Novel Methods For Improving Ionospheric Delay Correction In Orbit Determination With Single-frequence Space-borne GPS Observation

With the development of aerospace science and technology,minisatellites with dozens of merits such as low cost,light weight,good performance,short development cycle,fast,flexible,etc,have received increasing attention from home and abroad.Within 10 years,the number of minisatellite launched is more than 500,and most of them carry significant scientific mission such as atmospheric remote sensing,gravity probe,sea altimetry,etc.Because of the constraints of satellite volume,load,cost,etc,minisatellites can only be configured with a single frequency GPS receiver.Correspondingly,real-time orbit determination using single frequency space-borne GPS receiver is usually the most prevailing way to control and monitor minisatellites.For space-borne GPS real-time orbit determination systems,Ionophere is one of the most critical source errors that worsen the precision of space-borne single-frequency real-time orbit determination(SATODS-S).Thus,it has a great significance for improving the orbital precision by studying how to eliminate or reduce the effection of ionosphere.Given this,some methods for improving the ionospheric correction in SATODS-S were studied in this paper.The contrete contents as well as main contributions are summarized as follows:1.After introducing the basic theory of real-time orbit determination,the key problems were discussed including mapping function,effective height,scale factor in ionospheric models,which were applied in SATODS-S.Then,an optimal combination of the former two was determined through simulation experiments.Finally,in terms of the inaccuracy of scale factor calculated by Chapman function,a novel method for estimating scale factor in Kalman filter was proposed and this method was tested by simulation experiments.The results show that the orbital precision using proposed method is better than that using Chapman by 0.1 to 0.3m under the same condition that precise ephemeris and global ionosphere maps(GIM)were used.2.In light of the fatal problems exiting in ionospheric model,a three-dimensional model named NeQuick2 which is applied in GALILEO system was introduced.After presenting the basic algorithm of NeQuick2,the precision of NeQuick2 model from 2005 to 2017 was evaluated by setting GIM as well as GPS observation as references and carrying out simulation experiments using GPS observation from different LEO.The results indicate that the precision of NeQuick2 model is generally better than modified Klobchar model by approximately 2TECU,and when LEO's heights are below 500 km,the orbital precision using NeQuick2 is better than that using modified Klobchar model by 0.2m,which is nearly as high as that using double-frequency pseudorange measurements.3.For the issue that F10.7 aquaired from NASA GSFC can not reflect the intensity of ionosphere accurately,a novel method that calculates optimal F10.7 by absorbing GIM was presented.Then,precision of NeQuick2 using optimal F10.7 was assessed.The results suggest that the precision of NeQuick2 using optimal F10.7 is better than that using released F10.7.In addition,the higher Solar activity parameters are,the more significant improvement is.4.Since the limited precision of NeQuick2 and unstable orbital results by GRAPHIC method due to cycle slip,a new method was proposed that used carrier phase to improve differential ionospheric variations.Then,its effectiveness was assessed by simulation experiments using space-borne observation data in 2002.The results demonstrate that the orbital precision by this method is nearly 1m,which is better than that using NeQuick2 by 0.2m.The reseach regarding real-time orbit determination using single frequency space-borne GPS receiver could contribute to the development of earth observation and low-altitude detection industry in our country.