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.