Modeling and Simulation of GPS Using Software Signal Generation and Digital signal Reconstruction

GPS satellite simulator test facilities are used to checkout the software and hardware of GPS receivers and their augmentation systems. In general, they include satellite signal simulators, differential augmentation simulators and interface emulators. All testing is run in real time and requires considerable hardware and human assets. Initially, the dynamic scenario to be simulated must be designed and stored in the simulator control computer. This scenario specifies all relevant factors including the trajectory of the GPS receiver(s), the satellite constellation, the signal power and waveforms, environmental factors including propagation errors, and jamming effects. In general, there is considerable equipment setup and calibrations that must be performed prior to actually exercising the scenario. In addition, the satellite simulator and control equipment are expensive resources typically costing on the order of one million dollars. NAVSYS have developed a MATLAB product that allows software simulation of complex GPS environments to be constructed. This provides a high fidelity model of the GPS signal environment and can also model the effects of interfering signal sources, antenna and receiver characteristics on the received GPS signals. The output of the software simulation is a digital file for the selected scenario that represents the digitally sampled GPS signals from a GPS receiver in that environment. The digital file can be played directly into a digital receiver to evaluate its performance under the originating environment or, alternatively, can be remodulated on a RF carrier to provide digital reconstruction of the simulated signal. The advantages of this approach can be summarized as follows: • Enables follow-on tests at different time and locations without most expensive resources and preparation; • Simulates nearly exact, repeatable RF conditions as many times as necessary; • Saves exact RF conditions for historical and legal purposes. In this paper, the software signal generation tool and digital signal reconstruction capability developed by NAVSYS is described and test results are include demonstrating its application for historical and legal purposes.