page_title: Radar astronomy
section_title: History
hierarchical_section_title: Radar astronomy / History
caption_attribution_description: Radar images and computer model of asteroid 1999 JM8 Top row: Raw Radar images of asteroid 1999 JM8 Bottom row: 3-D computer model of the asteroid Middle row: Expected radar echo, as computed from the model, for comparison with the real radar images. Deutsch: Asteroid (53319) 1999 JM8: Radardaten (oben), das daraus abgeletete 3-D-Modell (unten), und das aus dem Modell errechnete, erwartete Radarecho (mitte). English: Radar images and computer models of 1999 JM₈
context_page_description: Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the reflections. This research has been conducted for six decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an active one. Radar systems have been used for a wide range of solar system studies. The radar transmission may either be pulsed or continuous.
  The strength of the radar return signal is proportional to the inverse fourth-power of the distance. Upgraded facilities, increased transceiver power, and improved apparatus have increased observational opportunities.
  Radar techniques provide information unavailable by other means, such as testing general relativity by observing Mercury and providing a refined value for the astronomical unit. Radar images provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques.
  Relying upon high-powered terrestrial radars, radar astronomy is able to provide extremely accurate astrometric information on the structure, composition and movement of Solar System objects.
context_section_description: The Moon is comparatively close and was detected by radar soon after the invention of the technique in 1946. Measurements included surface roughness and later mapping of shadowed regions near the poles.
  The next easiest target is Venus.  This was a target of great scientific value, since it could provide an unambiguous way to measure the size of the astronomical unit, which was needed for the nascent field of interplanetary spacecraft.  In addition such technical prowess had great public relations value, and was an excellent demonstration to funding agencies.  So there was considerable pressure to squeeze a scientific result from weak and noisy data, which was accomplished by heavy post-processing of the results, utilizing the expected value to tell where to look.  This led to early claims (from Lincoln Laboratory, Jodrell Bank, and Vladimir A. Kotelnikov of the USSR) which are now known to be incorrect.  All of these agreed with each other and the conventional value of AU at the time, 149467000 km.
  The first unambiguous detection of Venus was made by the Jet Propulsion Laboratory on 10 March 1961. JPL established contact with the planet Venus using a planetary radar system from 10 March to 10 May 1961. Using both velocity and range data, a new value of 149598500±500 km was determined for the astronomical unit.  Once the correct value was known, other groups found echos in their archived data that agreed with these results.
  The following is a list of planetary bodies that have been observed by this means:
  Mercury - Improved value for the distance from the earth observed (GR test). Rotational period, libration, surface mapping, esp. of polar regions.
  Venus - first radar detection in 1961. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.
  Earth - numerous airborne and spacecraft radars have mapped the entire planet, for various purposes.  One example is the Shuttle Radar Topography Mission, which mapped large parts of the surface of Earth at 30 m resolution.
  Mars - Mapping of surface roughness from Arecibo Observatory. The Mars Express mission carries a ground-penetrating radar.
  Jupiter System - Galilean satellites
  Saturn System  - Rings and Titan from Arecibo Observatory, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.
caption: Radar images and computer model of asteroid 1999 JM8
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