WAAS电离层格网播发特性及其性能评估

Broadcast Characteristics and Performance Evaluation of WAAS Ionospheric Grid

电离层格网信息决定了RTCA协议下的SBAS增强系统的服务等级。通过对WAAS 3颗GEO卫星播发的电离层格网信息进行长期分析,获得了RTCA协议下格网电离层在电文编排、更新周期、格网分布等方面的详细播发特性。通过解析实际WAAS电离层格网电文,设置不同的可用性条件,详细分析了每个格网点的可用性,得到当以GIV EI=15为门限值时,WAAS播发263个可用性为100%的电离层格网信息,覆盖包括整个北美大陆及海岸线20°以外的大部分地区。通过对比WAAS采用的Kriging和部分其他增强系统采用的IDWK插值方法的格网可用性,得出结论:IDWK方法仅对参考站布设范围内的格网点具有较高的可用性,Kriging插值方法以增加GIVE为代价,大幅拓展了可用格网点覆盖范围,但也造成脱离大陆的夏威夷群岛上空的格网点完全不可用。利用CODE发布的GIM模型对WAAS可用格网点的格网改正精度进行分析,得到,当GIV EI<14时,不同GIVEI格网电离层垂直延迟的改正精度差异较小,GIM评估的格网电离层垂直延迟的改正精度与纬度相关性较大。在中低纬地区虽然穿刺点分布密集,但其格网改正精度低于高纬地区。

Grid ionosphere information determines the service level of SBAS with RTCA protocol.Based on the long-term analysis of the ionosphere information broadcast by three WAAS GEO satellites,we obtain the detailed broadcast characteristics of grid ionosphere with RTCA protocol in terms of message arrangement,update period,grid distribution,etc.According to different availability conditions,the availability of each grid is analyzed in detail.When GIV EI=15 is the threshold value,WAAS broadcasts 263 grid ionosphere information with 100% availability,covering the whole North American continent and most areas beyond the coastline of 20°.By comparing the grid availability of WAAS with Kriging interpolation and other systems with IDWK interpolation,it is found that IDWK method has high availability only for grid within the layout range of reference stations.Kriging interpolation method greatly expands the coverage of available grid at the cost of increasing GIVE,but also causes the grid over the Hawaiian Islands which are separated from the mainland to be completely unavailable.Based on the GIM model published by CODE,the correction accuracy of the available grid of WAAS are analyzed.When GIV EI is less than 14,the difference correction accuracy of the ionospheric vertical delay between different GIV EI is relatively small,and the correction accuracy of the grid ionospheric vertical delay evaluated by GIM is relatively large in relation to latitude.Although the ionospheric puncture points are densely distributed in the middle and low latitude area,the correction accuracy is lower than that in the high latitude area.