Incident waves on a GPS antenna on mobile terminals suffer from multipath effects due to reflection, diffraction and scattering from environmental objects including buildings, trees, vehicles and ground. A traditional electromagnetic approach to analyse the GPS antenna performance is therefore unable to give a correct account of the antenna working in a real multipath environment. A new technique to characterise these environmental effects on the GPS antenna is presented in this paper, defined by three parameters; GPS Mean Effective Gain (MEGGPS), GPS Angle of Arrival (AoAGPS) distribution and GPS Coverage Efficiency (etac).