The Total Electron Content (TEC) from a spatially distributed network of Global Posi-rntioning System (GPS) receivers can show the temporal, seasonal, latitudinal and longi-rntudinal variations of the ionospheric electron distribution. However, it does not conveyrnany information about ionospheric electron density structure along the vertical direction.rnComputerized Ionospheric Tomography (CIT) is a method to investigate vertical iono-rnspheric electron density pro le in two or three dimensions. In this work, ground basedrnGPS receivers over Ethiopia have been used to record signals transmitted from GPS satel-rnlites and the computed pseudoranges and phase values are used to calculate Total ElectronrnContent (TEC). In two dimensions (2D), the vertical plane along 40:02610E longitude isrndiscretized into pixels and the distance traveled in each pixel crossed by GPS signals tornfour receivers located close to the vertical plane is calculated in a selected reference frame.rnUsing TEC from these stations a 2D, altitude versus latitude, distribution of electron den-rnsity is inverted with in each pixel. In three dimensions (3D), the volumetric ionospherernover Ethiopia is discretized in to numerous small voxels and the distance traveled byrnGPS signals in each voxel is calculated from the precise position of the GPS satellites torn nd the coe cient matrix in an appropriate reference frame. A 3D ionospheric electronrndensity distribution is then obtained using TEC data from GPS stations located all overrnthe country. In both 2D and 3D tomographic reconstructions damped least square inver-rnsion algorithm with second order Tikhonov regularization has been employed. We usedrnthe L-curve method to determine the regularization parameter. The performance of therninversion algorithm has been numerically validated with a simulated TEC from IRI2007rnmodel. The numerical experiments show that the algorithm produced results with 2-normrnrelative reconstruction errors 0:04 and 0:02 for the 2D and 3D inversions respectively