Diploma presents relativistic eﬀects on GPS observations. First and second chapter are a short introduction to special and general theory of relativity, where notions of proper time, Lorentz transform, tensor and manifold are presented. Famous Einstein ﬁeld equations follow and I give a brief explanation of physical backgroung. A detailed derivation of Doppler eﬀect and proper time equations in Schwartzschild metric follows, which are then used in connection with satellite and reciver, where the reciver is supposed to be placed on geoid. An important chapter on clock synchronisation is also included, methods described there are important in construction of coordinate time. Last chapter deals with code and phase observations, and an important conclusion is that relativistic eﬀects on these two diﬀerent types of observations are basically the same.