Research on the Ionospheric Delay Correction Methods in SBAS of Compass

As is known to all, GPS has revolutionized the field of navigation, surveillance and communication. However, the standalone GPS cannot meet the accuracy, availability and integrity required in civil aviation, especially for precision approach (PA) landing phase. Hence it should be augmented for such special applications. As a means of augmenting GPS, Satellite Based Augmentation Systems (SBAS) are widely developed to improve the performance by many countries, such as the Wide Area Augmentation System (WAAS) for US and Puerto Rico, the European Geostationary Navigation Overlay Service (EGNOS) for Europe, the Multi-Functional Satellite Augmentation System (MSAS) for Japan and the GPS and GEO Augmented Navigation (GAGAN) for India. Also, China is developing his global satellite navigation system named BeiDou Navigation Satellite System (BDS) or Compass as well as its SBAS. Similarly to GPS, ionospheric delay is also the largest error source in BDS positioning, thus modeling of ionospheric delay is one of the major challenges in developing precise and reliable SBAS of BDS. However, there are some difference between SBAS of GPS and that of BDS. Firstly, the height of ionospheric thin shell model defined in SBAS of BDS is 375km, while this value is 350km in SBAS of GPS. Secondly, the Ionospheric Grid Points (IGPs) definition for BDS is 2.5 degrees in latitude multiply 5 degrees in longitude, while the definition for GPS is 5 degrees in latitude multiply 5 degrees in longitude. Thus the ionospheric delay correction method for SBAS of GPS may not be suitable to SBAS of BDS, and researching for the appropriate ionospheric delay correction method for SBAS in China is necessary and of significance. In this paper, three ionospheric delay correction methods are evaluated for SBAS of BDS: the Inverse Distance Weighted (IDW) method, the planar fit method and the Kalman filtering method. According to Signal Space Interface Control Document for Open Service Signal B1I and B2I of BDS, which is published in December 2013, there are 320 Ionospheric Grid Points (IGPs) being defined on the thin shell model over China, from 70 degrees East longitude to 145 degrees East longitude and from 7.5 degrees North latitude to 55 degrees North latitude. Both Grid Ionospheric Vertical Delay (GIVD) and Grid Ionosperic Vertical Error (GIVE) are calculated by way of the three methods, upon which User's Ionospheric Vertical Error (UIVE) is evaluated. Through simulation, we can get the conclusion that the IDW method produces tighter UIVE than the other two methods under the situation that the Ionospheric Pierce Points (IPPs) formed by BDS are not as few as that of GPS; however, the number of usable IGPs calculated through IDW method is less than that calculated through the other two methods, which will decrease the system availability. Also we can see that without exactly knowing noise variance value, the planar fit method is more beneficial to system availability.TelecommunicationsEICPCI-S(ISTP)