# Solar Radiation Pressure Models for BeiDou-3 I2-S Satellite: Comparison and Augmentation
## 1 Introduction  
Five years after the official announcement of providing the positioning, navigation and timing (PNT) service around Asia-Pacific region, Chinese BeiDou Navigation Satellite System (BeiDou) has stepped into a new era recently. On 6 November 2017, two new-generation BeiDou satellites (BeiDou-3) have been successfully launched to medium earth orbit (MEO) to provide the global PNT service. Once the space segment is successfully deployed, BeiDou will possess global service capability comprising totally 35 satellites distributed in geostationary orbit (GEO), inclined geosynchronous orbit (IGSO) and MEO around 2020. In order to validate the new features of BeiDou-3 satellites, including new signals, inter-satellite link technology, and onboard frequency standards, two IGSO and three MEO experimental satellites have been launched. Up to now, the published in-orbit-validation (IOV) results have demonstrated the significant improvements of BeiDou-3 satellites w.r.t the regional BeiDou satellites (BeiDou-2). The inter-satellite link technology does not only facilitate high accuracy precise orbit determination (POD), but also enables autonomous navigation as well as improves the independency and stability of BeiDou system [1]. The quality of BeiDou-3 signals is comparable to that of GPS L1/L2/L5 and GalileoE1/E5a/E5b signals, and the elevation-dependent code biases, which have been previously identified to exist in the code observations of the BeiDou-2 satellites, seem to be not obvious for BeiDou-3 satellites, as demonstrated in [2]. These improvements ease the precise data processing of BeiDou. The stability of onboard frequency standards has been improved by a factor of 10, and can be compared to the latest type of rubidium atomic frequency standards employed onboard the GPS IIF satellites, as well as the passive hydrogen masers used onboard the Galileo satellites. A continuous yaw-steering (YS) attitude model is used to avoid the significant orbit accuracy degeneration when a satellite switches its attitude mode [3]. Thanks to the above improvements, the better POD performance has been achieved for BeiDou-3 experimental satellites, except for BeiDou-3 I2-S (PRN C32) satellite, for which an apparent linear sun elongation angle (\(\varepsilon\) angle) dependent systematic error has been observed in the satellite laser ranging (SLR) residuals. Without any doubt, the systematic error is original form the deficiency of non-gravitational force models, particularly in the solar radiation pressure (SRP) model. The similar error was also observed for European Galileo and Japanese QZSS Michibiki satellite's orbits determined with the 5-parameter extended CODE orbit model (called ECOM1 in this study) [4; 5]. To reduce the deficiency of the ECOM1 for the Galileo satellites, an a priori SRP model based on a generic box-wing model (hereafter called cuboid model) in YS mode to augment the ECOM1 was established [6]. With this a priori model, the peak magnitude of radial orbit errors was reduced from presently 20 cm down to 5 cm for Galileo IOV satellites outside eclipse phases, and similar performance has also been achieved for Galileo Full-Operational-Capability (FOC) satellites [7]. With approximate body dimensions, optical properties as well as mass, the QZSS Michibiki's a priori cuboid model has been obtained, which improves the orbit to a better than 10 cm RMS consistency with SLR measurements [8]. The reasons for that type of model are that Galileo and QZSS Michibiki satellites have a markedly elongated shape instead of a cuboid one, as well as a large area-to-mass ratio. Alternatively, the revised version of ECOM (ECOM2) also has the potential to reduce such kind of error, i.e., for Galileo and QZSS Michibiki satellites [9], although the primer aim of the model is to reduce the draconitic errors in GNSS geodetic products [10].