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Synthetic-aperture radar
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<p>Synthetic-aperture radar (SAR) is a form of <a href="/facts/Radar/jk9BTbFA">radar</a> that is used to create two-dimensional images or <a href="/facts/3D_reconstruction/MlnAUm3l">three-dimensional reconstructions</a> of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer <a href="/facts/Spatial_resolution/ll8LNYxN">spatial resolution</a> than conventional stationary 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 <a href="/facts/Side_looking_airborne_radar/HpWHSqBR">side looking airborne radar</a> (SLAR). The distance the SAR device travels over a target during the period when the target scene is illuminated creates the large <i>synthetic</i> <a href="/facts/Antenna_aperture/rAHTKdP6">antenna aperture</a> (the <i>size</i> of the antenna). Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical (a large antenna) or synthetic (a moving antenna) – this allows SAR to create high-resolution images with comparatively small physical antennas. For a fixed antenna size and orientation, objects which are further away remain illuminated longer – therefore SAR has the property of creating larger synthetic apertures for more distant objects, which results in a consistent spatial resolution over a range of viewing distances. </p><p>To create a SAR image, successive pulses of <a href="/facts/Radio_wave/XmB6WPpF">radio waves</a> are transmitted to "illuminate" a target scene, and the <a href="/facts/Echo/MCYU8q1S">echo</a> of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single <a href="/facts/Beamforming/4VpeHoGc">beam-forming</a> antenna, with <a href="/facts/Wavelength/kcJU0ymz">wavelengths</a> of a meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. <a href="/facts/Signal_processing/Npaja6zb">Signal processing</a> of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions. This process forms the <i>synthetic antenna aperture</i> and allows the creation of higher-resolution images than would otherwise be possible with a given physical antenna. </p>