A Software Signal Simulation of Low Earth Orbit Satellites for Investigative Analysis

Abstract

Simulation tools are an important part of the engineering design process by allowing changes in algorithms and processes before implementation. Current global navigation satellite systems (GNSSs) are mainly located in medium Earth orbit (MEO) and have been used for many decades. GNSSs are used for positioning applications all across the globe from civilian to military applications. These satellite systems work well in most applications, however they are susceptible to interferences due to low signal power. Low Earth orbit (LEO) satellites have gained interest as a possible alternative source of position, navigation, and timing (PNT). This is because LEO satellites have higher received signal power compared to standard GNSSs. One issue with LEO satellites is that the signal was not initially designed for navigation and most of the messages are unknown.

This thesis describes the design of a software signal simulation tool for LEO satellites for investigative purposes. The tool is modular to allow for simulation blocks to be changed quickly and efficiently. This design also allows for new pieces to be written and integrated easily. The MATLAB simulation is capable of generating complex in-phase and quadrature signals for time division multiple access signals for current and hypothetical LEO constellations. A customized navigation message was developed to account for multiple user selected formats. The signals generated by this simulation tool can be passed through a software receiver and through hardware by means of a universal software radio peripheral. By generating simulated signals, receivers can be designed and tested. To test the generated signals, a specialized receiver was designed. The output navigation data is used to produce position estimates for static receiver positions using batched Doppler and pseudorange based least squares algorithms. The simulation tool is shown to be flexible in order to examine varying test plans.