Control over unmanned space probes and artificial satellites is maintained from the ground at control centers, where huge electronic computers analyze data and determine the exact moment when a change should be made. These instructions are relayed to the spacecraft by signals carried on certain radio frequencies. Instruments inside the craft also use radio signals to send data back to earth. Radio contact with spacecraft divides naturally into three categories: tracking, telemetry, and control. Tracking is the continuous reporting of a satellite's or space probe's position in space. Telemetry is the transmission of data back to earth by an on-board instrument (e.g., camera, Geiger counter, or magnetometer). Control includes the overall direction of a spacecraft to achieve the intended trajectory. Commands are specific control signals that order execution of a specific maneuver, such as turning on a camera or firing a retro-rocket
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