Synthetic Aperture Radar

Why in News?

ISRO has completed development of a Synthetic Aperture Radar capable of producing high-resolution images for a joint earth observation satellite mission with the U.S. space agency NASA. The mission is targeted to be launched in 2022.

About:

• Synthetic-aperture radar (SAR) 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 (SLAR).
• 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 (the size 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.
• 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.
• The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths 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. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions.
• This process forms the synthetic antenna aperture and allows the creation of higher-resolution images than would otherwise be possible with a given physical antenna.

Applications

• SAR is capable of high-resolution remote sensing, independent of flight altitude, and independent of weather, as SAR can select frequencies to avoid weather-caused signal attenuation. SAR has day and night imaging capability as illumination is provided by the SAR.
• SAR images have wide applications in remote sensing and mapping of surfaces of the Earth and other planets.
•  Applications of SAR include topography, oceanography, glaciology, geology (for example, terrain discrimination and subsurface imaging), and forestry, including forest height, biomass, deforestation.
• Volcano and earthquake monitoring use differential interferometry. SAR can also be applied for monitoring civil infrastructure stability such as bridges.
• SAR is useful in environment monitoring such as oil spills, flooding, urban growth, global change and military surveillance, including strategic policy and tactical assessment.
• SAR can be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna.