Quality Analysis Of GPS Data Onboard GRACE And GRACE-FO And Their Precise Orbit Determination

As the gravity formation flying spacecraft jointly developed by National Aeronautics and Space Administration(NASA)and German Research Centre for Geosciences(GFZ),the Gravity Recovery and Climate Experiment(GRACE)and GRACE Follow-On(GRACE-FO)satellites adopt the same K-band ranging(KBR)system and orbit design in order to measure the Earth's gravity field information.Different from the BlackJack receiver onboard GRACE,a TriG GNSS receiver is used in the GRACE-FO satellites.The orbit determination strategy of GRACE is very mature,but for GRACE-FO there is no comprehensive assessment of the orbit accuracy.We discuss the quality of Global Positioning System(GPS)data and effects of phase center variation(PCV)model and attitude for GRACE and GRACE-FO,and the main research contents and conclusions are as follows:(1)An EEM(Enhance Error Method)for detecting the small phase cycle slip of space-borne GPS is proposed.Compared with the traditional TurboEdit method,it is found that EEM has more outstanding ability to detect cycle slips of GPS satellite signals.Considering the number of GPS observations,the detection results show that the occurrence of cycle slips of GRACE-FO satellite is only slightly different from that of GRACE.(2)The quality of space-borne GPS data for GRACE and GRACE-FO satellites is compared and analyzed,and the similarities and differences between the errors such as IOD and multipath effect are found.Although the observation arcs of GRACE and GRACE-FO are different,there is no significant difference in the terms of the rate of IOD change.The cycle slip is further verified by TurboEdit method,the fast-varying IOD indicates the occurrence of a cycle slip.The mean difference between GRACE-A MP1 and MP2 is largest,which can reach 23 cm.The MP1 of GRACE-FO is more susceptible to low elevation angle.Moreover,several quality indicators of GPS observations are summarized,and it can be concluded that both GRACE and GRACE-FO satellite receivers perform well.(3)The PCV corrections of GRACE and GRACE-FO satellites are estimated by residual method.It is found that the influence of signal crosstalk error on Trig receiver is less.When the occultation receiver of GRACE-D is used to detect atmospheric temperature and humidity information,the space-borne GNSS signals will not be disturbed greatly.Hence,the multipath errors of GRACE-C and GRACE-D are similar.(4)The effects of PCV corrections,attitude data and gravity field model on the GRACE and GRACE-FO orbits are analyzed,and the precise orbit determination strategy of GRACE-FO satellite is optimized.After using the in-flight PCV corrections,the RMS errors of kinematic and reduced-dynamic(RD)orbit determination solutions decrease by 0.4-0.5 mm and 0.6-0.9 mm,respectively.External validation result for RD orbits confirms the reliability of the GPS observations and orbit solutions.The nominal attitudes are estimated by LEO satellite position and velocity vectors,and the mean differences between orbits using nominal and measured attitudes are similar.Hence,it can be confirmed that nominal attitudes can be used in precise orbit determination of GRACE-FO satellites.The influence of different types and orders of gravity field models on orbits is further studied.It is found that the EGM2008 and EIGEN-6C4 models with over 110 degrees and orders are more suitable for GRACE and GRACE-FO RD orbit determination.