This document analyzes the spectral characteristics of four Global Navigation Satellite Systems: GPS, Galileo, GLONASS, and Compass/Beidou. It compares the center frequencies, modulations, and signal structures of different signals within each system, including GPS L1 C/A, L5, Galileo E1 and E5, GLONASS L1/L2, and Compass B1. Simulation results of the power spectra and autocorrelations of various signals are presented, showing that signals from different systems can coexist without significant interference due to their distinct spectral properties. The analysis allows researchers to study interoperability and compatibility among global navigation satellite systems.