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Satellites are celestial bodies that orbit around planets in space. The main purposes of satellites can be divided into three categories: scientific research satellites, technology test satellites, and application satellites (82%). Application satellites are mainly divided into communication satellites (about 49%), remote sensing satellites (27%), and navigation satellites (7%). Remote sensing satellites (RS remote sensing) are one of the most important types of application satellites, which refer to sensors installed on a platform that can obtain target object feature information without direct contact with the target object.
According to the imaging principle, remote sensing satellites can be divided into two types: optical and microwave. Remote sensing satellites can be divided into optical sensors (over 50%) and microwave sensors (10%) according to their sensor performance. The main difference is the range of electromagnetic waves they receive. Optical sensors receive visible light to infrared regions, while microwave sensors mainly receive microwave regions.
The main technical performance indicators of optical imaging include four dimensions: spatial resolution, spectral resolution, temporal resolution, and radiometric resolution. Spatial resolution, spectral resolution, and temporal resolution are the most important parameters for describing optical imaging and the main direction of satellite technology upgrading and differentiation competition. Currently, the main optical imaging satellites are high-resolution satellites and hyperspectral satellites.
Optical imaging satellite refers to an application satellite that uses optical sensors to detect information and obtain and analyze the optical feature information of the observed objects through imaging observation in the visible, near-infrared, and short-wave infrared electromagnetic spectrum. Currently, optical imaging satellites account for 52.7% of the total number of remote sensing satellite and are the most important remote sensing satellites.
Radar imaging, also known as microwave remote sensing satellites, is an active satellite, such as synthetic aperture radar (SAR) satellites. Compared with passive receiving optical sensors of light sources, its advantages of all-weather, all-time imaging can penetrate some ground objects. The parameters that describe its performance are generally the operating band, polarization mode, and full-color resolution. Canada's Radarsat SAR series satellite is one of the earliest satellites equipped with SAR devices in the world. Radarsat-2 satellite can provide higher quality SAR images and 11 beam modes, three polarization modes, widened swept frequency, and large-capacity solid-state recorder, making the operation of Radarsat-2 more flexible and convenient.
According to downstream customers' classification, remote sensing satellites can be divided into civilian and military satellites.
In the field of civilian commercial remote sensing satellites, the United States leads, and China's commercial remote sensing has developed rapidly, and the spatial resolution has reached the international leading level. In terms of military remote sensing satellites, China's "Pointer" and "Vanguard" series have reached the international top level. Among foreign military high-resolution remote sensing satellites, the Keyhole 12 (KH-12) satellite of the United States is the most prominent, with a resolution of 0.1m. In addition, the Kobalt-M satellite of Russia is 0.3m, the new generation of military reconnaissance satellite IGS of Japan is 0.3m, and the Helios 2 satellite of France is 0.35m. China's existing military remote sensing satellites include the "Pointer" series reconnaissance satellites and the "Vanguard" series infrared warning satellites.