A New Method to Make Ionospheric Delay Corrections in SBAS for GPS and Compass Dual Constellations

S. Wang, B. Zhu

Abstract:	In these years, several countries have been developing Satellite Based Augmentation Systems (SBAS) to improve positioning accuracy and integrity of Global Positioning System (GPS). With the development of Global Navigation Satellite System (GNSS), China is also accelerating his Compass satellite system, which will operating together with GPS. Thus SBAS for dual constellations is becoming an inexorable trend. As the largest error source for positioning solution in these systems, ionospheric delay is also an important component affecting the system availability. Therefore in this paper, we will propose a new method to make more accurate Grid Ionospheric Vertical Delay (GIVD) and tighter Grid Ionospheric Vertical Error (GIVE) in SBAS under GPS and Compass dual constellations, which will give rise to system accuracy and integrity. Under Compass constellation, the most significant characteristic for ionospheric delay correction is that ionospheric pierce points (IPPs) formed by GEO satellites are motionless, thus the IPPs in an update period could just reflect the ionospheric temporal variation. Since Kalman filter is the minimum mean square error estimation, it is adopted to calculate the GIVD and GIVE at IGPs. Through the simulated data as well as measured data from Crustal Movement Observation Network of China(CMONOC), we can conclude that the GIVE is obviously decreased by means of the new method, simultaneously the accuracy of ionospheric delay correction is more or less improved, both of which contribute to system accuracy and availability. Five sections are included in the paper and the corresponding contents are as follows: In section 1, we give the background information about SBAS and its ionospheric delay correction theory for single GPS constellation. When both GPS and Compass are available, modeling of ionospheric effects is also one of the major challenges in developing precise and reliable SBAS. At that time, more IPPs will be formed on the ionospheric thin shell, which will contribute to the accuracy and integrity of ionospheric delay correction. Specially, there will be 5 GEO satellites in Compass, which will contribute to some stationary IPPs. Thus how to generate more accurate ionospheric delay correction as well as tighter error bounds under dual constellations will be of significance. In section 2, firstly, we make a correlation analysis between IPPs, then it is treated as the weighted function in planar fit method, finally we propose the Kalman filtering method for GIVD and GIVE estimation at ionospheric grid points (IGPs). In order to estimate the ionospheric delay at IGPs, the ionosperic delays at IPPs should firstly be modeled. Thus firstly we should research on the characteristic of ionospheric delay. According to the work done for WAAS, we make spatial correlation analysis on the ionospheric measurements taken by the SBAS reference stations in China which are equipped with dual frequency receivers. Then based on the analysis result, we take a planar fit model on the IGPs at each sampling time point in an update interval. Since some IPPs formed by GEO satellites in Compass are motionless, for each IPP serial formed by the same satellite and reference station in an update interval, we make a filtering process to calculate the GIVD and GIVE at IGPs. In addition that both GIVD and GIVE calculated through planar fit method are treated as the initial value for the Kalman filtering. In section 3, the simulation and corresponding results are shown. In this paper, we use the Klobuchar model as the ionosphere delay over Chinese Mainland. The slant delays are computed by a distribution of 27 reference stations spreading over China. We do the chi-square test for each IGP that has been calculated, if the statistical value is below the threshold, the planar fit values are set to be the initial values in Kalman filter. After that, we proceed the filtering for 5 minutes with 30 seconds interval, get the GIVD and GIVE at each IGP. From the simulation we can see that the GIVE in filtering method bound the ionospheric error tighter and smoother than planar fit method at each IGP; the accuracy between the two methods is not as significant, the former does a bit better than the latter. In order to test the filtering method proposed in the paper, we also do experiment with actually measured data in section 4. Up to now the data from Compass can not be available, therefore we could only do the test with observation data from GPS satellites. We treat the 27 reference stations in CMONOC as the reference network in SBAS, and process the dual frequency observation data to get the original ionospheric delay value both in quiet and stormy days. Then the planar fit method and filtering method are processed to generate ionospheric delay correction on IGPs. As a result, it also can be seen that the proposed method does better than the planar fit method. Section 5 is the conclusion and the potential problems we should go on study. From the statistical analysis, we can see that the correlation of ionospheric delay is looser in the day than that at night, furthermore, the correlation decreases with distance apart throughout the whole day. Through simulation and observation data, it is suggested that the filtering method bound the user's ionospheric delay error tighter and smoother than planar fit method; however, the accuracy of correction between these two methods is not as significant, the former method does a bit better than the latter. Thus in the future, when GPS and Compass dual constellations coexist in the sky, no matter the ionosphere is quiet or stormy, we could use the filtering method to calculate ionospheric delay, then more accurate ionospheric delay correction and tighter ionospheric delay correction error bound can be gained, which will improve the system accuracy and availability. This could be an important recommendation to the SBAS construction in China. For the following work, we will better verify the method proposed in the paper via dual frequency observation data from Compass.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	865 - 874
Cite this article:	Wang, S., Zhu, B., "A New Method to Make Ionospheric Delay Corrections in SBAS for GPS and Compass Dual Constellations," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 865-874.
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