What is Remote Sensing?
Remote sensing is the process of obtaining information regarding phenomena that cannot be directly observed. It is a method used to collect, store and analyze data on the Earth’s surface in real-time, such remote collection of data provides information on the Earth’s surface. Remote sensing uses sensors and instruments to gather such information from various remote locations.
Remote sensing may be conducted by collecting light reflected or emitted from the Earth’s surface and in some cases shining different wavelengths of electromagnetic energy (such as infrared) through the atmosphere at various heights to ascertain what can be seen in different parts of the earth. The majority of remote sensing applications use some kind of sensor or instrument that collects the information that needs to be collected. There are many types of sensors, each with its advantages and disadvantages.
Applications of Remote Sensing.
Remote sensing is analysing and acquiring information that varies in time and space from the real-world area of interest. It can be used for studying natural phenomena or to analyse or monitor a particular action at a remote site. The different types of remote sensing include television mapping, satellite imaging, and aerial photography – each has its benefits as well as drawbacks.
Sensing comes in many forms, including radio, radar (including LIDAR, SAR and hyperspectral imaging), sonar and infrasound, artificial eyes such as microscopes and telescopes, chemical sensors, seismic waves and gravitational sensors. The field of remote sensing encompasses five areas: imagery; spectroscopy; radar; electro-magnetism; and environmental monitoring. The collected data can be used to map objects on Earth, detect climate changes, monitor land degradation, identify security threats and examine outer space.
Electromagnetic Spectrum
Methods of energy transfer and electromagnetic spectrum
To understand remote sensing, we should also discuss how energy travels in different mediums as the devices used in remote sensing essentially just record such energy released by different surfaces. The main source of this energy is from the sun. In simple terms, sensors absorb the energy received from the sun and help us get a better view remotely.
How energy is transferred?
Transfer of energy takes place in three ways depending on the medium
Conduction: In solids
Convection: In fludis
Radiation: No medium is required
In conduction and convection, a medium is required and energy transfer takes place through particles but in radiation, the transfer of energy takes place through waves. This means that energy from sun travels in waveform until it comes in contact with the earth and its atmosphere. Once this energy interacts with the earth and its atmosphere its nature changes such changes can be recorded by our sensors and analysed.
What is the electromagnetic spectrum?
Since outer space is empty and there are no particles to conduct or convect energy, energy travels in the form of waves. This energy that travels from the sun is in waveform and has different frequencies and wavelengths. Each frequency has different characteristics and based on those characteristics it is named, for example, the frequency with the longest wavelength is known as a Radio wave.
What is electromagnetic radiation (EMR)?
Electromagnetic radiation is generated when an electrical charge is accelerated. Sun emits electromagnetic radiation ranging from Gamma waves to Radio waves. This energy takes 8 minutes to reach our planet since it is 150 million kilometres from the sun.
Properties of EM radiation
In the 1860’s James Maxwell (1831-79) conceptualised that EM moves through space at the speed of light.
EM consists of two oscillating fields electric and magnetic. Both are perpendicular to the direction of propagation of radiation.
Many physicists today would say that “Light is a particular kind of matter” or discrete packets of energy or quantum of energy.
Previously light was thought to be a smooth and continuous wave, then Albert Einstein found that, when light interacts with matter, it behaves as if it is composed of individual bodies called Photons
Two important properties of EM radiation are
(1) Wave
(2) Particle
And a photon comprises of radiation emitted, reflected, or absorbed
Wavelength, Amplitude & Frequency
>Wavelength is the distance between two peaks or crests
>Amplitude is the maximum extent of an oscillation from equilibrium
>Frequency is the number of crests or waves that move past a given point in a unit of time
Light Interactions
Atmosphere and Surface Interactions with EMR
Remote sensing sensors installed on satellites capture light or energy coming from the Earth. This energy/light coming from the sun has to pass through the atmosphere twice before it reaches the sensors. Therefore, there are two classifications of radiation i.e., the incoming solar radiation known as shortwave radiation, and the one that is radiated from the earth is known as longwave radiation.
Shortwave Radiation coming from the sun have a higher frequency (wavelength is shorter than 3 micrometre) and carries more energy but cannot be absorbed by the atmosphere directly, but a major part of this is reflected by our atmosphere (examples- UV, Visible light).
Above figure shows the distribution of Shortwave Radiations coming from the Sun
Longwave Radiation is emitted from the earth after it absorbs the shortwave radiation. These radiations have a lower frequency (wavelength is higher than 3 micrometres), carry less energy, and are responsible for heating our atmosphere directly.
Above figure shows the distribution of Longwave Radiation emitted from Earth
Atmospheric Window
All wavelengths cannot pass through the Earth’s atmosphere, visible light, infrared and radio are the three wavelengths that form the atmospheric window.
The figure shows absorption bands of Earth’s atmosphere (grey colour) delimit its atmospheric windows (middle panel) and the effect they have on both downgoing solar radiation and upgoing thermal radiation emitted near the surface is shown in the top panel. The individual absorption spectra of major greenhouse gases plus Rayleigh scattering are shown in the lower panel
Solar Energy Interactions:
The adjoining diagram explains the interaction of solar energy radiation with the Remote Sensing System.
- The Solar Irradiance E0 is incident on the area of interest at an angle theta not (specific solar zenith angle). the atmospheric transmittance is T theta not which is defined as the ratio of the radiation reaching the earth to the total radiation emitted by the Sun. Its value can lie between 0 to 1.
- Spectral Diffuse Irradiance Ed does not reach the area of interest due to scattering in the atmosphere. Since blue light is scattered the most (according to Rayleigh Scattering), the blue band image produced by the sensor is much brighter as compared to other bands. It also contains a lot of unwanted diffuse sky irradiance. This is referred to Upward Reflectance of the atmosphere (Edu).
- Energy reaches the target area after some scattering/absorption and varies in spectral composition as compared to (1) hence is known as Modified Energy or Downward Reflectance of the atmosphere (Edd).
- Some radiation is also reflected from the neighboring areas which are not the area of interest.
- Some energy does get reflected from nearby terrain into the atmosphere but then gets scattered or reflected onto the target area.
Radiation from Paths 1,3 and 5 combine to form Target Radiance LT which is desirable on the sensor. Radiation from Path 2 and 4 combine to form Path Radiance LP. It is not desirable on the sensor, hence its effects need to be minimized.
The total radiance recorded by the sensor becomes LS = LT + LP
