DGPS positional improvement by mitigating the ionospheric horizontal gradient and elevation angle effects

Over the equatorial region, the inhomogeneous formation of the ionosphere introduces greater ionospheric horizontal gradient which could give greater positioning error for a user station in a DGPS system. Furthermore, the difference in the elevation angle of the user station that ‘views’ the same GPS satellite as the reference station also introduces range measurement errors at mm to cm levels. In order to mitigate those two effects in a DGPS system, the Jones 3D ray tracing program was used to show final positioning improvement at the user station. Group delay and carrier phase advance measurements from 8 GPS satellites were used to determine and mitigate the effects of ionospheric horizontal gradient and the elevation angle. Positioning improvement of about 7 cm has been obtained by using both group delay and carrier phase advance measurements. This shows a good positioning improvement for the user station in a DGPS system that is located over the equatorial region.