This paper 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 and analysis conducted in the GPS and Vehicle Dynamics Laboratory (GAVLAB) 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.

A GPS IF-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 signal simulator which was also recorded with a USRP. Position and velocity 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 for single antenna applications.

A four-element CRPA interference simulation was generated by the software simulator and analyzed with a software receiver that was modified to include multi-antenna interference mitigation techniques. Results provided from the CRPA simulation by the modified software receiver illustrate that a software GPS simulator is a powerful tool for rapid development of GPS interference mitigation techniques in the software domain. 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.

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

In this thesis methods of spoofing prevention are developed to detect, identify, and mitigate an attack against both networked and standalone GPS receivers. A network based detection algorithm is introduced which combines existing network data and GPS receiver outputs to create a dynamic threshold used as an indication of a spoofing attack. Attack mitigation is accomplished in the development of an interference cancellation algorithm. In the event of an attack, correlators are designated to track the attacking signal and extract critical parameters describing its power, phase, and frequency. These parameters are used to create a replica of the incoming signal which is then subtracted from the buffered raw data. This process removes the interfering signal allowing recovery of the authentic signal and computation of true receiver position. The anti-spoofing routines evaluated in this thesis have an advantage over other methods due to their robustness in a wide variety of situations and their ability to mitigate an attack without any prior knowledge of the spoofer or the spoofed signal characteristics. Testing of the algorithms developed in this thesis is accomplished using various types of simulated GPS data since live-sky testing in the GPS frequency band is restricted by the Federal Communications Commission. Actual GPS measurements are collected and modified to simulate spoofing in tests of the detection algorithms. Sets of simulated GPS data files are combined in software to simulate spoofing at the signal level. These data sets are used to test the interference cancellation algorithm’s effectiveness at removing a spoofed signal in the intermediate frequency (IF) stage. The detection and suppression algorithms are demonstrated to effectively alert the user to an attack and mitigate its effect in IF stage generating a cleaned data set for acquisition and tracking of the authentic GPS signal.

Enhancement and Defense of GPS Navigation Using Signal Processing Techniques

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Enhancement and Defense of GPS Navigation Using Signal Processing Techniques