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In this paper we are presenting a methodology, which can remove the effect of ionospheric refraction (ionospheric error) on the coordinates derived by single frequency GPS receivers up to 85 percent at a radius of 1400 km around the computational point. The method is based on the GPS observations being made at the permanent GPS stations, according to the following procedure: (i) The real-time GPS observations being made at the permanent GPS station is compared, epoch- wise, with precisely known coordinates of the permanent station and the time series of coordinate variations in x, y, and z component {?xi,?x,?z} is derived. (ii) Fast Fourier technique is applied in order to transfer each components of the variations vector {?x,?y,?z} from the time domain into frequency domain. (iii) The plot of the resulted periodogram provided us with the frequency vector {fx,fy,fz} of the ionospheric refraction
(ionospheric error) with nearly 24 hour period. (iv) Having derived the frequency vector {fx,fy,fz} of the ionospheric refraction, a sinusoidal models of the form
?xi= ?i0 + ?i1 cos(wit — ?i) (where w = 2?f) is fitted to the coordinate differences {?x,?y,?z} via least squares technique. As a case study, 25 day observations of the permanent GPS station of Tehran National Cartographic Center (NCC) is used.