Definitions list
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radarOriginally an acronym for RAdio Detection And Ranging. Technique used to detect, locate, track and identify objects of various kinds by transmitting electromagnetic waves and observing the echoes returned from them. The round-trip time of the radar signal makes it possible to calculate the distance or range of the object.
Also, a device that uses this technique.
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radar altimeterRadar used to measure the distance (range) from a satellite to the ocean surface. See altimetry
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radiationEnergy emitted from a source and propagated in the form of waves or particles.
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radio waveElectromagnetic wave at a frequency less than 3,000 GHz.
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RadiobroadcastingTechnique used to transmit information by radio waves, usually for the general public.
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radiocommunicationTelecommunication using radio waves.
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radiometerPassive receiver capable of collecting and measuring energy from a source of electromagnetic radiation, for example, natural radiation from the Earth’s surface or from clouds. Radiometers are used on many Earth-observation, weather and planetary exploration satellites. Measurements can be digitized, processed and transmitted automatically as radio waves to generate a wide range of products on the ground.
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RadiopositioningPosition-fixing using radio waves.
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rangingMeasurement of distance using optical, radioelectric or acoustic processes.
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RedundancyA system or signal has redundancy if it has two or more means of accomplishing a function.
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reference ellipsoidArbitrary reference surface that is a first approximation of the shape of the Earth, which is a sphere flattened at the poles.
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remote sensingTechnique that involves sensing electromagnetic radiation coming from an object, without physical contact, in order to determine its features. It is especially useful for collecting information about a planet’s atmosphere and surface.
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rendezvous
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repeat cycle / revisit intervalFrequency with which an Earth-observation system or any other remote-sensing system passes over the same point.
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resolutionAbility of a measuring instrument to discriminate two close values. In a satellite image, spatial resolution refers to the size of the smallest object or ground feature that can be distinguished.
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retrorocketRocket engine designed to produce thrust in a direction opposite to a spacecraft’s direction of motion. Normally used to slow a capsule in readiness for deorbiting or for a soft landing of a planetary probe.
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ringA usually circular accretion of small, solid bodies—dust, grains, fragments or large blocks of rock or ice—each moving at their own speed around the larger planets of the Solar System.
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Roche limitMinimum distance to which a natural satellite can approach its primary body without being torn apart by tidal forces. Named after the French astronomer Édouard Roche (1820-83).
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rocketSpace vehicle that propels itself by reaction. The term is used in everyday language as synonymous with launch vehicle. It is also used to designate low-thrust rocket engines.
How launchers work
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rocket engineJet engine capable of operating in a vacuum, used to propel launch vehicles and manoeuvre satellites. It uses only onboard propellants.
How launchers work
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rocket propulsionMode of propulsion in which a spacecraft is propelled by ejecting matter.
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rollMotion of a body about its roll axis, usually its longitudinal axis.