# The Impact of Eclipsing GNSS Satellites on the Precise Point Positioning
## 1 Introduction  
The satellite attitudes usually satisfy two basic requirements: the transmitting navigation antenna is pointing towards to the geocenter and the solar panel axis is perpendicular to the incident direction of the sun. These two requirements can be realized by the satellite Attitude Control Subsystem (ACS) using solar sensors mounted on the solar panels. The nominal yaw attitude is defined by the satellite body-fixed axes: the \(Z\)-axis is pointing towards the geocenter, the \(Y\)-axis is parallel to the rotation axis of the solar panels, and then the \(X\)-axis completes the right-handed coordinate system. Most of the time, the satellite can remain the nominal attitude. However, two singularities when satellite-earth-sun are collinear have broken down the nominal attitude, causing two 180\({}^{\circ}\) discontinuities. At this time, the satellite will experience noon maneuver, shadow crossing or post-shadow recovery until the actualyaw angle is equal to its nominal value [1]. Since the body-fixed \(Z\)-axis is always pointing to the earth, the determination of the attitude is mainly to determine the orientation of body-fixed \(X\)-axis. The attitude of GNSS satellites are the critical information for GNSS Precise Orbit Determination (POD) and PPP. The influences of the attitude mainly include three aspects: the solar radiation pressure model, the phase wind-up model and the corrections of the antenna PCO. Since the POD is beyond the scope of this contribution, only the latter two aspects are considered. When the orbit products are referred to the Center of Mass (CoM), the corrections of the antenna PCO should be required [2]. In general, the post-processing orbit products are referred to the CoM. For the real-time products, as in case of CNES real-time multi-GNSS corrections ([[https://igsac-cnes.cls.fr/](https://igsac-cnes.cls.fr/)]([https://igsac-cnes.cls.fr/](https://igsac-cnes.cls.fr/))), the values are referred to the satellite antenna phase center and only the phase wind-up corrections are needed. The wind-up corrections because of the attitude error can reach up to 10 cm. While the magnitude of the corrections of PCO because of the attitude error is closely related to the magnitude of the antenna eccentricity in X and Y direction. This error does not exist for GPS IIR-A/IIR-B/IIR-M and GLONASS-K1 satellites since their horizontal antenna eccentricity is near-zero. However, this error should be carefully considered for GPS IIF, GLONASS-M and all BDS satellites [3]. In recent years, GNSS has developed towards a new stage with more systems and frequencies [4]. Different types of satellites usually adopt different attitude control models [3]. Therefore, it is necessary to establish the proper attitude model for different satellite types. For GPS II/IIA satellites, a rigorous deterministic model, called GYM95 (GPS Yaw attitude Model-95), for noon maneuver and shadow crossing was developed [1]. During the period of the post shadow recovery, deleting 30 min of data was recommended because of the unpredictable yaw attitude. A simplified yaw attitude model, consistent with GYM95, was implemented for GPS II/IIA and IIR satellites [5]. The attitude of GPS IIF satellites was analyzed by Reverse Kinematic Precise Point Positioning (RKPPP) and the anomalous noon turn directions for small negative sun angles (\(\geq\)\(-\)0.9\({}^{\circ}\)) had been found [6;7]. The modeled and estimated yaw angles of eight GPS IIF satellites with small sun angles were compared by RKPPP [8]. It had been found that all the GPS IIF satellites with sun angles smaller than \(-\)0.7\({}^{\circ}\) had the correct noon turn directions with only two exceptions (both for SVN62). For GLONASS-M satellites, the attitude model for noon maneuver and shadow crossing was different from GPS satellites [9]. The Galileo IOV (In-Orbit Validation) satellites metadata and the official attitude law of GALILEO FOC (Full Operational Capability) satellites were released and updated on 6 October 2017 [10].