Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technology to detect and classify objects on Earth (both on the surface, and in the atmosphere and ocean) by means of propagated signals (e.g. electromagnetic radiation emitted from aircraft or satellites).

    There are two main types or remote sensing: passive remote sensing and active remote sensing. Passive sensors detect natural radiation that is emitted or reflected by the objects or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive censors. Examples of passive remote sensors include film photography, infra-red, charge-coupled devices, and radiometers. Active collection, on the other hand, emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or backscattered from the target. RADAR and LIDAR are examples of active remote sensing where the time delay between emission and return is measured, stabilizing the location, height, speed and direction of an object.

What is the main idea of the second paragraph?