Sensor is a device that measures and records the reflected or emitted electromagnetic radiation. Sensors can be divided into two group’s namely passive and active sensor.
So far, throughout this chapter, we have made various references to the sun as a source of energy or radiation. The sun provides a very convenient source of energy for remote sensing. The sun’s energy is either reflected, as it is for visible wavelengths, or absorbed and then re-emitted, as it is for thermal infrared wavelengths.
In order to assist you in making an informed choice, we will address the issue of deciding between active and passive sensors for remote sensing, explore the difficulties encountered, and offer a solution.
A. Passive sensors
Remote sensing systems which measure energy that is naturally available are called passive sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded. Example of passive sensor includes Aerial Camera, Thermal scanner, Gamma-ray spectrometer, Video camera, imaging spectrometer etc.
Passive sensors are used in various remote sensing applications, such as:
- Cameras for capturing visible light.
- Infrared sensors for detecting heat signatures.
- Multispectral sensors for capturing images in different spectral bands.
Passive sensors are preferred for remote sensing applications where the target is easily accessible or when the target emits enough natural energy to be detected.
Example of Passive Sensor In Remote Sensing
An example of passive sensor in remote sensing is a multispectral or hyperspectral imager, which measures the natural radiation emitted or reflected by the target at various wavelengths to determine its temperature, color, and chemical composition.
These sensors are used for identifying land cover, mapping vegetation, monitoring ocean color, and detecting atmospheric gases. Another example is a thermal imaging camera, which detects infrared radiation emitted by the target to measure its temperature and identify sources of heat.
Advantages of Passive Sensors
- Passive sensors do not require a high energy source to detect the target.
- Passive sensors can be less expensive to operate.
- Passive sensors are not affected by weather conditions or atmospheric interference.
- Passive sensors can capture images with high spatial resolution.
- Passive sensors can provide accurate information about the natural energy emitted or reflected by the target.
Disadvantages of Passive Sensors
- Passive sensors cannot penetrate clouds or smoke to detect the target.
- Passive sensors cannot be used to detect objects hidden behind other objects.
- Passive sensors are limited to the amount of natural energy emitted or reflected by the target.
B.Active sensors, on the other hand, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Active sensors can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and synthetic aperture radar (SAR).
The main advantage of active sensor systems is that they can be operated day and night and have a controlled illuminating signal.
Active sensors are used in various remote sensing applications like:
- RADAR (Radio Detection and Ranging) for detecting objects and their positions.
- LIDAR (Light Detection and Ranging) for detecting the distance between the sensor and the target using laser beams.
- SONAR (Sound Navigation and Ranging) for detecting underwater objects and their positions.
Active sensors are preferred for remote sensing applications where the target is difficult to access or when data needs to be acquired quickly.
Example of Active Sensor In Remote Sensing
An example of active sensor in remote sensing is the radar, which emits microwaves and measures the time delay and intensity of the reflected signal to determine the location, shape, and composition of the target.
Radars are used for mapping topography, detecting targets such as ships and aircraft, and monitoring the movement of glaciers and ice sheets.
Advantages of Active Sensors
- Active sensors can penetrate clouds and smoke to detect the target.
- Active sensors can be used to detect objects hidden behind other objects.
- Active sensors can operate at any time of day or night.
- Active sensors can generate 3D images of the target.
Disadvantages of Active Sensors
- Active sensors require a high energy source to generate the energy required to detect the target.
- Active sensors can be affected by external factors such as weather conditions and atmospheric interference.
- Active sensors can be expensive to operate.
In summary, both passive and active sensors offer advantages and disadvantages when it comes to remote sensing applications. Applications where the target is hard to reach or where data must be obtained fast are better suited for active sensors. Applications where the target is easily accessible or where it releases enough natural energy to be detected are better suited for passive sensors. The particular needs of the application determine which sensor is best.
FAQs: Sensors: Active vs. Passive
What distinguishes an active sensor from a passive sensor?
While passive sensors monitor the radiation that a target emits or reflects, active sensors use energy to identify targets.
In remote sensing, how does an active sensor function?
Active sensors, like radar or lidar, release energy and use the intensity and time delay of the reflected signal to identify the target’s composition, location, and form.
In remote sensing, how do passive sensors operate?
In order to ascertain the target’s temperature, color, and chemical makeup, passive sensors measure the radiation that the target emits or reflects, such as visual, infrared, or microwave radiation.
What impact does sensor selection have on remote sensing’s efficacy?
The kind is influenced by the sensor selection.
