GPS navigation signal includes vital information such as orbital parameters, clock error coefficients etc. This received signal which is extremely weak is affected by several errors during its propagation and is of the order of 10^-16 W. The noise floor of this signal is 400 times higher than the transmitted signal. The situation becomes much worse particularly when the GPS receiver is located at urban areas where the multipath effect is predominant in the code and carrier phase measurements. GPS usage is not limited to the aircraft en-route navigation and missile guidance where the user receives the satellite signals from the open sky. At the present time, it has become an essential utility in the car navigation, mobile phones, surveying and aircraft landing application. The signal propagation characteristics particularly the short-term variations severely affect the quality, availability and continuity of the system. In this paper, short-term propagation characteristics of GPS signal are modeled and analyzed. Short-term variations are mainly due to multipath reflections and Doppler shift which degrades the quality of received signal particularly in urban environments. The variation of signal quality with respect to user velocity is observed using Rayleigh and Rician fading models.