Precise Orbit Determination And The Earth Gravity Field Recovery By Acceleration Approach For Swarm

CHAMP,GRACE and GOCE satellites had been successfully launched into space.They provide important survey methods for low and middle order global gravity field and time-variable signal detection,especially the GRACE satellite,which made great contributions to detect the Earth time-variable gravity signal.The CHAMP and GOCE satellite missions had been completed,the GRACE satellite has been extended service,which may be terminated at any time,while the GRACE Follow-On is expected to launch in 2018,in the meantime,Swarm regards as an only satellite gravity survey plan,which will fill the gap of satellite gravity survey,and contribute to the refinement of the Earth gravity field model and time-variable gravity field research.Precise Orbit Determination and the Earth Gravity Field Recovery by acceleration approach for Swarm are deeply studied,the accuracy of Swarm kinematic and reduced-dynamic orbits can reach centimeter level,then we use acceleration approach to recovery the Swarm gravity field model.The main contents of this paper are as follows:The coordinate system and time system used in satellite precise orbit determination and gravity field recovery are deeply studied.The coordinate system conversion and time system conversion used in the satellite precise orbit determination and gravity field recovery are realized.The related theories and methods of precise orbit determination for LEO are introduced.In-depth study of LEO GPS observation model,space-borne dual-frequency GPS zero-differenced data quality control,dynamic model,least square estimation principle and orbit evaluation method et al.The above contents lay a good theoretical foundation for the next Swarm precise orbit determination and accuracy assessment.Precise orbit determination for the Swarm satellites based on Swarm space-borne GPS dual-frequency carrier phase and pseudo-range observation is proposed.In order to avoid solving ambiguity parameters.First of all,we use Swarm space-borne dual-frequency P-code ionosphere-free combination to compute Swarm orbit.Then we use Swarm space-borne dual-frequency carrier phase ionosphere-free combination to compute the phase difference between two adjacent epochs,we get the orbit difference between the two adjacent epochs of the Swarm satellites.Last but not least,we combine the pseudo-range with phase difference between two adjacent epochs to obtain Swarm kinematic precise orbit.The experimental results show that even if the ionospheric activity is high,the RMS of GPS phase observation residual for Swarm is about 5 mm level;compared with the reference orbit,the RMS of radial orbit difference is about 4-5 cm level,the RMS of tangential orbit difference is about 3-4 cm level,the RMS of normal orbit difference is about 3-5 cm level,the RMS of SLR residuals for Swarm kinematic orbit is about 4-5 cm level.In the process of Swarm reduced-dynamic orbit determination,the gravity field model directly affects the reduced-dynamic orbit determination accuracy for Swarm satellites.According to the type and order of the gravity field model,the influence of gravity field model on the Swarm reduced-dynamic orbit determination is studied.The results show that the high-precision gravity field model can effectively improve the Swarm reduced-dynamic orbit determination,when the order of gravity field model is lower than 30,the accuracy of Swarm reduced-dynamic orbit determination is very low,which is decimeter level,when the order of gravity field model is high,the accuracy of Swarm reduced-dynamic orbit determination is high and stable.During the period of Swarm reduced-dynamic orbit determination,the reasonable selection of pseudo-stochastic parameters(pseudo-stochastic pulse and piecewise constant acceleration)has a significant contribution to improve the accuracy of the Swarm reduced-dynamic orbit determination.According to the characteristics of pseudo-stochastic parameters,the corresponding design scheme is given,and the pseudo-stochastic parameters are optimized.The results show that setting the pseudo-stochastic parameter as a pseudo-stochastic pulse,the optimization of pseudo-stochastic pulse prior standard deviation is 10-6m/s,when setting the pseudo-stochastic parameter as a piecewise constant acceleration,the optimization of piecewise constant acceleration prior standard deviation is 10-9m/s2.In order to test the orbit determination accuracy of the optimized pseudo-stochastic parameters in the harsh environment,we choose Swarm satellite data during high ionospheric activity period,the pseudo-stochastic parameter is introduced to optimize the Swarm reduced-dynamic orbit determination and the analysis of accuracy.The results show that the RMS of space-borne GPS phase observation residual is about 8-9 mm level;compared with the reference orbit,the RMS of radial orbit difference is about 1 cm level,the RMS of tangential and normal orbit difference is about 1-2 cm level;the RMS of SLR residuals for Swarm reduced-dynamic orbit is about 2-3 cm level.During high ionospheric activity period,based on the Swarm kinematic orbit and reduced-dynamic orbit which is calculated in this paper,the Swarm gravity field model is recovered by the acceleration approach and the accuracy is evaluated.The results show that the Swarm gravity field model is in good agreement with the EGM2008 model in the 40 order during high ionospheric activity period.Therefore,the Swarm satellites have the full capability to detect the earth's gravity field signals,then to fill in the gaps during the absence of GRACE and before the launch of GRACE Follow on,and continue to monitor the earth's gravity field signal.