**4. Spaceborne sensors**

Spaceborne sensors have been developed for over 40 years. Currently, approximately 50 countries are operating remote sensing satellites [9]. There are more than 1000 remote sensing satellites available in space, and among these, approximately 593 are from the USA, over 135 are from Russia, and approximately 192 are from China [27].

Conventionally, remote sensors are divided into two groups: passive sensors and active sensors, as we described in the first section. However, as sensor technology has advanced, nothing has been absolute. For example, an imaging camera is usually regarded as a passive sensor. However, in 2013, a new approach that integrates active and passive infrared imaging capability into a single chip was developed. This sensor enables lighter, simpler dual-mode active/passive cameras with lower power dissipation [28]. Alternatively, remote sensing sensors can be classified into imaging sensors and non-imaging sensors. In terms of their spectral characteristics, the imaging sensors include optical imaging sensors, thermal imaging sensors, and radar imaging sensors. **Figure 6** illustrates the category in terms of imaging sensors and non-imaging sensors.
