At present,the main problem faced by ground-based augment system (GBAS) is that though carrier smoothing filter and local dif-ferential global positioning system (LDGPS) improve the accuracy of the pseudorange by reducing the noise in it and eliminating almost all the common errors between the user and the reference station,they also cause extra errors on account of the effects of the ionosphere temporal and spatial gradients.Based on the analysis of these errors as well as the smoothing noise,this article suggests a new algorithm to design the optimal Hatch filter,whose smoothing window width varies real-time with the satellite elevation,ionosphere variation,and distance from the user to the reference station.By conducting the positioning process in the GBAS emulation platform for several hours and after its comparison with the performances of traditional Hatch filters,it is found that the errors in the differential correction become smaller and the positioning accuracy gets heightened with this new method.
RAIM可用性预测系统是在飞机起飞前对飞机即将飞行的整个过程的机载GPS接收机的RAIM可用性进行预测,辅助空管和签派放行许可,是民航应用GPS导航的必要程序。SA(Selective Availability)取消前,SA是SPS(Standard Position Service)用户的最大误差源,各颗卫星的UERE(User Equivalent Range Error)基本接近,在进行RAIM可用性预测时可认为所有可见卫星的UERE相同;SA取消后,电离层成为SPS用户的最大误差源,与用户到卫星的仰角紧密相关,相同UERE的假设将降低RAIM的可用性。文中针对此特性,基于传统的RAIM可用性预测方法,提出了加权RAIM可用性预测方法,并对权值的选择进行分析。计算机仿真结果表明,加权方法提高了RAIM可用性,缩短了RAIM不可用时段。算法已经应用于民航的RAIM预测系统,运行良好。
