An improved method on ionospheric delay calculation in multi-constellation navigation systems

Since there will be several satellite constellations working simultaneously, combining signals from these systems to get better performance is significant. With the development of GNSS, Satellite Based Augmentation Systems (SBAS) are under development to ensure the integrity, continuity and availability of GPS. Ionospheric delays, as the main error in pseudo-range, need to be accurately calibrated. Therefore in this paper, we make an investigation on how to calculate the ionospheric delay of single frequency users in multiple satellite constellations augmented by SBAS. Signals from different satellite constellations are in various frequencies. Ionospheric delay is in verse proportion to the square of frequency. Hence, frequency should be focused on in signal combination. In the ionospheric delay calculations currently, IPPs (ionospheric pierce points) form with a SBAS station distribution and visible GPS satellites. Each ionospheric grid point (IGP) delay value is the distance based weighted average value from the IPPs less than 556km away from it. When there are Compass satellites working simultaneously, the number of IPPs is far more enlarged. Furthermore, the variance of smoothed pseudorange noise, which is mostly related to the signal transmitting frequency if smoothed in same way, varies between different systems. Therefore, when we combine IPPs from these constellations to improve the ionospheric correction service, the conventional algorithm need to be modified. This paper will give an analysis and propose improved algorithms for the IGP delay calculation in multi-constellation navigation systems. From the results, we can get the conclusions that when combining the GPS and Compass satellites, the average of usable IGPs can be improved, three kinds of the weighted methods mentioned in dual constellation have almost the same performance in ionospheric delay correction accuracy; however, they are all better than the single GPS constellation. Users in the middle of China have better accuracy improvement than that on the fringe of China, where little accuracy enhancement can be gained.