Remote Sensing refers to the acquisition of information about an distant object or phenomenon without coming into physical contact with the object. It refers to satellite based sensing technology that helps to analyse and interpret as well as to gather information about concerned object or phenomenon. Here our topic of interest is remote sensor, and the main types of remote sensor.


Sensor (satellite sensor) plays a significant role in remote sensing process. Sensor is a device, onboard an earth-orbiting artificial satellite, detects or captures and records the reflected electromagnetic radiation from the object of interest, and transmits the acquired datasets to the ground based receiving station.

Sensor is the most important component of remote sensing. It acts as a mediator to help the propagation of electromagnetic radiation between object and ground based receiving station.

Types of Sensors

Primarily remote sensing sensors are classified into two types. These are as follows–
  • Optical Sensor
  • Microwave Sensor

The flow chart is showing the main types of sensors used in remote sensing

1. Optical Sensor

The sensor which can detect the visible portion of the electromagnetic radiation, and three types of infrared radiation (e.g., near infrared, intermediate infrared, and thermal infrared), that is called the optical sensor.

Thus optical sensor carries out two types of remote sensing, viz.,

i) Visible Radiation or Near Infrared Remote Sensing :
When optical sensor detects the visible radiation and near infrared (NIR) radiation of sunlight which is back-scattered by the object surface.

The strength of reflection helps to study the surface features of the object of interest. But this sensing method has some constraints, such as, interruption occurs during dense cloud cover, or darkness.

ii) Thermal Infrared Remote Sensing :
Similarly, when optical sensor detects the thermal infrared radiation that emits from the object surface in the form of heat.

This sensing method helps to study any phenomenon related to heat emission, e.g., wildfire, volcanic eruption, surface temperature of any object etc. Optical sensors can detect thermal radiation even at night, but cloud cover also affects this observation method.


2. Microwave Sensor

The sensor which can detect and record microwaves, that is called the microwave sensor. Microwave radiation has longer wavelength than that of the visible or thermal infrared radiations, and the sensing method is not affected by cloud cover, or day-night, etc.

Two types of sensors play the role to detect and record microwave radiations, viz.

i) Active Sensor :
Active sensor provides its own source of electromagnetic radiation to illuminate the object of interest. The sensor itself transmits radiation towards the object or phenomenon, and again detects and records that radiation when reflected or back-scattered from the target.

Active remote sensors include Laser altimeter, Radar, LIDAR or Light Detection and Ranging, Ranging instrument, Sounder, Scatterometer, etc.

ii) Passive Sensor :
But in the case of passive sensor, there is no artificial source of energy or electromagnetic radiation. Rather, a passive sensor detects and records natural energy or electromagnetic radiation which is reflected, or emitted from the object of interest or the phenomenon it observes. Sunlight (solar radiation) is the usual source of radiation that is measured by a passive sensor.

The passive remote sensors include Accelerometer, Hyperspectral radiometer, Spectrometer, Imaging radiometer, Sounder, Spectroradiometer, Radiometer, etc.

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