## 2 Multipath Error Principle and Mitigation Methods
### Multipath Error Mitigation Methods  
The research on multipath error mitigation methods from the station side should be distinguished between occurrences in a line-of-sight (LOS) environment and a non-line-of-sight (NLOS) environment. This paper primarily elucidates the approach to mitigating carrier-phase multipath errors from the station side in a line-of-sight environment. At present, multipath error mitigation methods can be broadly categorized into three groups: site selection, methods based on hardware improvement, and methods based on software data processing. Site selection involves opting for an open and suitable geographical location to set up a GNSS receiver [15]. The presence of water bodies with high reflection coefficients can significantly impact the multipath error when they surround the station. When there is a large water body around the station, it seriously affects the multipath error. The reflection coefficient of ground vegetation, such as grassland, is low, the reflection signal is weak, and the multipath error generated is small. The antenna suppression plate in the receiver can block most of the reflected signals from the horizontal ground, but when there are high-rise buildings or high-terrain environments around the station, the satellite signal is reflected by the vertical surface and enters the receiver, causing multipath errors. When the measuring station is set on a hillside and the slope is large, it affects the receiver's reception of satellite signals and produces multipath effects. In station selection, it is advisable to avoid surroundings with high reflection coefficients, such as water bodies and glass, and to steer clear of challenging terrains, like slopes, valleys, and urban canyons. While a judicious site selection can diminish the impact of multipath effects [16], it is subject to relatively significant limitations. The method of mitigating multipath errors through hardware enhancements encompasses two primary aspects: the antenna method and the receiver method. The antenna method is mainly used to improve the antenna in order to suppress the multipath effect. GNSS signals are right-handed circularly polarized signals, but most of the reflected signals are left-handed circularly polarized signals. The GNSS antenna is designed as a right-handed circularly polarized receiving antenna, which can effectively suppress the reflected signal, because the gain of the incident right-handed circularly polarized signal is larger than that of the left-handed circularly polarized signal [17]. [PERSON] et al. [18] proposed a choke ring technology for GPS dual-frequency signals in 1998, which can optimize L1 and L2 frequency signals. [PERSON] et al. [19] suppressed low- or negative-altitude signals by adding choke rings. [PERSON] et al. [20] proposed a new antenna technique using a compression wheel to control the receive pattern in order to reduce multipath effects in GNSS signals. [PERSON] et al. [21] proposed a choke ring. [PERSON] et al. [22] proposed a dual-polarized antenna technique using the polarization characteristics of GNSS original and reflected signals. [PERSON] et al. [23] proposed compact cross-plate reflection planes with a low multipath effect. [PERSON] et al. [24] proposed a single orthogonal dual linearly polarized GNSS technology to suppress multipath effects through dual linearly polarized antennas. The multipath suppression method based on the antenna technology has a poor multipath suppression effect on low-altitude observation satellites. The receiver method is mainly used to improve the receiver in order to suppress the multipath effect. [PERSON] [25] proposed a multipath elimination delay-locked loop technology to identify and suppress multipath signals in the carrier-phase-locked stage, thereby improving the accuracy of original observations. [PERSON] et al. [26] proposed an improved group-delay multiple amplitude phase-locked loop technology to further improve the multipath suppression effect at the receiver. [PERSON] et al.