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Title of Journal: GPS Solut

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Abbravation: GPS Solutions

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Springer Berlin Heidelberg

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Enhanced orbit determination for BeiDou satellites with FengYun-3C onboard GNSS data

Authors: Qile Zhao, Chen Wang, Jing Guo, Guanglin Yang, Mi Liao, Hongyang Ma, Jingnan Liu,

Publish Date: 2017/02/03
Volume: 21, Issue:3, Pages: 1179-1190
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A key limitation for precise orbit determination of BeiDou satellites, particularly for satellites in geostationary orbit (GEO), is the relative weak geometry of ground stations. Fortunately, data from a low earth orbiting satellite with an onboard GNSS receiver can improve the geometry of GNSS orbit determination compared to using only ground data. The Chinese FengYun-3C (FY3C) satellite carries the GNSS Occultation Sounder equipment with both dual-frequency GPS (L1 and L2) and BeiDou (B1 and B2) tracking capacity. The satellite-induced variations in pseudoranges have been estimated from multipath observables using an elevation-dependent piece-wise linear model, in which the constant biases, i.e., ambiguities and hardware delays, have been removed. For IGSO and MEO satellites, these variations can be seen in onboard B1 and B2 code measurements with elevation above 40°. For GEO satellites, a different behavior has been observed for these signals. The GEO B2 pseudoranges variations are similar to those of IGSO satellites, but no elevation-dependent variations have been identified for GEO B1. A possible cause is contamination of the larger noise in GEO B1 signals. Two sets of precise orbits were determined for FY3C in March 2015 using onboard GPS-only data and onboard BeiDou-only data, respectively. The 3D RMS (Root Mean Square) of overlapping orbit differences (OODs) is 2.3 cm for GPS-only solution. The 3D RMS of orbit differences between BeiDou-only and GPS-only solutions is 15.8 cm. Also, precise orbits and clocks for BeiDou satellites were determined based on 97 global (termed GN) or 15 regional (termed RN) ground stations. Furthermore, also using FY3C onboard BeiDou data, two additional sets of BeiDou orbit and clock products are determined with the data from global (termed GW) or regional (termed RW) stations. In general, the OODs decrease for BeiDou satellites, particularly for GEO satellites, when the FY3C onboard BeiDou data are added. The 3D OODs reductions are 10.0 and 291.2 cm for GW and RW GEO solution with respect to GN and RN solution, respectively. Since the OODs in the along-track direction dominate the OODs reduction, no improvement has been observed by satellite laser ranging, which mainly validates the accuracy of the radial orbital component. With the GW BeiDou orbit and clock products, the FY3C orbits determined with onboard BeiDou-only data also show improvement in comparison with those determined with BeiDou GN products.Currently, the Chinese BeiDou Navigation Satellite System (BeiDou) consists of Geostationary Earth Orbit (GEO) satellites C01, C02, C03, C04 and C05, Inclined Geosynchronous Orbit (IGSO) satellites C06, C07, C08, C09 and C10, and Medium Earth Orbit (MEO) satellites C11, C12 and C14. BeiDou satellite orbits, as determined with observations from ground stations, suffer from problems in attitude control mode (Montenbruck et al. 2015), solar radiation pressure (SRP) modeling (Guo et al. 2016a), systematic errors in pseudoranges (Wanninger and Beer 2015), and geometry conditions (Zhao et al. 2013).For BeiDou IGSO and MEO satellites, which use two attitude modes, namely yaw-steering (YS) and orbit-normal (ON) mode, dramatic orbit degradation can be observed when satellites switch the attitude mode or are in the ON mode. Based on studies with the yaw attitude model for BeiDou IGSO and MEO satellites (Feng et al. 2014; Guo et al. 2016b), efforts have been made to construct a better SRP model for these satellites in ON mode and at the attitude transit epoch (Guo 2014; Guo et al. 2016a; Prange et al. 2016). Although the orbit accuracy in ON mode can be improved, the orbit quality for the orbital arc containing the attitude transit epoch is still poor. Guo et al. (2016a) further identified the deficiency of the purely empirical CODE SRP model (Beutler et al. 1994; Springer et al. 1999) for BeiDou IGSO satellites in YS mode, and proposed the box-wing model as a priori SRP model to improve the CODE SRP model. In addition to this attitude and SRP issues, Wanninger and Beer (2015) identified satellite-induced variations in code measurements, termed code biases hereafter, which limit the ambiguity resolution when using the geometry-free approach. Furthermore, they also proposed an elevation-dependent model to correct the satellite-deduced code biases.Compared with IGSO and MEO satellites, the BeiDou GEO orbits have relative poor quality as shown in Guo et al. (2016b). The main reason is that the GEO satellite ground tracks are relatively static, resulting in almost static observation geometry. Also, the ON model has been applied to GEO satellites (Montenbruck et al. 2015), but it makes the SRP acting on the satellites hard to model. Guo et al. (2016b) identified errors that noticeable depend on the orbital angle, i.e., the argument angle of the satellite with respect to the midnight point in the orbit plane, and large bias of about −40 cm in satellite laser ranging (SLR) residuals. These errors are a result of using the empirical CODE SRP model. Liu et al. (2016) reported that GEO orbits could be improved by estimating six parameters instead of the typical five parameters of the empirical CODE SRP model. With this model, the bias of SLR residuals is only about −7.8 cm for 28-day solutions. However, no significant improvement has been observed after incorporating the model into the Position And Navigation Data Analyst (PANDA) software (Liu and Ge 2003). Although elevation-dependent satellite-induced code biases were identified for IGSO and MEO satellites (Wanninger and Beer 2015), such biases cannot be seen for GEO satellites using ground tracking data only. The cause is mainly the relative static geometry between GEO satellites and ground stations. Hence, it should be possible to improve the GEO orbits with onboard BeiDou tracking data from Low Earth Orbiters (LEOs), since the relative movement between GEO and LEOs results in the desired rapid change of observation geometry.The idea of overcoming GNSS POD weakness due to small number of ground stations, poor distribution of ground stations, and poor geometry condition using LEOs onboard tracking data has been assessed previously (Geng et al. 2011; Zoulida et al. 2016). When onboard GPS data of GRACE A are combined with data from 43 ground stations, the 1D GPS orbit differences with respect to the IGS final orbit decreased to 5.5 from 8.0 cm (Geng et al. 2011). Also, when more LEOs are used, less ground stations are needed to achieve similar orbit accuracy as obtained without LEOs onboard data (Geng et al. 2011). The LEO onboard GPS data can also be used to estimate phase center corrections or to improve the reference frame (Haines et al. 2015). Previous research has focused on LEO onboard GPS data since there were no onboard data available for other GNSS systems. Thanks to the FengYun-3C (FY3C) satellite, onboard BeiDou data are collected and can be used for these investigations.The aim of this study is to improve the BeiDou orbits by combining ground data and FY3C onboard tracking data, called here enhanced POD, and to analyze the elevation-dependent code biases of BeiDou satellites, particularly for GEO satellites. Following the introduction of the FY3C satellite and its onboard GNSS receiver, the quality of the collected data and the multipath errors will be investigated and analyzed. Afterward, the POD for FY3C with onboard GPS-only data or onboard BeiDou-only data will be presented, followed by a section on POD for BeiDou satellites combining ground BeiDou data and FY3C onboard BeiDou data. The thus determined BeiDou orbits and clocks will be validated by overlapping orbit comparison, by SLR, and by determination of FY3C orbits.The FY3C satellite was launched on September 23, 2013, and developed by the Meteorological Administration/National Satellite Meteorological Center (CMA/NSMC) of China. This satellite is in a sun-synchronous orbit with orbit altitude and inclination of about 836 km and 98.75°. The primary mission of FY3C is scientific investigation of atmospheric physics, weather, climate, electron density, magnetosphere, and troposphere as well as stratosphere exchanges (Bi et al. 2012). A GNSS Occultation Sounder (GNOS) has been placed on the satellite to ensure that the objectives can be achieved.The GNOS instrument was developed by the Center for Space Science and Applied Research (CSSAR) of the Chinese Academy of Sciences (CAS). Three GNSS antennas, namely the PA (Positioning Antenna), the ROA (Rising Occultation Antenna), and the SOA (Setting Occultation Antenna) were installed on GNOS. The PA can track up to six BeiDou satellites and more than eight GPS satellites. The collected pseudoranges and carrier phases are used for real-time navigation, positioning, and POD of FY3C. The along-velocity viewing antenna ROA and anti-velocity viewing antennas SOA are used for rising and setting occultation tracking; however, only four BeiDou and six GPS occultations can be tracked simultaneously (Bai et al. 2014). In this study, we only used data from the PA for analysis.



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