## 3 Research Status of the Multipath Error Data Processing Algorithm
### Multipath Error Mitigation Method Based on the Function Model
#### 3.2.2 Multipath Error Mitigation Method Based on the Observation Domain
The experimental results showed that the performance of KF-RTSS was better than that of the wavelet technique and EMD. [PERSON] and [PERSON] [66] proposed a general and robust three-stage wavelet denoising method for repetitive time-based carrier-phase multipath filtering in relative positioning. [PERSON] et al. [67] used the satellite elevation as the constraint information of the wavelet transform to extract an accurate multipath error model and then used SF for multipath error correction. The space part of the BDS system consists of GEO, IGSO, and MEO to form a hybrid heterogeneous constellation. There are many high-orbit satellites in BDS that have a strong anti-blocking ability. Due to the unique constellation design of BDS, many scholars havefound that the multipath effect characteristics of BDS satellites are significantly different from those of other systems; thus, considerable research has been conducted. [PERSON] et al. [68] proposed a real-time multipath correction algorithm for IGSO and GEO satellites, called code noise and multipath correction (CNMC), which reduces the influence of multipath errors, thus ensuring the performance of the system. [PERSON] et al. [69] applied the sidereal filtering method based on the observation domain to BDS and considered the repeat period of each satellite independently, successfully weakening the BDS carrier-phase multipath error. The results showed that the combined system of GPS and BDS has a higher reliability and higher accuracy than a single system. [PERSON] et al. [70] analyzed the ambiguity fixation rate of BDS and GPS in different resolution modes and weakened the influence of wide-lane multipath effects through the sidereal filtering method in the observation domain. [PERSON] et al. [71] analyzed the multipath repeatability of three types of BeiDou satellites in different orbits by calculating the residual sequence of double-difference observations, and on this basis, studied the influence of multipath effects on the accuracy of the BDS static baseline solution. The results showed that, in a strong multipath environment, the impact of BDS multipath errors on its static solution reaches the centimeter level. [PERSON] et al. [72] used SF based on the coordinate domain to weaken the multipath error of the BeiDou satellite deformation monitoring data, which effectively improved the accuracy of the plane and elevation directions. Based on single-difference residual reconstruction and wavelet decomposition, [PERSON] et al. [73] proposed a BDS system multipath mitigation method combining sidereal filtering and wavelet. [PERSON] et al. [74] showed that, in the relative positioning of the BDS-2 and BDS-3 systems, MHM can effectively correct the multipath error of low-elevation satellites and improve the success rate of ambiguity resolution, and the positioning accuracy of the floating-point solution and the fixed-point solution can be improved. The Galileo satellite orbit repeat period is ten days, and the existing SF and MHM need a large amount of data for modeling when dealing with Galileo multipath errors, which is not applicable. The elevation and azimuth between adjacent epochs do not change considerably; thus, it can be considered that the low-frequency phase multipath effect at adjacent epochs is unchanging. Based on this, [PERSON] et al. [75] proposed a random walk multipath method (RWM), which can alleviate Galileo multipath errors and improve the localization accuracy without using historical data. [PERSON] et al. [76] proposed a least-squares collocation (LSC) method based on the covariance function, which can effectively weaken the multipath error when applied to the baseline positioning and precise single-point positioning of GPS and Galileo ([PERSON] and [PERSON] et al. (2023)). This method has a higher modeling efficiency in the Galileo system. [PERSON] et al.