2011 International Technical Meeting
Session A1: QZSS

Title: Ionospheric Delay Estimation using the Modified IDW Grid Model for MWAAS
Author(s): J. Joo, J. Cho, M. Heo, Korea Aerospace Research Institute, Republic of Korea

The Global Positioning System (GPS) is a satellite based navigation system. It provides better positional accuracy of the user worldwide at any time. In general, GPS meets the requirements of position determination at various user platforms. However, the standalone GPS cannot meet the accuracy, availability and integrity required in civil aviation, especially for precision approach (PA) landing phase and hence it should be augmented for such critical applications. The first satellite-based augmentation system (SBAS) was initiated by USA in early 1990s for providing coverage of Continental United States (CONUS) region. The European Geo-stationary Navigation Overlay System (EGNOS) developed by European Space Agency is being operational officially since 2010. The Japanese MTSAT Satellite Augmentation System (MSAS) for aviation use was already commissioned in 2007. Other countries such as Russia, China and India are also developing their own SBAS. Moreover, as per the directive of International Civil Aviation Organization (ICAO), every member country has to a primary future system for aviation. The Minister of Land, Transport and Maritime Affairs (MLTM) of South Korea is also going to begin to implement Multipurpose Wide Area Augmentation System (MWAAS) such as US SBAS, WAAS, from 2014. The objectives of MWAAS are to provide the navigation performance parameters such as accuracy, integrity, time to alert, continuity and availability to a variety of Global Navigation Satellite System (GNSS) users such as land, sea and aviation users over the Korean service region. The largest source of positioning error for single-frequency users of the GPS is typically the radio delay caused by the ionosphere. To allow the user to compensate for this error, the SBAS broadcasts both estimates of ionospheric delay and error bounds that circumscribe the uncertainty in these estimates. The nominal or quiet ionosphere is smooth and easily estimated, but unfortunately there are times when the ionosphere is more difficult to estimate, particularly during geomagnetic and ionospheric storms. Under these disturbed conditions, smaller scale features may be difficult to observe or estimate. Therefore, success of MWAAS depends on proper real-time estimation of ionospheric corrections over the Korean service area continuously. Fortunately, since South Korea is located in the mid-latitude region, above disturbed conditions such as the large diurnal and seasonal variability and intense ionosphere irregularities including equatorial anomaly conditions may be rarely occurred, the problem will be solved using thin shell approximation. Up to now, even though several analytic function based models are available for ionospheric error corrections, grid based models are preferred due to their greater estimation accuracy and less complexity. One of the ionospheric grid models used for the US WAAS is the Inverse Distance Weighted (IDW) model weighted with Klobuchar ionspheric model. In this paper, delays at the Ionospheric Grid Points (IGPs), user's Ionospheric Pierce Points (IPPs) and corresponding error bound Grid Ionospheric Vertical Errors (GIVEs) using the data from of various candidate reference stations of MWAAS are estimated by the IDW based grid model and the analysis results are firstly presented. The used simulation data are generated by the Deimos's GRANADA software simulator on analysis test scenarios considering the number of reference stations and their coordinates. 24 channel GPS signals and 27 channel Galileo signals are also generated. The accuracy of modeled vertical ionospheric delays at the IGPs, user's IPPs and GIVE are the functions of IPP density, grid size, input data and measurement update rates. Therefore, we analyze these relationships by simulation tests and propose the appropriated results of the number of reference stations, their locations and grid resolutions for a suitable regional ionospheric grid model of MWAAS using the relation analysis results. Secondly, in this paper, we also proposed a modified IDW estimation method using adaptable cut-off radius for MWAAS. The Klobuchar time delay model is considered with IDW model to take into account the temporal variations of the IPP delays. This model estimates the vertical delays at the selected IGP by measuring the delays of surrounding IPPs and computing their weights. According to IDW, the weight will be more to an IPP nearer to the IGP as compared to the father IPP, which has minimum correlation or nearly no correlation on the IGP delay. The contribution by distant IPPs is very less. Therefore, the IPPs lying within a specified maximum distance centering the IGP are only considered in the delay estimation. Because MWAAS target service area is local, smaller grid size is appropriate to improve the integrity performance of system level. When we simulated in 1 degree grid resolution condition over the Korean service region, IPP density at any IGP was 10 and less and IPP densities at other IGPs were enough to compute the delay. When the number of reference stations and their coordinate are restricted, proper IPP densities at all IGPs is required to meet the accuracy and availability performance requirements of system. Our proposed method is the function of the IPP density and its variation rate and control the cut-off radius until proper IPP density is maintained. In the simulation results, it demonstrated that the proposed scheme is able to meet the expected performance of MWAAS on the grid model with high grid resolution.

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