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The Deep Space Network, NASA's international collection of giant radio antennas used to communicate with spacecraft at the Moon and beyond, helps scientists and engineers use gravity and radio science experiments to learn more about our planetary neighborhood. After reaching a spacecraft reaches its destination, it uses radio antennas to communicate with the Deep Space Network, which in turn transmits radio signals back to the spacecraft. Every spacecraft travels in a predetermined path emitting radio signals as it orbits around its target. Scientists and engineers can infer the spacecraft's location and how fast it's going by measuring changes in the spacecraft's radio signal frequency. This is made possible by the Doppler effect, the same phenomenon that causes a siren to sound different as it travels towards and away from you.