In this paper, a microstrip antenna is designed using a rectangular patch. To find characteristics of the antenna, computer simulations of the rectangular single microstrip patch antenna are performed with changing width and feed line. And we found characteristics of RHCP using axial ratio. Through the results, we found that the Finite Difference Time Domain(FDTD) method is an effective method for designing microstrip patch antenna. According to simulation the resonant point has been found it in the frequency received from GPS satellite. And these results were in relatively good accordance with the measured values.