Version SAR -Satellites Acquire Images

All satellites equipped with SAR sensors orbit the Earth on a near-polar orbit at an altitude ranging from 500 to 800 km above the Earth’s surface. The time taken for a satellite to re-pass over the same area is called the ‘revisiting time’. Since the launch of ERS satellites in 1992, numerous satellites have been orbiting the Earth, providing higher resolution images, faster repeat times and data redundancy for many parts of the world.

All SAR satellites travel from the north pole towards the south pole for half of their trajectory. This direction is referred to as their descending orbit. Conversely, when satellites travel from the south towards the north pole, it is said to be in an ascending orbit. The same area is revisited along the two orbits. As a consequence, ascending and descending imageries are collected over the same area.

A satellite continuously emits millions of radar signals toward the Earth’s surface along the radar beam’s line of sight (LOS). The angle at which the sensor is pointed toward the Earth’s surface is referred to as the off-nadir angle (or look angle). The off-nadir angle ranges from values of 20° to 50° according to the satellite platform. This ability to vary the off-nadir angle is important to adjust it in case of hilly or mountainous terrain (potential impediments to InSAR), if the relationship between viewing geometry and terrain slope is not optimal.

Radar signals are characterized by two fundamental properties: amplitude and phase. Amplitude is related to the energy of the backscattered signal. Metal and hard objects (natural or artificial) have a high reflective quality and therefore the amplitude of the reflected signal is high. The amplitude characteristics of signals can be visualized in black and white. Phase is related to the sensor-to-target distance. It is this specific property of the radar signal that is used in estimating displacement in interferometric applications.