Research On The Retrieval And Recovery Of Beidou Real-time Precise Orbit/Clock Products

In December 2019,China Satellite Navigation Office issued "Bei Dou Navigation Satellite System(BDS)Signal In Space Interface Control Document-Precise Point Positioning Service Signal PPP-B2 b ",which indicates that BDS will provide satellitebased augumentation services to users through BDS satellites.BDS satellite-based augumentation service includes orbit/clock corrections.Users can obtain real-time high-precision satellite orbit/clock by using this service,and then realize real-time precise point positioning.In order to make full use of BDS satellite-based augumentation services,it is necessary to study BDS real-time precise point positioning.BDS real-time precise point positioning requires real-time precise orbit/clock products.Since IGS(International GNSS service)provides real-time service,only a few Analysis Centers(ACs)provide BDS-2 real-time precise orbit/clock corrections.At present,the research on BDS real-time orbit/clock correction is still limited.Several problems related to the usage of BDS real-time orbit/clock corrections remain unsolved.Two major problems are listed below: 1)Due to the unstandarlized encoding method for BDS real-time orbit/clock corrections,users cannot be compatible with the realtime products of Beidou of different ACs.2)Due to the outages of BDS real-time orbit/clock corrections caused by network interruption,users have trouble in obtaining precise real-time orbit/clock corrections.In order to solve these problems,some effort has been made and the corresponding conclusions are obtained as listed below:(1)The Ntrip(Networked Transport of RTCM via Internet Protocol)protocol is integrated with RTCM(Radio Technical Commission for Maritime services)message decoding module to design and implement a BDS real-time orbit/clock correction receiving and decoding software.The software is applicable to BDS real-time orbit/clock corrections provided by IGS ACs.The BDS real-time precise orbit/clock corrections provided by Wuhan University(WHU),Deutsches Zentrum für Luft-und Raumfahrt(DLR),Geo Forschungs Zentrum Potsdam(GFZ)and Centre National d'Etudes Spatiales(CNE)were then analyzed for 11 consecutive days.In terms of the products outages,WHU is the highest with 8.9% outages.DLR is the lowest with 1.6%outages.GFZ and CNE are with 1.8% and 2.8% outages,respectively.It should be metioned that DLR does note provide GEO satellite precise products,CNE does not provide C16 sa In terms of orbit accuracy,the average real-time orbit accuracy of GEO satellites provided by three analysis centers follow the order from high to low: CNE(2.423 m),WHU(2.506 m),GFZ(3.176 m).For IGSO/MEO satellites,the order turns to: CNE(0.296/0.285m),GFZ(0.319/0.357 m),WHU(0.804/0.487 m),DLR(1.077/0.521 m).In terms of clock precision,the average real-time clock precision of GEO satellites provided by three analysis centers follow the order from hight to low:CNE(1.12 ns),WHU(1.46 ns),GFZ(1.65 ns).For IGSO/MEO satellites,the order turns to: CNE(0.32/0.40 ns),WHU(0.51 / 0.49 ns),GFZ(0.55/0.93 ns),DLR(2.53/1.96 ns).(2)A simplified BDS real-time orbit/correction and broadcast ephemeris matching method is proposed.The method enables users recover BDS precise orbit/clock products provided by different AC.Compared with the existing matching method based on IODCRC calculation,the proposed matching method improves the recovery efficiency by 4.50-10.84% under the condition of ensuring the same number of recovery epoch.(3)A set of BDS real-time orbit and clock correction prediction method is proposed which can be adpted during real-time correction outages.For the BDS realtime orbit/clock correction,linear prediction model and the polynomial prediction model considering the periodic term are established respectively.The prediction method can keep the predicted orbit accuracy consistent with the actual real-time orbit within one hour.The IGSO/MEO orbit accuracy difference between predicted and actual real-time orbit is below 10 cm.The predicted GEO/IGSO/MEO clock precision is about 1.32/0.96/1.00 ns within one hour.(4)The BDS real-time orbit/clock products provided by GFZ are applied to the real-time precise point positioning.Static and kinematic solutions of 47 MGEX stations in the Asia Pacific region were carried out.An average of 1.2-hour convergence time is required for 20/50 cm horizontal/vertical accuracy in static mode.The static solutions show that the average precision of E/N/U/3D is 4.7/2.9/8.7/10.5 cm after convergence,respectively.An average of 2.1-hour convergence time is required for kinematic mode.The kinematic solutions show that the average precision of E/N/U/3D is11.6/8.5/27.1/31.1 cm after convergence,respectively.