The Global Positioning System (GPS) is a space-based radio positioning system which is capable of providing continuous position, velocity and time information to users anywhere on, or near, the surface of the Earth. The positional accuracy of GPS is limited by several sources of error, such as satellite and receiver clock offsets, signal propagation delays (due to the ionosphere and troposphere), multipath, receiver measurement noise, and instrument bias. The ionospheric delay is the predominant error source. The main objective of this work was to estimate the regional vertical Total Electron Content (vTEC) densities, using a linear combination of L1 and L2 carrier phase for higher order time delays at different thin-shell layers of the ionosphere (i.e. 60 km, 90 km, 150 km, 200 km and 450 km) over GPS stations in Ethiopia. Due to the high equatorial ionization anomaly and irregularity of the electron density distribution, we selected four GPS stations across the country. We studied longitudinal, latitudinal, and altitudinal variations in the ionosphere using GPS observables extracted by precise geodetic GAMIT-GLOBK software package. We obtained data from the 2013 to 2015 period for all stations. For daily data processing in GAMIT, we switched the International Geomagnetic Reference Field 2012 (IGRF-12) model (which consists of spherical harmonic coefficients) on and off, representing the Earth's main field and its secular variation and ionospheric electron content along the signal path (available from the Centre for Orbit Determination in Europe (CODE)). Our results confirmed that there is latitudinal, longitudinal and altitudinal variation in the ionosphere. The density of total electron content and GPS positional error due to higher-order time delays have a positive correlation (more than 95%). The positional error, due to the higher order ionospheric delay, exceeds 4 mm for all GPS stations and reached up to 8 mm. Finally, we observed that the time delay due to the higher-order ionospheric effect in the L2 signal is twice that in the L1. We conclude that the GPS signal is affected by the higher order ionospheric delay by up to several centimetres due to its electron content and the Earth's magnetic field.
- Total electron content