5.13. Different Types of Sensors

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 essential tools in remote sensing that capture energy from objects on the Earth’s surface. They record various types of electromagnetic radiation (EMR), including visible light and infrared radiation. The sensors convert these captured analog signals into digital data which can be processed to create images and gather information about the Earth’s features. Since different types of radiation provide different insights, satellites often carry multiple sensors to capture a broader spectrum of data.

Think of sensors as a camera that not only takes pictures in visible light, like our eyes, but can also capture heat signatures and other types of light that we can't see. Just like a photographer uses different lenses for various lighting conditions, satellites use different sensors for capturing diverse kinds of information about the Earth.

1. Passive Sensors 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 capture data. These sensors detect the sunlight reflected off the Earth’s surface. Since they depend on sunlight, they can only collect data during the day and cannot operate in the dark. An example of a passive sensor is the Landsat Multispectral Scanner, which uses reflected sunlight to collect information about land use, vegetation, and water bodies.

Imagine a photographer taking pictures outside during the day. The photographer uses sunlight to illuminate the scenes they capture. Similarly, passive sensors need sunlight to record information about the Earth, just like a camera needs light to take photographs.

2. Active Sensors 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 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 sensors generate their own energy to illuminate the target, allowing them to capture data at any time, day or night. This is particularly useful in situations where natural light is not available, such as during the night or through cloud cover. A common type of active sensor is the Synthetic Aperture Radar (SAR), which sends out microwave signals and measures the echoes that bounce back to determine the characteristics of the Earth's surface.

Think of active sensors like a flashlight in a dark room. The flashlight shines light to illuminate objects around it, allowing you to see them even when there’s no natural light. Active sensors work similarly, using their own energy source to 'light up' the target, whether it’s day or night.

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.

Sensors can be categorized not only as passive or active but also as scanning or non-scanning systems. Non-scanning sensors capture data from a single point without moving, while scanning sensors move across the surface, capturing data in a line. Scanning systems can provide a broader view and gather more comprehensive data over an area, like the way a photographer might pan their camera across a landscape to capture a panoramic image.

Consider the differences between a single-point camera that takes a snapshot and a video camera that scans back and forth to capture motion. Non-scanning sensors are like the snapshot camera, capturing a fixed view, while scanning sensors are like the video camera, continuously recording sequences to provide detailed images.

1. Optical Sensors The optical remote sensing devices operate in the visible, near infrared, middle infrared and short wave infrared portions of the EMS within a range 0.30 µm to 3.0 µm, e.g., bands of IRS P6 LISS IV sensor work in optical range of EMS.

2. Thermal Sensors Thermal infrared energy is emitted as heat by all the objects, both natural and manmade, that have a temperature greater than absolute zero. Even in complete darkness and poor weather conditions, thermal imaging is able to detect small temperature differences. The 3 to 5 μm range is related to high temperature phenomenon, like forest fire, while 8 to 14 μm range is related with the general Earth features having lower temperatures.

3. Microwave Sensors The microwave region falls between the IR and radio wavelengths, and has a long range extending from approximately 0.1 cm to 1 m. The microwave sensors, e.g., RADARSAT, having their own sources of energy, record the backscattered microwaves and operate independent of weather and solar energy.

Sensors can also be classified based on the wavelengths they operate in: optical, thermal, and microwave. Optical sensors capture visible and near-infrared light, while thermal sensors detect the heat emitted by objects. On the other hand, microwave sensors utilize longer wavelengths and can penetrate clouds and operate in any weather conditions, making them versatile in remote sensing applications.

Think of optical sensors as the colorful lenses of a pair of sunglasses, enhancing the light they capture. Thermal sensors act like a temperature gun that can measure the warmth of objects, no matter how dark it is. Lastly, microwave sensors resemble a radar system that can 'see' through fog or rain, making them invaluable for all-weather surveillance.

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