Types of Remote Sensing

Remote sensing refers to the technology of gathering information about the Earth’s surface without direct physical contact. There are two main types of remote sensing:

1. Passive sensors
2. Active sensors

Passive Sensors

Passive sensors detect natural energy (radiation) emitted or reflected by the object or scene being observed. The most common source of radiation measured by passive sensors is reflected sunlight. In contrast, sensing done via active sensors is called active sensing, for which they often require additional electrical power for excitation. Passive sensors can only detect the energy that naturally occurs, and they gather data during the day when electromagnetic energy is available.

Unlike active sensors, passive sensors do not emit energy and instead wait for data requests. They are vulnerable to external disturbances. The transducer of a passive sensor causes a change in a passive electrical quantity such as capacitance, resistance, or inductance as a result of the stimulation.

Passive sensors measure using electromagnetic radiation that is naturally emitted within their field of view. Examples of passive sensors include satellites used for remote sensing, such as SPOT-1 and LANDSAT-1.

Active Sensors

Active sensors transmit their own signal and measure the energy that is reflected, transmitted back, or scattered back from the target. Examples include radar and sonar. Sensing done via passive sensors is called passive sensing. An active sensor’s transducer directly produces electric current or voltage in response to external stimulus. It generates its own electromagnetic (EM) energy, sends it toward the Earth, and then collects the energy reflected back from the planet. Electromagnetic (EM) radiation received is used for measurement.

Active sensors can self-destruct during hijack attempts and can take measurements at any time. They measure and transmit electromagnetic energy. Unlike passive sensors, active sensors actively communicate measurements to ground stations regardless of whether the on-duty employees want the data or not. They provide their own energy source for illumination. Examples of active sensors include communication satellites, earth observation satellites, and LISS-1.

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