# Characterization of Inter-System Biases in GPS + BDS Precise Point Positioning  
[PERSON]  
1 College of Geomatics, Shandong University of Science and Technology, Qingdao 266590, China; [EMAIL_ADDRESS] (C.K. ); [EMAIL_ADDRESS] (Y.Z.) [PERSON]  
1 College of Geomatics, Shandong University of Science and Technology, Qingdao 266590, China; [EMAIL_ADDRESS] (C.K. ); [EMAIL_ADDRESS] (Y.Z.) [PERSON]  
2 College of Ocean Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China 2*Correspondence: [EMAIL_ADDRESS]; Tel. : +86-135-8929-5632221  
The influence of intersystem bias (ISB) cannot be ignored in multi-GNSS PPP and unreasonable modeling of ISB will adversely affect the positioning accuracy and convergence speed [9]. In order to explore a reliable processing strategy of ISB, further investigation of its characteristic is necessary. After rigorous formula derivation, the ISB estimated by PPP contains the satellite clock datum and the receiver-code biases. Many agencies providing precise products have their own processing strategy in orbit determination and clock estimation, resulting in different satellite clock datums. Moreover, new efforts are made to rethink the assumption of constant receiver-code bias, which is closely related to temperature [10]. Therefore, to analyze the characterization of ISBs, a proper way is considering different satellite clock datums in precise products, the receiver (and antenna) type and other related factors such like the ambient environment (temperature) of the receiver. Extensive studies have reported the influences of the abovementioned factors upon the ISB estimation. The receiver-code bias remarkably affects the ISB in the relative positioning [11,12]. The ISB between GPS and GLONASS is related to receiver type, antenna type and time system offset [13]. The code ISBs of BDS GEO, IGSO and MEO satellites to GPS estimated by single point positioning (SPP) are consistent [14]. The purpose of analyzing the characterization of ISBs is to model them more properly in PPP, many investigations provide their suggested strategies. A simple way is modeling the ISBs as constant process in one day [15, 16, 17, 18], while some studies model the ISB as random walk process [19]. There also exist some investigations which model the ISB as white noise in the estimation [20]. In more detail, [PERSON] et al. believes that estimating ISBs as random walk or white noise process is much more reliable in multi-GNSS PPP processing [21]. [PERSON] et al. find that the ISBs estimated with the center for orbit determination (CODE) and Wuhan University (WHU) final precise products are stable, which estimated with Deutsches GeoForschungsZentrum (GFZ) final precise products vary to a significant extent. Therefore, they recommends that, in GPS/BDS PPP, modeling the ISB as constant process when using precise products from the center for orbit determination (CODE) and Wuhan University (WHU) and modeling ISB as random walk when using precise products from Deutsches GeoForschungsZentrum (GFZ) [21]. In addition, some studies propose a short-term ISB model to predict the ISB in GPS/BDS-combined PPP [19]. To the best of our knowledge, there are few studies take the variation of the receiver-code bias into consideration. Whereas the receiver-code bias is affected by temperature and even exists a variation of ten meters. As we know, the ISB estimated by PPP contains the receiver-code bias. The question is how the variation of receiver-code bias influences the ISB. In this work, we further investigate the characterization of GPS/BDS ISBs in different scenarios. Except for estimating ISBs based on precise products from CODE, WHU and GFZ, we select the stations equipped with receivers of the same type in different areas to study whether or not the ambient environment (typically the temperature) of the receiver affects the ISB. Furthermore, the ultra-short baselines with the same ambient environment were applied to analyze the influence of receiver and antenna type in the ISB.