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In GPS (the Global Positioning System) pseudorange positioning is a method of calculating a receivers geographical location by interpreting satellite data, combined with the time taken for the data to reach the receiver. In order for a satellite to provide location and timing information to a GPS receiver, it transmits two signals. The initial signal identifies the satellite using a unique code, and indicates the precise time the message was sent. This time signal is based on the satellite’s internal atomic clock, which is regularly updated from the Air Force’s ground control station. The satellite then sends a second signal, indicating its ephemeris (orbital position) at the time the first message was sent. The GPS receiver then calculates the distance from the satellite by comparing the time the initial message was sent with the time it was received (based on the GPS receiver's own internal clock). The receiver’s possible position must therefore be somewhere on the circle of positions that match those data. This circle of positions is known as the pseudorange. The receiver requires data from multiple satellites to derive its position. The point at which pseudoranges from these multiple satellites intersect provides the receiver's exact position.