Sensors are the key component of remote sensing systems, as they are responsible for collecting the data that is used to create images and other forms of information. There are many different types of sensors used in remote sensing, each with their own unique characteristics and capabilities.
Sensors are instruments that collect data about Earth processes or atmospheric components. Along with being carried aboard satellites or aircraft, sensors also can be installed on the ground.
A Sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
“A sensor is a device that detects and responds to some type of input from the physical environment. The specific input could be light, heat, motion, moisture, pressure, or any one of a great number of other environmental phenomena”.
The output is generally a signal that is converted to human-readable display at the sensor location or transmitted electronically over a network for reading or further processing.
Types of sensors
1) On the Basis of Light Source
2) On the basis of data acquisition
3) On the basis of scanning
On the Basis of Light Source
There are two types of sensors:
- Active Sensors
- Passive Sensors
Active sensors provide their own source of energy to illuminate the objects they observe.
Active Sensor is source of light or illumination and its sensor measures reflected energy. The energy is generated and sent from the Remote Sensing platform towards the targets. Radar is an example of Active Sensor.
An active sensor emits radiation in the direction of the target to be investigated. The sensor then detects and measures the radiation that is reflected or backscattered from the target.
The majority of active sensors operate in the microwave portion of the electromagnetic spectrum, which makes them able to penetrate the atmosphere under most conditions.
Example of ARS Laser altimeter, Lidar, Radar, Ranging Instrument, Scatterometer
Active remote sensing system carries onboard its own electromagnetic radiation source. This electromagnetic radiation is directed at the surface and the energy that is scattered back from the surface is recorded.
A camera is normally a passive means of recording data. If the photograph is obtained in daylight, the electromagnetic radiation within the visible waveband is reflected from the object and recorded by the film. However, if the photograph is obtained at night, an electronic flash is initially fired, which directs an electromagnetic pulse at the target in order to illuminate it and the reflected radiation is received and recorded back at the camera. It is then called Active Remote Sensing. (Ref. 1)
An Active Remote Sensing System Produces its own electromagnetic Radiation
The electromagnetic (EM) spectrum is the range of all types of EM radiation
Electromagnetic spectrum, the entire distribution of electromagnetic radiation according to frequency or wavelength.
Active Remote Sensing Instruments
- Laser altimeter
- Ranging instruments
Passive Sensor is source of energy is that naturally available of the Sun. Most of the Remote Sensing systems work in passive mode using solar energy as the source of EMR.
Sunlight is the most common source of radiation measured by passive sensors.
Passive sensors gather radiation that is emitted or reflected by the object or surrounding areas.
Most passive systems used in remote sensing applications operate in the visible, infrared, thermal infrared, and microwave portions of the electromagnetic spectrum.
Example of PRS Accelerometer, Hyperspectral radiometer, Imaging radiometer, Radiometer, Spectrometer.
When the source of energy is external such as the Sun, Remote Sensing System is called Passive. It detects the solar energy reflected form the target (earth). When the energy is produced by the remote sensing system itself then it is called Active; it senses the energy reflected from the target. (Ref.1)
Passive Remote Sensing System
Passive Remote Sensing Instruments
- Hyperspectral radiometer
- Imaging radiometer
On the Basis of Data Acquisition
- Imaging Sensors
- Non-Imaging Sensors
Imaging sensors are sensors that build up a digital image of the field of view, with some information about how the input varies in space, not just in strength.
Generally this is done with some sort of pixelated sensor, like a CCD camera.
The spatial distribution of the signal strength will be recorded in the spatial distribution of the sensor’s response.
Imaging Sensor is the electrons released are used to excite or ionize a substance, like silver in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors.
Optical Imaging Sensors
Optical imaging sensors operate in the visible and reflective IR ranges.
Typical optical imaging systems on space platform include panchromatic systems, multispectral systems, and hyperspectral systems.
In a panchromatic system, the sensor is a monospectral channel detector that is sensitive to radiation within a broad wavelength range. The image is black and white or gray scale.
A multispectral sensor is a multichannel detector with a few spectral bands.
A hyperspectral sensor collects and processes information from 10 to 100 of spectral bands. A hyperspectral image consists of a set of images.
Thermal IR Imaging Sensors
It operates in the electromagnetic spectrum between the mid-to-far-infrared and microwave ranges, roughly between 9 and 14 μm.
Any object with a temperature above zero can emit infrared radiation and produce a thermal image.
A warm object emits more thermal energy than a cooler object. Therefore, the object becomes more visible in an image.
This is especially useful in tracking a living creature, including animals and the human body, and detecting volcanos and forest fires.
Radar Imaging Sensors
A radar (microwave) imaging sensor is usually an active sensor, operating in an electromagnetic spectrum range of 1 mm–1 m.
The sensor transmits light to the ground, and the energy is reflected from the target to the radar antenna to produce an image at microwave wavelengths.
Unlike infrared data that help us identify different minerals and vegetation types from reflected sunlight, radar only shows the difference in the surface roughness and geometry and the moisture content of the ground
Optical and Thermal Image
A non-imaging sensor measures a signal based on the intensity of the whole field of view, mainly as a profile recorder.
In contrast with imaging sensors, this type of sensor does not record how the input varies across the field of view.
In the remote sensing field, the commonly used non-imaging sensors include radiometers, altimeters, spectrometers, spectroradiometers, and LIDAR.
The applications for non-imaging sensors mainly focus on height, temperature, wind speed, and other atmospheric parameter measurements.
Non-imaging Sensor is measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance.
Measures the intensity of radiation in multiple wavelength bands (i.e., multispectral). Many times the bands are of high-spectral resolution, designed for remotely sensing specific geophysical parameters
Measurement of spectral output of different radiation sources (often for plant or human lighting), and reflectance and transmittance measurements of natural and synthetic surfaces and materials (often plant leaves and canopies).
An instrument that quantitatively measures the intensity of electromagnetic radiation in some bands within the spectrum. Usually, a radiometer is further identified by the portion of the spectrum it covers; for example, visible, infrared, or microwave
Lasers have been applied in measuring the distance and height of targets in the remote sensing field.
Generally call a laser scanning system as LIDAR (light detection and ranging) system.
Satellite LIDAR, airborne LIDAR, mobile mapping LIDAR, and terrestrial LIDAR are different carrier platforms.
On the Basis of Scanning
Sensors can also be classified based on the way they scan the Earth’s surface. The two main types of scanning methods are push-broom and whisk-broom.
Across Track Scanning Sensor (Whisk-broom)
Whisk-broom sensors, also known as frame-scan sensors, collect data in a rectangular pattern. They scan the Earth’s surface by moving in a continuous pattern, and the data is collected as a complete image or frame. Examples of whisk-broom sensors include cameras and multispectral sensors.
Whisk-broom sensors, on the other hand, are typically used for detailed mapping and monitoring of specific areas or features. They are also useful for monitoring dynamic events, such as natural disasters or urban development.
Scanning in the series of line
The lines are oriented perpendicular to the direction of motion of the sensor platform
Due to rotational mirror scan of satellite
Airborne scanning angle- (90-120 degree)
Satellite scanning angle- (10-20 degree)
Along Track Scanning Sensor (Push-broom)
Push-broom sensors, also known as line-scan sensors, collect data in a linear pattern. They scan the Earth’s surface by moving in a single direction, and the data is collected as a series of lines or strips. Examples of push-broom sensors include Landsat, Sentinel, and MODIS.
Push-broom sensors are typically used for large-scale mapping and monitoring of the Earth’s surface, as they can cover large areas in a single pass. They are also useful for monitoring changes over time, as multiple images can be collected and compared.
The most common types of sensors used in remote sensing include:
Which detect and measure the electromagnetic energy in the visible and near-infrared regions of the spectrum. These sensors are typically used to create images and maps of the Earth’s surface, and include cameras and multispectral sensors.
Which emit and detect radio waves to measure the reflection or backscatter of energy. These sensors are typically used to create images and maps of the Earth’s surface and atmosphere, and can be used in all-weather conditions.
Which emit laser pulses and detect the reflection of the energy to measure the distance to an object. These sensors are typically used to create 3D maps and models of the Earth’s surface, and can be used to measure vegetation height, building height, and other features.
Which detect and measure the electromagnetic energy in the infrared regions of the spectrum. These sensors are typically used to measure temperature and other thermal properties of the Earth’s surface and atmosphere.
Which detect and measure the electromagnetic energy in the microwave regions of the spectrum. These sensors are typically used to measure the precipitation, sea surface temperature and other properties of the Earth’s surface and atmosphere.
Sensors are also classified based on the way they collect data, Passive sensors, which detect the naturally occurring radiation emitted or reflected by the Earth’s surface and Active sensors which emit the energy and measure the reflection or backscatter.
Overall, the selection of the sensor depends on the application and the information required. Remote sensing systems are designed to collect data from different regions of the electromagnetic spectrum, and the sensor must be selected to match the desired wavelength and resolution.
- R.P. Misra, Fundamentals of Cartography, Second Revised and Enlarged Edition.