The four global navigation systems — GPS (US), GLONASS (Russia), Galileo (EU) and BeiDou (China) — each broadcast their position and a very accurate time from MEO (see 2.3). A receiver that hears four satellites calculates its position and the time from this. That time is at least as important: data links, radio nets and financial systems run on it.
The signal is extremely weak by the time it reaches the Earth, and therefore easy to jam or falsify. That side — jamming, spoofing, military codes and protected antennas — is covered in detail in the EW Academy (§ 14).
You can view the constellations live on N2YO (GPS, GLONASS, Galileo, BeiDou) and on Orbital Radar (GPS, Galileo); see 10.6.