Free levelled English reading practice. Read for meaning first, then use the full Speed Reading tool to time yourself and track your progress.
The Global Positioning System determines location by measuring signals from satellites carrying extremely accurate atomic clocks. A receiver compares the arrival times of signals from multiple satellites and calculates its distance from each. With enough measurements, it can estimate position and correct its own clock error. GPS originated as a US military system, with the first experimental satellites launched in the 1970s and full operational capability declared in the 1990s. Civilian access eventually transformed navigation, surveying, agriculture, logistics and smartphone services. Relativity even matters: satellite clocks require corrections because motion and weaker gravity alter the rate at which time passes.
