Over the span of time, remote sensing technology found its applications in various spheres of human life. Farmers benefit from the types of remote sensing satellites on a daily basis. Many important decisions ground on the data from RADARSAT, TerraSAR-X, SRTM, EOS, ERS, Sentinel, and LANDSAT, among others.
There are various types of remote sensing. It can be classified on the basis of the following grounds
- Based on the source of energy
- Based on the range of electro-magnetic spectrum
Based on the source of energy
There are two types of remote sensing
- Active remote sensing
- Passive remote sensing
1. Active remote sensing
Contrary to ‘Passive Remote Sensing’ the Remote Sensing performed with the help of artificial energy sources like electric bulbs tube lights, etc. is called Active Remote Sensing. It is generally done when there is dark or during the period of night.
To be active remote sensing they must offer their own source of energy in order to illuminate the items they are studying.
An remote active sensor emits radiation in the direction of the studied target. The sensor then detects and measures reflected or backscattered radiation from the target.
And also active remote sensing has been employed for a number of security purposes, including monitoring in the ocean and the Arctic. The double-bounce dispersion has supplied important information in search and rescue missions.
Remote sensing sensors which have the advantage of obtaining data any time of day or season are known as Active Remote Sensing.
What Is Active Remote Sensing?
There exist two main types of remote sensing classified according to the source of signal they use to explore the object, active vs. passive. Active remote sensing instruments operate with their own source of emission or light, while passive ones rely on the reflected one. Radiation also differs by wavelengths that fall into short (visible, NIR, MIR) and long (microwave).
Active Remote Sensing Instruments
Each active sensor in remote sensing directs its signal to the object and then checks the response – the received quantity. The majority of devices employ microwaves since they are relatively immune to weather conditions. Active remote sensing techniques differ by what they transmit (light or waves) and what they determine (e.g., distance, height, atmospheric conditions, etc.).
Radar
is a sensor assisting in ranging with radio signals. Its specific feature is the antenna emitting impulses. When the energy flow in radar active remote sensing meets an obstacle, it scatters back to the sensor to some degree. Based on its amount and traveling time, it is possible to estimate how far the target is.
Lidar
determines distance with light. Lidar active remote sensing implies transmitting light impulses and checking the quantity retrieved. The target location and distance are understood by multiplying the time by the speed of light.
Laser altimeter
measures elevation with lidar.
Ranging instruments
estimate the range either with one or two identical devices on different platforms sending signals to each other.
Sounder
studies weather conditions vertically by emitting impulses, in case it falls to the active category.
Scatterometer
is a specific device to measure bounced (backscattered) radiation.
Applications And Benefits
Apart from a variety of implementations, active remote sensors basically have no restrictions as to research conditions. Active types of remote sensing systems fully function at any time of the day as they do not require sunlight, and they are relatively independent of atmospheric scatterings.
Data acquired with remote sensing instruments serve agriculturalists and foresters. They are critical in hard-to-reach places in marine sciences and rescue missions.
what are the advantages?
provides two primary benefits
1. The ability to collect imagery at all hours of the day and night.
2. Clouds and bad weather don’t seem to bother it.
2. passive remote sensing
The sun is the most convenient source of energy for Remote Sensing. This sun’s energy is either reflected or absorbed and then re-emitted. We can see any object through the reflected energy of visible wavelength. Remote Sensing systems may major the naturally available energy through sensors. These systems are called Passive Sensors and the remote sensing done through naturally available energy sources like the sun is called Passive Remote Sensing. But the sensors can measure the energy of the sun only during daytime when it is illuminating the Earth. The emitted energy from the Earth can be detected by the sensor for day and night for 24 hours.
Passive remote sensing is the study of the interaction between a light source and various Earth surface characteristics, each of which has its own spectral response.
Radiation is supplied by two sources. The sun is the source of visible to shortwave infrared light gathered by remote sensing systems.
A portion of the radiation received by a sensor was reflected at the earth’s surface, while the remainder was scattered by the atmosphere and never reached the earth.
Thermal radiation released directly by materials on the earth interacts with self-emitted thermal radiation in the atmosphere as it propagates upward in the thermal infrared.
Example of Passive Remote Sensors
Film photography, infrared, charge-coupled devices, and radiometers are examples of passive remote sensors.
What Is Passive Remote Sensing?
Passive sensors in remote sensing do not streamline energy of their own to the researched object or surface, unlike active ones. Passive remote sensing depends on natural energy (sunrays) bounced by the target. For this reason, it can be applied only with proper sunlight, otherwise, there will be nothing to reflect.
Passive remote sensing employs multispectral or hyperspectral sensors that measure the acquired quantity with multiple band combinations. These combinations differ by the number of channels (two wavelengths and more). The scope of bands includes spectra within and beyond human vision (visible, IR, NIR, TIR, microwave).
Passive Remote Sensing Devices
The most popular passive remote sensing examples of devices are various types of radiometers or spectrometers.
Instrument names clearly identify what they measure:
Spectrometer
Distinguishes and analyzes spectral bands.
Radiometer
Determines the power of radiation emitted by the object in particular band ranges (visible, IR, microwave).
Spectroradiometer
Finds out the power of radiation in several band ranges.
Hyperspectral radiometer
Operates with the most accurate type of passive sensor that is used in remote sensing. Due to extremely high resolution, it differentiates hundreds of ultimately narrow spectral bands within visible, NIR, and MIR regions.
Imaging radiometer
Scans the object or a surface to reproduce the image.
Sounder
Senses the atmospheric conditions vertically.
Accelerometer
Detects changes in speed per unit of time (e.g., linear or rotational).
Applications And Advantages
Among examples of passive sensors in remote sensing, Landsat definitely stands out as the most long-lasting Earth-observing mission. It monitored our planet and recorded the obtained data enabling us to analyze the way it changed within a 40-year span. The mission’s great plus is that the information is accessible to the public, with interpretations applied in geology, mapping, ecology, forestry and agriculture, marine sciences, meteorology, etc.
In agriculture, remote sensing utilizes the reflectance properties of vegetation, measuring them, and assessing crop health with vegetation indices. It is possible because specific values of vegetation indices correlate with certain species at a certain growth stage. Our EOSDA Crop Monitoring app helps agro businesses worldwide to manage their daily tasks, as well as check the fields’ state and maintain healthy vegetation on them.
Disadvantages
- Difficult to record a data during night time
- Some of a sensor may fail because of energy shortage
- Atmospheric impact is high
Based on the range of electro-magnetic spectrum
Optical remote sensing
It is the remote sensing done in visible, near infra-red and middle infra-red ranging between the wavelengths from 0.3 um to 3 um.
Thermal remote sensing
Remote sensing of the emitted radiation in the range of 3 um to 5.5 um and 8 to 14 um.
Microwave remote sensing
Remote sensing of higher wavelengths ranging between 1 mm to 1 m. These higher wavelengths can penetrate clouds.