A Multiple-Antenna Software GPS Signal Simulator for Rapid Testing of Interference Mitigation Techniques

Abstract

This thesis details the design of a software GPS signal simulator and illustrates how it can be used to support rapid testing of GPS interference mitigation research. The MATLAB-based signal simulator is capable of generating digital GPS signals at intermediate frequencies (IF) for multiple-element, controlled-radiation-pattern-antenna (CRPA) configurations in jamming environments. The simulator was developed to support novel GPS interference mitigation research conducted in the GPS and Vehicle Dynamics Laboratory at Auburn University. The digital signals can be analyzed with software acquisition, tracking, and positioning techniques or can be converted to analog signals at the GPS radio frequencies and played to multiple receivers via cabling using the Universal Software Radio Peripheral (USRP) software-defined-radio (SDR) platform.

An IF GPS signal for a single antenna generated by the software simulator was played-back and recorded simultaneously with USRPs and compared to an IF signal for the same scenario generated by a hardware GPS simulator that was also recorded with a USRP. Position, velocity, pseudorange, Doppler frequency, and carrier-to-noise ratio measurements calculated by a software receiver from the simulated IF signal, the played-back and recorded IF signal, and the signal generated by the hardware simulator are compared to evaluate the baseline performance of the software simulator. Results of this experiment illustrate that a software simulator paired with the USRP is comparable in performance to the hardware simulator. Additionally, a dynamic scenario generated by the software simulator was played-back and recorded with USRPs and also played to a hardware GPS receiver. Position results calculated by a software receiver from the simulated and played-back and recorded signals were compared to results calculated by the hardware receiver to further evaluate the performance of the software simulator. These results illustrate that the software simulator is capable of generating GPS signals for dynamic trajectories.

To illustrate the capabilities provided by a software-based GPS simulator, signals along with three types of simulated jammers were generated and are illustrated in the frequency, time, and histogram domains . Each interference signal was played-back and recorded with USRPs, and results are provided to illustrate the steps that must be taken to capture the effects of simulated jammers in a USRP playback or record. Additionally, a 4-element CRPA interference simulation was generated by the software simulator and analyzed with a software receiver that was modified to include several interference mitigation techniques. Results provided by the modified software receiver illustrate that a software GPS simulator is a powerful tool for developing GPS interference mitigation techniques. Lastly, the simulated CRPA signals were played-back and recorded simultaneously with multiple USRPs to examine the feasibility of using relatively inexpensive SDRs to simulate multiple-antenna GPS scenarios. Results from this test illustrate that a multiple-USRP setup is a useful tool for testing robust interference mitigation techniques.