section_path: Applications/Space communication
section_name: Space communication
paragraph_text: Space communication
  
  This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft. In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high-gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L, C, S, k and k band are allocated for space communication. A radio link that transmits data from the Earth's surface to a spacecraft is called an uplink, while a link that transmits data from the spacecraft to the ground is called a downlink.