User Guides - Sentinel-1 SAR - Geophysical Measurement - Sentinel Online
SENTINEL-1's C-band SAR active sensor can observe the Earth's surface at any time of the day or night, regardless of weather and environmental conditions. SAR has the advantage of operating at wavelengths not impeded by cloud cover or lack of illumination.
Unlike passive optical sensors that require the sun's illumination, an active SAR instrument transmits its own microwave signal to illuminate the Earth's surface at an angle. SAR actively transmits microwave signals towards the Earth and receives a portion of transmitted energy as backscatter from the ground. The returned backscatter echo of the scene is received by the instrument's antenna a short time later at a slightly different location, as the satellite travels along its orbit. The brightness amplitude of the returned signal, along with its phase information, is recorded to construct an image of the scene.
The SAR instrument provides radar backscatter measurements influenced by the terrain structure and surface roughness. Generally, the more roughness or structure on the ground, the greater the backscatter. Rough surfaces will scatter the energy and return a significant amount back to the antenna resulting in a bright feature. Flat surfaces reflect the signal away resulting in a dark feature. Likewise, more structurally complex targets such as forests will appear brighter as signal interaction with the leaves, branches and trunks will result in a higher proportion of the signal being transmitted back to the sensor.
The dielectric constant of the materials on the ground also play a factor in the measurements. In the microwave region of the spectrum, most natural materials have a dielectric constant in the range of 3 to 8 when dry, whereas water has a dielectric constant of approximately 80. The dielectric constant indicates the reflectivity and conductivity of materials. Therefore, the presence of moisture in soil and vegetation results in significant reflectivity.
These properties make SAR very suitable for various applications, including geology and geomorphology, soil moisture, land cover, oceanography and maritime applications and in particular for applications exploiting SAR's polarimetric and interferometric properties.