Multipath is defined by echoes associated with the fields reflected by the surrounding environment. In multipath situations, the received signal is not only the direct one but also a sum of attenuated and delayed versions of the direct signal. For this reason, a multipath channel is usually characterized by a set of four parameters: the amplitude, the delay, the phase shift and the Doppler shift of each multipath. When characterizing a multipath channel, the main issue is the prediction of these parameters. In this paper, we propose a GPS multipath prediction simulator adapted to airport navigation which can predict the GPS multipath error in static and dynamic configurations. This simulator consists in a multipath generator and a GPS receiver simulator. It takes into account the 3D environment, the satellite and receiver positions. Concerning the electromagnetic modeling we show that ray methods should not be used for GPS multipath prediction if few-meters objects are present in the environment. Next, the choice of physical optics (PO) is numerically validated via comparisons with the method of moments (MoM). We also show that the computation of multiple reflections up to order 2 is sufficient. We present techniques that improve the computation efficiency of the simulator. Finally a test case is exposed.