These sources provide a comprehensive overview of Global Navigation Satellite Systems (GNSS), explaining how they function through orbiting satellites and receivers to provide position, navigation, and timing services crucial for modern society on land and water. The text details the four major global constellations: GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China), outlining their unique features like accuracy levels, signal technologies, and special services such as messaging or search and rescue functions. It also examines regional systems (India's NavIC, Japan's QZSS) and augmentation technologies (SBAS, RTK, PPP) that enhance performance. Ultimately, the sources emphasize the growing importance of multi-GNSS receivers for improved reliability and accuracy in diverse applications.