Definitions list
Click one of the letters above to advance the page to terms beginning with that letter.
  • radar
    Originally 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.
  • radar altimeter
    Radar used to measure the distance (range) from a satellite to the ocean surface. See altimetry
  • radiation
    Energy emitted from a source and propagated in the form of waves or particles.
  • radio wave
    Electromagnetic wave at a frequency less than 3,000 GHz.
  • Radiobroadcasting
    Technique used to transmit information by radio waves, usually for the general public.
  • radiocommunication
    Telecommunication using radio waves.
  • radiometer
    Passive 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.
  • Radiopositioning
    Position-fixing using radio waves.
  • ranging
    Measurement of distance using optical, radioelectric or acoustic processes.
  • Redundancy
    A system or signal has redundancy if it has two or more means of accomplishing a function.
  • reference ellipsoid
    Arbitrary reference surface that is a first approximation of the shape of the Earth, which is a sphere flattened at the poles.
  • remote sensing
    Technique 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.
  • rendezvous
    Bringing together of two separately launched spacecraft.
    How orbital manoeuvres work
  • repeat cycle / revisit interval
    Frequency with which an Earth-observation system or any other remote-sensing system passes over the same point.
  • resolution
    Ability 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.
  • retrorocket
    Rocket 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.
  • ring
    A 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.
  • Roche limit
    Minimum 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).
  • rocket
    Space 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
  • rocket engine
    Jet engine capable of operating in a vacuum, used to propel launch vehicles and manoeuvre satellites. It uses only onboard propellants.
    How launchers work
  • rocket propulsion
    Mode of propulsion in which a spacecraft is propelled by ejecting matter.
  • roll
    Motion of a body about its roll axis, usually its longitudinal axis.