The previous study has shown that in the spaceborne iGNSS-R altimetry scenario, in order to achieve an altimetric precision better than 20 cm, the signal processing time required for a single height retrieval is approximately 10 s, and the along-track spatial resolution is about 65 km [8]. If the iGNSS-R ocean altimetry with high precision and high along-track spatial resolution can be realized, it would provide critical data and information resources for small and medium scale ocean phenomena monitoring, high temporal and spatial resolution ocean gravity field model establishment, and other earth science research. One of the data products of GNSS-R observation is the delay-Doppler maps (DDM). [PERSON] et al. detailed the corresponding relationship between the DDM and its spatial location [11]. The waveform corresponding to zero Doppler is extracted from the calibrated DDM waveform for GNSS-R code-delay altimetry, and the delay information of the reflected signal is extracted from this waveform. Commonly used retrieval algorithms include the peak point of the first derivative (DER), the peak of the waveform (MAX), and the half-power point (HALF) [8; 12; 13]. Obtaining the inversion delay requires correction of numerous parameters including the receiver and transmitter orbits, ionospheric errors, tropospheric errors, and antenna baseline offsets [14].