Before understanding satellite remote sensing, it is necessary to understand what remote sensing is. The term remote sensing was proposed by American geographer Evelyn Pruitt in 1962. Broadly speaking, it can be understood as remote perception, referring to all remote sensing without direct contact; narrowly speaking, it can be understood as a science and technology that uses instruments sensitive to electromagnetic waves to detect electromagnetic waves reflected, radiated, or scattered by target under non-contact conditions and process them to obtain target information. In contrast to remote sensing is proximity sensing (also known as on-site measurement), which is a detection method that involves direct contact with the object.
For example, inserting a thermometer into the soil to measure soil temperature is proximity sensing; using instruments such as infrared radiometer or microwave radiometer positioned several meters (on the ground or on vehicles, etc.), several thousand kilometers (on airplanes), or several hundred thousand kilometers (on satellites) from the soil to measure soil temperature is remote sensing. Similarly, mapping on the ground using geometric triangulation is proximity sensing, while mapping using survey cameras loaded on airplanes or satellites is remote sensing.
The basis of remote sensing mainly relies on electromagnetic waves to achieve contactless detection. The principle of remote sensing is: electromagnetic waves interact with objects, causing them to carry information about the objects; remote sensors sensitive to electromagnetic waves receive electromagnetic waves carrying information, obtain remote sensing data containing information; and after processing, inversion, and interpretation, the information contained in the object is extracted.
The satellite remote sensing system is mainly composed of remote sensors used to obtain remote sensing data, satellite platforms loaded with remote sensors to ensure their normal operation, and facilities for receiving, processing, completing information extraction, and generating remote sensing information products.
Remote sensing has different classification methods:
According to the working mechanism, it can be divided into passive remote sensing and active remote sensing;
According to the detection spectral range, it can be divided into optical remote sensing and microwave remote sensing;
According to the form of data representation, it can be divided into imaging remote sensing and non-imaging According to the platform loaded with remote sensors, it can be divided into ground-based remote sensing, aerial remote sensing, near-space remote sensing, and space remote sensing, etc.
The remote sensing sensors of passive remote sensing only have the function of passive receiving of electromagnetic waves, and receive electromagnetic radiation from nature (such as sunlight) interacting with objects. Representative remote sensing sensors include visible light and infrared cameras, microwave radiometers, etc.
The remote sensing sensors of active remote sensing have the function of actively emitting and receiving electromagnetic waves. First, they actively emit electromagnetic waves to illuminate the object to be detected, and then receive the electromagnetic waves interacting with the object. Typical remote sensing sensors include synthetic aperture radar (SAR), lidar, and microwave scatterometer, etc.
Optical remote sensing can also be divided into ultraviolet, visible light, and infrared remote sensing. Microwave remote sensing can also be divided according to spectral bands, ranging from meter waves, decimeter waves, centimeter waves, millimeter waves, to terahertz waves.