## 2 Methodology
### Weighting Model with C/N0 for the Sun-Pointing Attitude Mode  
One of the most significant characteristics for the onboard GNSS receiver of the LEO satellite with the sun-pointing attitude mode is that the antenna boresight direction of the GNSS receiver depends on the satellite attitude mode. The GNSS antenna is installed on the same side as the solar panel and fixed to the LEO satellite. The antenna boresight direction can be defined in the satellite body-fixed frame and always points to the sun. Figure 1 shows the impact of the sun-pointing satellite attitude. The local Cartesian coordinate system with the origin at the GNSS phase center of the LEO satellite is used to calculate the nominal elevation. The up direction points towards outer space, as shown in Figure 1. The GNSS antenna boresight for ground-based applications normally coincides with the up direction, while the onboard GNSS antenna boresight depends on the satellite attitude. For the earth-pointing attitude mode, the boresight direction may be the same as the up direction. For the sun-pointing attitude mode, the boresight direction depends on the geometrical relationship between the satellite, the earth, and the sun. The motion of the LEO satellite around the earth produces a changing discrepancy between the antenna boresight direction and the up direction. The boresight direction presents a large discrepancy with the up direction, which has a strong impact on the GNSS signal. The impact depends on the antenna radiation pattern, which is illustrated in Figure 2.  
Figure 1: Impact of the sun-pointing satellite attitude on the relationship between the GNSS antenna boresight direction (green arrow) and the up direction (red arrow). Since the antenna radiation pattern is almost the same at different azimuth angles, we only present the antenna radiation pattern at azimuth zero for clarity. The red curve denotes the gain variation of signal strength as the variation of the elevation angle at azimuth zero. The radiation pattern indicates that the antenna gain decreases from the antenna boresight direction to the low-elevation angle. Consequently, the low-elevation GNSS signal has lower signal strength and larger noise than the high-elevation signal. However, this is not the case for the onboard GNSS antenna due to its sun-pointing attitude mode. The impact of the satellite attitude on the onboard GNSS signal tracking are as follows: (1) some low-elevation angle signals become trackable, and correspondingly, a portion of high-elevation angle signals becomes 'invisible'; (2) the relationship between the observation noise and the elevation angle is changed due to the radiation pattern rotation. The radiation pattern is fixed to the antenna and rotates along with the satellite platform, which changes the theoretical carrier-to-noise ratio (C/N0) of the tracked signals and changes the noise characteristics correspondingly. In this study, the satellite attitude impact on the GNSS observations is described using a suitable weighting model. Observation weighting is an essential component of the weighting model and plays a key role in reliable parameter estimation [37]. For the ground receiver, the elevation-dependent function may be reasonable to capture the observation noise characteristics, while it may not always remain valid for the onboard GNSS receivers. For example, some low-cost LEO satellites may have no rotation angle information of the antenna plane so that true elevation cannot be computed. The nominal elevation is computed relative to the local horizontal plane and the negative elevation may occur. Many observations below the elevation mask will be excluded even though they have good noise characteristics. When the boresight direction of the GNSS antenna does not coincide with the zenith direction, the weighting model based on the elevation angle becomes unrealistic for data processing. Instead, the C/N0 can always reflect the strength of the signal and can be used as a measurement noise indicator.