GPS Ray Tracing to show the effect of ionospheric horizontal gradeint to L1 and L2 at ionospheric pierce point

Abstract

GPS ray propagates at two carrier frequencies; L1 at 1575.42 MHz and L2 at 1227.6 MHz. In this project, Jones 3D Ray Tracing program being used to determine the characteristics of the GPS ray as it propagates through the ionosphere. A numerical mathematical model which is continuous and contains no singularities has been used to represent the actual formation of the ionosphere in 3 dimensional (latitude, longitude and altitude). By using GPS ray tracing, it has been proved that the path of propagation for both L1 and L2 in the ionosphere is different. The point (latitude and longitude) where L1 intersects at IPP is different than the point (latitude and longitude) of intersection of L2. IPP or Ionospheric Pierce Point is the altitude in the ionosphere where the composition of electron density is greatest. It is also shown that the distance between the L1 and L2 at IPP over the equatorial region is greater than in the mid-latitude region. This is due to the difference in the composition of electron density in both these regions and also due to the presence of ionospheric horizontal gradient. In GPS, the presence of ionospheric horizontal gradient can effect the final GPS positioning if its' being ignored, especially the GPS positioning over the equatorial region.