5.13.1. Based on the source of illumination

Sensor is a device that captures the reflected/emitted radiations from the objects, and coverts these radiations (analog signals) into digital signals. Satellite sensors record the reflected and emitted radiations not only the visible spectrum but also infrared, near infrared, and thermal infrared bands, and microwave bands. So, normally more than one sensor is deployed in a satellite.

Sensors are tools that detect light or other electromagnetic radiation from objects on Earth. They transform these signals into a digital format for analysis. Different types of sensors can capture various wavelengths of radiation, allowing them to gather comprehensive information about the Earth's surface.

Think of a camera. When you take a picture, the camera sensor captures light reflecting from objects and converts this light into a digital image. Similarly, remote sensing satellites have sensors that capture not just visible light but also infrared and thermal energy.

The Sun provides a useful and important source of natural energy for remote sensing applications. The sensors record the reflected Sun energy from the objects. A sensor that depends on an external (natural) source to illuminate the target to be sensed is called a passive sensor. In visible light, the Sun illuminates the target/object, and the reflected light from the target is detected by the sensor. Most sun-synchronous (Polar) satellites employ passive sensors with them. Landsat MSS is an example of passive sensor. Passive remote sensing images have been used frequently for natural resource mapping and monitoring.

Passive sensors rely on external sources of light, primarily the sun, to illuminate objects on Earth. These sensors detect the light that is reflected off these objects. For example, satellites that use passive sensors can capture images of landscapes during daylight. An example of a passive sensor is the Landsat MSS, which is often used for mapping natural resources.

Imagine taking a photograph outside during the day. You’re using natural sunlight to see everything around you. In the same way, passive sensors on satellites take images during sunny days, using sunlight to illuminate the ground.

The Sun’s energy is available only in the sunlit portion of Earth and not in the other half dark portion of Earth surface, at any given time. So, when taking images in dark or poor weather conditions, sensors carry their own source of energy to illuminate the objects and record reflected and emitted energy from these objects. A sensor that consists of both the source to illuminate the targets and sensor to record the reflected and emitted energy from the objects/targets is called active sensor. The majority of active sensors operate in the microwave portion of the EMS, and are able to penetrate the atmosphere all the time under any conditions. These sensors have the advantage of obtaining data any time of day or season. The Synthetic Aperture Radar (SAR) is a good example of active sensor which transmits microwave pulses from its transmitter antenna to illuminate the target and receives the return signals by its receiver antenna. Active remote sensing images can be used for a variety of applications, such as soil moisture, agriculture, geology, vegetation, including marine, and search and rescue missions.

Active sensors use their own light source to illuminate targets, allowing them to capture data at any time, including during the night or in cloudy conditions. They produce their own signals—often in the microwave range—and analyze the reflections of these signals to gather information about the Earth's surface. A common type of active sensor is the Synthetic Aperture Radar (SAR), which can provide detailed images of the landscape regardless of weather or lighting conditions.

Think of a flashlight. If you walk into a dark room and turn on your flashlight, you can see everything around you even when there's no natural light. Similarly, active sensors like SAR send out their own energy to light up the target, allowing them to capture images in darkness or cloudy weather.

Passive and active sensors both are further divided into scanning and non-scanning systems. A sensor classified as a combination of passive, non-scanning and non-imaging method is a type of profile recorder, for example a microwave radiometer. A sensor classified as passive, non-scanning and imaging method, is a camera, such as an aerial survey camera or a space camera. Sensors classified as a combination of passive, scanning and imaging are classified further into image plane scanning sensors, such as TV cameras and solid state scanners, and object plane scanning sensors, such as multispectral scanners (optical-mechanical scanner) and scanning microwave radiometers.

Both passive and active sensors can be designed to operate in different ways. Some sensors continuously scan across the ground to capture images (scanning systems), while others take images in discrete steps (non-scanning systems). Examples include microwave radiometers for non-scanning measures and various types of cameras for imaging.

Imagine taking video of a parade. If you move your camera side to side smoothly while recording, that's like a scanning sensor. If you take pictures at intervals, that's more like a non-scanning sensor that captures specific moments instead of a continuous view.

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