Ground simulation is essential for assessing the performance of space-borne Global Navigation Satellite System (GNSS) based positioning algorithm and hardware. SPIRENT GNSS simulator is widely used for establishing the simulation environment. Precise Orbit Determination (POD) data can be used to define exact motion of the corresponding satellite. However, publicly available POD data format is sometimes not compatible with SPIRENT simulator. This paper presents the procedures of using a SPIRENT GNSS simulator to simulate space-borne GNSS based positioning algorithm. A methodology is developed in MATLAB to convert the available POD data to SPIRENT-compatible format. POD data of TANDEM-X satellite was used to define the user satellite motion. Recent GPS navigation data is used to define the motion of the GPS satellites and hypothetical Galileo constellation orbit is used to define the motion of the Galileo satellites. This setup is used to simulate multi-GNSS based navigation scenario. Code and carrier phase measurements are recorded and the measurements are used in a MATLAB based Kalman Filter routine to estimate the position of the satellite.