The most widely used active remote sensing systems include:
• active microwave (RADAR), which is based on the transmission of long-wavelength microwaves (e.g., 1 – 100 cm) through the atmosphere and then recording the amount of energy back-scattered from the terrain;
• LIDAR, which is based on the transmission of relatively short-wavelength laser light (e.g., 0.90 mm) and then recording the amount of light back-scattered from the terrain; and
• SONAR, which is based on the transmission of sound waves through a water column and then recording the amount of energy back-scattered from the bottom or from objects within the water column.