page_title: Synthetic-aperture radar
section_title: Basic principle
hierarchical_section_title: Synthetic-aperture radar / Basic principle
caption_attribution_description: العربية: صُورة عالميَّة لِسطح كوكب الزُهرة عند خط الطول 180 درجة شرقًا. صُنعت هذه الصُورة بِمُحكاة الحاسوب من خِلال رسم فُسيفساء من بيانات رادار الفُتحة التركيبيَّة في المسبار الفضائي مجلَّان. مُلئت فجوات البيانات باستخدام بيانات مشروع پيونير الزُهرة، أو جُعلت قيمة متوسطة المجال ثابتة. استُعملت الألوان المُحاكية لتحسين دقَّة الأجسام الصغيرة الظاهرة في الصُورة. واستندت تلك الألوان على الصُور المُلوَّنة التي سجلها المسباران السوڤيتيَّان ڤينيرا 13 و14. English: This global view of the surface of Venus is centered at 180 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Data gaps are filled with Pioneer Venus Orbiter data, or a constant mid-range value. Simulated color is used to enhance small-scale structure. The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. The image was produced by the Solar System Visualization project and the Magellan science team at the JPL Multimission Image Processing Laboratory and is a single frame from a video released at the October 29, 1991, JPL news conference. It is important to note that Venus is completely shrouded in clouds. A bright elongated region in the center is Aphrodite Terra. Français&#160;: Vue globale de la surface de Vénus centrée sur 180° est de longitude. Cette vue est une mosaïque des images radar obtenues par la sonde spatiale Magellan lors de son premier cycle de cartographie. Les vides sont comblés par des images obtenues par la sonde Pioneer Venus orbiter. Les couleurs de la surfaces sont calculées à partir des données obtenues à la surface de Vénus par les atterrisseurs Venera 13 et 14. Cette image ne pourrait pas être vue par un observateur survolant Vénus, car la planète est constamment recouverte de nuages. La compilation des données en une image unique a été réalisée par le Solar System Visualization project et l'équipe scientifique du projet Magellan. Čeština: Radarový obraz povrchu Venuše směřovaný na 180 stupňů východní délky.
context_page_description: Synthetic-aperture radar is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer spatial resolution than conventional beam-scanning radars. SAR is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar. The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large synthetic antenna aperture. Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical or synthetic – this allows SAR to create high-resolution images with comparatively small physical antennas. Additionally, SAR has the property of having larger apertures for more distant objects, allowing consistent spatial resolution over a range of viewing distances.
  To create a SAR image, successive pulses of radio waves are transmitted to "illuminate" a target scene, and the echo of each pulse is received and recorded.
context_section_description: A synthetic-aperture radar is an imaging radar mounted on a moving platform. Electromagnetic waves are transmitted sequentially, the echoes are collected and the system electronics digitizes and stores the data for subsequent processing. As transmission and reception occur at different times, they map to different positions. The well ordered combination of the received signals builds a virtual aperture that is much longer than the physical antenna width. That is the source of the term "synthetic aperture," giving it the property of an imaging radar. The range direction is parallel to the flight track and perpendicular to the azimuth direction, which is also known as the along-track direction because it is in line with the position of the object within the antenna's field of view.
  The 3D processing is done in two stages. The azimuth and range direction are focused for the generation of 2D (azimuth-range) high-resolution images, after which a digital elevation model (DEM) is used to measure the phase differences between complex images, which is determined from different look angles to recover the height information. This height information, along with the azimuth-range coordinates provided by 2-D SAR focusing, gives the third dimension, which is the elevation. The first step requires only standard processing algorithms, for the second step, additional pre-processing such as image co-registration and phase calibration is used.
  In addition, multiple baselines can be used to extend 3D imaging to the time dimension. 4D and multi-D SAR imaging allows imaging of complex scenarios, such as urban areas, and has improved performance with respect to classical interferometric techniques such as persistent scatterer interferometry (PSI).
caption: The surface of Venus, as imaged by the Magellan probe using SAR
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