Modularized GPS software defined radio (SDR) has many advantages of applying and modifying algorithm. Hardware based GPS receiver uses many hardware parts (such as RF front, correlators, CPU and other peripherals) that process tracked signal and navigation data to calculate user position, while SDR uses software modules, which run on general purpose CPU platform or embedded DSP. SDR does not have to change hardware part and is not limited by hardware capability when new processing algorithm is applied. The weakness of SDR is that software correlation takes lots of processing time. However, in these days the evolution of processing power of MPU and DSP leads the competitiveness of SDR against the hardware GPS receiver. This paper shows a study of modulization of GPS software platform and it presents development of the GNSS software platform using MATLAB Simulink™. We focus on post processing SDR platform which is usually adapted in research area. The main functions of SDR are GPS signal acquisition, signal tracking, decoding navigation data and calculating stand alone user position from stored data that was down converted and sampled intermediate frequency (IF) data. Each module of SDR platform is categorized by function for applicability for applying for other frequency and GPS signal easily. The developed software platform is tested using stored data which is down-converted and sampled IF data file. The test results present that the software platform calculates user position properly.