The sensors located on a satellite can be active or passive.

The active sensors illuminate the target and then measure the electromagnetic radiation that is reflected from the target.

The passive sensors do not emit radiation and can only receive the electromagnetic radiation that is naturally emitted from the target or that is reflected from the target when it receives the radiation from other sources, such as the Sun.

There are two categories of Earth remote sensing passive systems:

• Instruments that collect the electromagnetic radiation of the Sun when the Earth reflects it. They operate in the visible (0.4 – 0.7) µm and in the medium infrared (3 – 6) µm

• Instruments that collect the electromagnetic radiation that is directly emitted by the Earth. They operate mainly in the thermal infrared (6 – 20) µm.

The remote sensing active systems can be divided into:

• Scattering systems. They work in the visible and in the infrared

• Radar systems. They work in the microwaves range.

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