Basics of Remote Sensing: Definitions

DEFINITIONS

1. Remote Sensing: Remote sensing is the science and technology of acquiring information about the Earth's surface and atmosphere using sensors mounted on platforms such as satellites, aircraft, drones, or ground-based instruments. It involves the detection, recording, and interpretation of electromagnetic radiation emitted, reflected, or scattered by objects and features on the Earth's surface.

   
   

   
   

2. Electromagnetic Radiation: Electromagnetic radiation (EMR) refers to the energy emitted by objects in the form of waves as a result of their temperature or interaction with other objects. EMR includes a wide range of wavelengths, from short wavelengths such as gamma rays and X-rays to long wavelengths such as radio waves and microwaves.

   
   

   
   

3. Wavelength: Wavelength is the distance between successive peaks or troughs of a wave. In remote sensing, electromagnetic radiation is characterized by its wavelength, which determines its position in the electromagnetic spectrum. Different wavelengths of EMR interact with the Earth's surface and atmosphere in unique ways, providing valuable information about the properties and features of the observed objects.

   
   

   
   

4. Sensors: Sensors are devices used to detect and measure electromagnetic radiation emitted, reflected, or scattered by objects on the Earth's surface. Remote sensing sensors may operate in various regions of the electromagnetic spectrum, such as visible, infrared, thermal, microwave, and radar wavelengths. They capture images and data that are used to create maps, monitor changes in the environment, and study Earth processes.

   
   

   
   

5. Resolution: Resolution refers to the level of detail or spatial accuracy of remote sensing images and data. It is determined by the sensor's spatial, spectral, radiometric, and temporal capabilities. Spatial resolution refers to the size of the smallest feature that can be resolved in an image, spectral resolution refers to the number and width of the spectral bands, radiometric resolution refers to the sensitivity to detect variations in brightness or intensity, and temporal resolution refers to the frequency of data acquisition over time.

   
   

   
   

6. Platform: A platform is the vehicle or platform from which remote sensing sensors are deployed to observe the Earth's surface. Common platforms include satellites, aircraft, drones (unmanned aerial vehicles), and ground-based instruments. Each platform has advantages and limitations in terms of spatial coverage, spatial resolution, revisit time, and cost.

   
   

   
   

7. Image Interpretation: Image interpretation is the process of visually analyzing remote sensing images to identify, classify, and extract information about features and phenomena on the Earth's surface. It involves interpreting patterns, shapes, colors, textures, and spatial relationships in the images to derive meaningful insights and make informed decisions.

These definitions provide a foundational understanding of key concepts and terminology in remote sensing, which is essential for effectively utilising remote sensing technology in various applications, including environmental monitoring, natural resource management, agriculture, urban planning, disaster management, and climate change research.

Principles and Processes of Remote Sensing

Principles of Remote Sensing

Different objects reflect or emit different amounts of energy in different bands of the electromagnetic spectrum. The amount of energy reflected or emitted depends on the properties of both the material and the incident energy (angle of incidence, intensity and wavelength). Detection and discrimination of objects or surface features is done through the uniqueness of the reflected or emitted electromagnetic radiation from the object.

A device to detect this reflected or emitted electro-magnetic radiation from an object is called a “sensor” (e.g., cameras and scanners). A vehicle used to carry the sensor is called a “platform” (e.g., aircrafts and satellites).

Main stages in remote sensing are the following:

A. Emission of electromagnetic radiation

# The Sun or an EMR source located on the platform

B. Transmission of energy from the source to the object

Absorption and scattering of the EMR while transmission

C. Interaction of EMR with the object and subsequent reflection and emission

D. Transmission of energy from the object to the sensor

E. Recording of energy by the sensor

# Photographic or non-photographic sensors

F. Transmission of the recorded information to the ground station

G. Processing of the data into digital or hard copy image

H. Analysis of data

Types of Remote Sensing: Advantages and Limitation

Depending on the source of electromagnetic energy, remote sensing can be classified as passive or active remote sensing. Passive and active remote sensing are two main approaches used to acquire information about the Earth's surface and atmosphere using remote sensing technology. Here's an overview of passive and active remote sensing:

1. PASSIVE REMOTE SENSING

   • In passive remote sensing, sensors detect natural electromagnetic radiation emitted, reflected, or scattered by objects on the Earth's surface and atmosphere. Passive sensors do not emit their own radiation but rely on the ambient energy sources, such as sunlight or thermal radiation, to illuminate the Earth's surface.
   • Examples of passive remote sensing include:
     • Optical sensors: These sensors capture sunlight reflected or scattered by objects on the Earth's surface in various spectral bands, such as visible, near-infrared, and thermal infrared wavelengths. Examples include multispectral and hyperspectral imaging sensors.
     • Passive microwave sensors: These sensors detect natural microwave radiation emitted by the Earth's surface and atmosphere, such as thermal emissions from the Earth's surface and atmospheric water vapor and precipitation.
   • Passive remote sensing is commonly used for applications such as land cover mapping, vegetation monitoring, oceanography, atmospheric studies, and climate research.
2. ACTIVE REMOTE SENSING
   
   • In active remote sensing, sensors emit their own electromagnetic radiation and measure the reflected or backscattered energy from objects on the Earth's surface and atmosphere. Active sensors actively illuminate the target with pulses of energy and measure the time it takes for the energy to return to the sensor.
   • Examples of active remote sensing include:
     • Radar (Radio Detection and Ranging): Radar sensors emit microwave or radio waves towards the Earth's surface and measure the strength and timing of the reflected signals to create images and maps of surface features. Different radar techniques, such as synthetic aperture radar (SAR) and interferometric SAR (InSAR), are used for various applications, including topographic mapping, terrain analysis, and monitoring of land deformation.
     • LiDAR (Light Detection and Ranging): LiDAR sensors emit laser pulses towards the Earth's surface and measure the time it takes for the pulses to return after reflecting off objects on the ground. LiDAR data provides highly accurate three-dimensional information about the Earth's surface, including terrain elevation, vegetation structure, and land cover.
   • Active remote sensing is widely used for applications such as topographic mapping, terrain modeling, forest inventory, urban planning, infrastructure monitoring, and disaster management

Both passive and active remote sensing techniques have their advantages and limitations, and the choice between them depends on factors such as the desired spatial resolution, spectral coverage, temporal frequency, and specific application requirements. By combining passive and active remote sensing approaches, researchers and practitioners can obtain complementary information and gain a comprehensive understanding of the Earth's surface and atmosphere.                      

EMR Spectrum and its properties

EMR SPECTRUM

Distribution of the continuum of radiant energy can be plotted as a function of wavelength (or frequency) and is known as the electromagnetic radiation (EMR) spectrum. EMR spectrum is divided into regions or intervals of different wavelengths and such regions are denoted by  different names. However, there is no strict dividing line between one spectral region and its adjacent one.

The EM spectrum ranges from gamma rays with very short wavelengths to radio waves with very long wavelengths. The EM spectrum is shown in a logarithmic scale in order to portray shorter wavelengths.

The visible region (human eye is sensitive to this region) occupies a very small region in the range between 0.4 and 0.7 μm. The approximate range of color “blue” is 0.4 – 0.5 μm, “green” is 0.5-0.6 μm and “red” is 0.6-0.7 μm. Ultraviolet (UV) region adjoins the blue end of the visible region and infrared (IR) region adjoins the red end.

The infrared (IR) region, spanning between 0.7 and 100 μm, has four subintervals of special interest for remote sensing:

(1) Reflected IR (0.7 - 3.0 μm)

(2) Film responsive subset, the photographic IR (0.7 - 0.9 μm)

(3)  Thermal bands at (3 - 5 μm) 

(4) (8 - 14 μm)

Longer wavelength intervals beyond this region are referred in units ranging from 0.1 to 100 cm. The microwave region spreads across 0.1 to 100 cm, which includes all the intervals used by radar systems. The radar systems generate their own active radiation and direct it towards the targets of interest. 

Energy in the gamma rays, X-rays and most of the UV rays are absorbed by the Earth’s atmosphere and hence not used in remote sensing. Most of the remote sensing systems operate in visible, infrared (IR) and microwave regions of the spectrum. Some systems use the long wave portion of the UV spectrum also. 

APPLICATIONS
 Different regions of the electromagnetic spectrum have diverse applications across various fields, including:

Communication: Radio waves and microwaves are used for wireless communication, broadcasting, and satellite communication.

Remote Sensing: Infrared, visible, and microwave radiation are used for remote sensing applications, such as weather forecasting, environmental monitoring, and agriculture.

Medical Imaging: X-rays and gamma rays are used for medical imaging techniques such as X-ray radiography, computed tomography (CT), and positron emission tomography (PET).

Astronomy: Different regions of the electromagnetic spectrum are used for astronomical observations, including radio astronomy, optical astronomy, and gamma-ray astronomy.

EMR interaction in the atmosphere and with earth’s surface

Electromagnetic radiation (EMR) interacts with the Earth's atmosphere and surface through various processes, each influencing the behaviour and properties of EMR in different ways. Here's an overview of EMR interactions in the atmosphere and with the Earth's surface:

1. ABSORPTION
   
   • Absorption occurs when electromagnetic radiation is absorbed by molecules or particles in the atmosphere or at the Earth's surface. Different substances absorb EMR at specific wavelengths, depending on their molecular structure and energy levels. For example, greenhouse gases such as carbon dioxide (CO2), water vapour (H2O), and methane (CH4) absorb infrared radiation, leading to the greenhouse effect and warming of the Earth's surface.

2. SCATTERING

   • Scattering occurs when EMR is deflected or redirected in different directions by particles or molecules in the atmosphere. There are three main types of scattering:
     • Rayleigh scattering: This occurs when EMR interacts with particles smaller than the wavelength of the radiation, such as gas molecules and aerosols. Rayleigh scattering is responsible for the blue color of the sky and the reddening of the sun during sunrise and sunset.
     • Mie scattering: This occurs when EMR interacts with particles similar in size to the wavelength of the radiation, such as large aerosols, dust, and water droplets. Mie scattering can cause haze, fog, and cloud formation.
     • Non-selective scattering: This occurs when EMR interacts with particles larger than the wavelength of the radiation, such as water droplets in clouds. Non-selective scattering results in diffuse reflection and reduces the intensity of direct sunlight.

3. REFLECTION

   • Reflection occurs when EMR bounces off the surface of materials without being absorbed. At the Earth's surface, reflection can be specular (mirror-like) or diffuse (scattered in different directions) depending on the surface roughness and angle of incidence. Different surfaces have varying degrees of reflectivity, with shiny surfaces reflecting more EMR than rough surfaces.

4. TRANSMISSION

   • Transmission occurs when EMR passes through transparent materials such as air, water, and glass with minimal absorption or scattering. At the Earth's surface, transmission of solar radiation through the atmosphere contributes to daylight and solar irradiance, providing energy for photosynthesis, heating, and other processes.

5. EMISSION

   • Emission occurs when materials emit electromagnetic radiation as a result of their temperature or energy state. At the Earth's surface, materials emit thermal radiation in the form of infrared radiation, depending on their temperature. This emitted radiation can be absorbed, scattered, or transmitted in the atmosphere, contributing to the Earth's energy balance and climate.

These interactions play a crucial role in shaping the distribution, intensity, and spectral properties of electromagnetic radiation in the Earth-atmosphere system, influencing climate, weather patterns, and environmental processes. Understanding these interactions is essential for various applications, including remote sensing, atmospheric science, climate modeling, and environmental monitoring.

Spectral Signatures and Atmospheric Windows

Spectral signatures and atmospheric windows are important concepts in remote sensing, particularly in understanding how electromagnetic radiation interacts with the Earth's atmosphere and surface.

1. SPECTRAL SIGNATURES:

   • Spectral signatures refer to the unique patterns of electromagnetic radiation reflected, emitted, or absorbed by different materials or substances at specific wavelengths across the electromagnetic spectrum. Each material has its own characteristic spectral signature, determined by its chemical composition, molecular structure, and physical properties.
   • Spectral signatures are often represented graphically as spectral reflectance or emission curves, showing the intensity of radiation as a function of wavelength. These curves exhibit distinctive features or peaks corresponding to specific absorption or reflection bands associated with molecular vibrations or electronic transitions in the material.
   • Spectral signatures are used in remote sensing to identify and classify surface features, such as vegetation, water bodies, soils, and urban areas, based on their unique spectral characteristics. By comparing the spectral signature of a target with known reference spectra, remote sensing analysts can infer the composition, health, and condition of the Earth's surface.

2. ATMOSPHERIC WINDOWS:

   • Atmospheric windows are specific wavelength ranges within the electromagnetic spectrum where electromagnetic radiation can penetrate the Earth's atmosphere with minimal absorption or attenuation. These windows correspond to regions where atmospheric gases, particularly water vapor and other greenhouse gases, have low absorption coefficients.
   • The main atmospheric windows include:
     • Visible (VIS) window: Wavelength range approximately 0.4 to 0.7 micrometers (µm), corresponding to the visible spectrum detectable by the human eye. Visible light can penetrate the atmosphere relatively unimpeded, allowing for direct observation and photography of the Earth's surface.
     • Near-Infrared (NIR) window: Wavelength range approximately 0.7 to 1.3 micrometers (µm), adjacent to the visible spectrum. Near-infrared radiation is also transmitted well through the atmosphere and is used in remote sensing for vegetation monitoring, land cover classification, and other applications.
     • Thermal-Infrared (TIR) window: Wavelength range approximately 8 to 14 micrometers (µm), corresponding to thermal radiation emitted by the Earth's surface. In this window, atmospheric absorption is minimal, allowing for the detection of thermal emissions from the Earth's surface for temperature mapping and thermal analysis.
   • Atmospheric windows are critical for remote sensing applications as they enable the acquisition of electromagnetic radiation at specific wavelengths relevant for surface observation and analysis. Remote sensing instruments and sensors are often designed to operate within these atmospheric windows to maximise data acquisition efficiency and quality.
   • Image showing Atmospheric window and Reflectance Curve                                                                                                                                                                                                                                                                          

Understanding spectral signatures and atmospheric windows is essential for selecting appropriate remote sensing techniques, wavelengths, and sensors for specific applications, as well as for interpreting remote sensing data accurately to extract information about the Earth's surface and atmosphere.

Physical Basis of Remote Sensing: Sources of Energy

Remote Sensing: Sources of Energy

Remote sensing relies on the detection and measurement of electromagnetic radiation emitted or reflected by objects on Earth's surface or in its atmosphere. This radiation, often referred to simply as "energy," is emitted by the sun and subsequently interacts with the Earth's surface and atmosphere.  The primary sources of energy involved in remote sensing are: 

1. SOLAR ENERGY: The sun is the primary source of energy for remote sensing. It emits electromagnetic radiation across a wide range of wavelengths, from gamma rays to radio waves. However, only a small portion of this radiation, known as the solar spectrum, is relevant for remote sensing applications. The solar spectrum includes ultraviolet (UV), visible, and near-infrared (NIR) wavelengths, which are essential for many remote sensing techniques.

   
   

   
   

2. REFLECTED SOLAR ENERGY: When sunlight strikes an object, some of the energy is absorbed, and the rest is reflected. The composition and properties of the object determine which wavelengths are absorbed and which are reflected. By measuring the reflected solar energy across different wavelengths, remote sensing instruments can gather information about the properties of the object, such as its composition, moisture content, and health.

   
   

   
   

3. THERMAL ENERGY: In addition to reflecting solar energy, objects also emit their own radiation, known as thermal or infrared radiation. This radiation is a result of the object's temperature. Everything above absolute zero (-273.15°C or 0 Kelvin) emits thermal radiation. Remote sensing instruments that detect thermal energy, such as thermal infrared sensors, can provide valuable information about the temperature and thermal properties of objects on the Earth's surface.

   
   

   
   

4. ARTIFICIAL SOURCES: In some cases, remote sensing relies on energy sources other than the sun. For example, active remote sensing techniques, such as radar and lidar, use their own energy sources to illuminate objects and measure the properties of the reflected or scattered energy. Radar systems emit microwaves, while lidar systems emit laser pulses. By analyzing the properties of the reflected or scattered energy, these techniques can provide detailed information about the shape, surface roughness, and other characteristics of objects on Earth's surface.

   
   

                                           

Radiation Laws

In the context of remote sensing, radiation laws play a vital role in understanding how electromagnetic radiation interacts with the Earth's surface, atmosphere, and remote sensing instruments. Here's how some of these laws apply specifically to remote sensing:

1. PLANCK'S LAW IN REMOTE SENSING: Planck's law helps to understand the spectral distribution of radiation emitted by different objects on the Earth's surface. By considering the temperature of the object, remote sensing instruments can predict the intensity of radiation emitted at various wavelengths. This information is crucial for thermal remote sensing, where the emitted radiation provides insights into the temperature of the surface or atmosphere.

   
   

   
   

2. STEFAN BOLTZMANN LAW IN REMOTE SENSING: The Stefan-Boltzmann law is applicable in thermal remote sensing for estimating the total radiant flux emitted by surfaces. By measuring the thermal radiation emitted by objects at different wavelengths, remote sensing instruments can estimate their temperatures. This is particularly useful for monitoring changes in land surface temperature, which can indicate phenomena like urban heat islands or agricultural productivity.

   
   

   
   

3. WEIN'S DISPLACEMENT LAW IN REMOTE SENSING: Wien's displacement law is relevant for determining the peak wavelength of emitted thermal radiation, which corresponds to the maximum intensity of emission. In thermal remote sensing, this law helps in selecting appropriate spectral bands for sensors to optimize temperature measurements. Understanding the relationship between temperature and peak wavelength assists in calibrating and interpreting thermal infrared imagery.

   
   

   
   

4. KIRCHOFF'S LAW OF THERMAL RADIATION IN REMOTE SENSING: Kirchhoff's law is essential for interpreting remote sensing data, especially in the thermal infrared region. It states that the emissivity of a material equals its absorptivity at a given wavelength and temperature. Remote sensing instruments exploit this principle to infer surface properties based on the spectral signatures of materials. By analyzing the reflectance and emissivity of surfaces across different wavelengths, remote sensing techniques can identify and classify surface features, such as vegetation types or soil compositions.

These radiation laws provide a theoretical foundation for interpreting remote sensing data and extracting valuable information about Earth's surface and atmosphere. By understanding how electromagnetic radiation behaves according to these laws, remote sensing scientists can develop and apply techniques to monitor and study various aspects of the Earth's environment.

Blackbody Concept Platforms & Sensors: Introduction

The blackbody concept is fundamental in remote sensing, particularly in the context of thermal infrared imaging and calibration. A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation across the entire electromagnetic spectrum, with none being reflected or transmitted. Moreover, a blackbody emits radiation over a wide range of wavelengths according to its temperature. This emission follows Planck's law, which describes the spectral radiance of blackbody radiation.

In remote sensing, blackbody calibration targets serve as reference sources for calibrating sensors, particularly those operating in the thermal infrared region. These targets are designed to approximate the behavior of a blackbody as closely as possible within a specific temperature range. By measuring the radiation emitted by a blackbody calibration target, sensors can be calibrated to accurately quantify the thermal radiation emitted by objects in the scene being observed.

Blackbody calibration targets are commonly used in various remote sensing platforms and sensors, including:

1. SATELLITE AND AIRBORNE SENSORS: Remote sensing satellites and airborne platforms equipped with thermal infrared sensors often utilize blackbody calibration targets to ensure the accuracy of temperature measurements. These targets may be deployed on the ground or mounted on the platform itself, allowing sensors to periodically observe them for calibration purposes.

   
   

   
   

2. LABORATORY INSTRUMENTS: In laboratory settings, instruments such as thermal infrared cameras or spectrometers may also employ blackbody calibration targets. These targets are used to calibrate the instruments before conducting experiments or measurements, ensuring the accuracy and reliability of the data collected.

   
   

   
   

3. FIELD INSTRUMENTS: Field-based remote sensing instruments, such as handheld thermal cameras or radiometers, may incorporate blackbody calibration capabilities for on-site calibration. This allows researchers or practitioners to calibrate their instruments in the field, ensuring accurate measurements in real-time applications such as environmental monitoring or industrial inspections.

Overall, the blackbody concept and blackbody calibration targets play a crucial role in ensuring the accuracy and reliability of thermal infrared remote sensing data. By providing a standardized reference source for calibration, blackbody targets enable remote sensing platforms and sensors to produce consistent and precise measurements of thermal radiation emitted by objects on Earth's surface and in its atmosphere.

Types and Characteristics of Sensors

Sensors are essential components in remote sensing systems, allowing the detection and measurement of electromagnetic radiation emitted or reflected by objects on Earth's surface and in its atmosphere. These sensors vary in terms of their design, technology, spectral sensitivity, and application. Here are some common types of sensors used in remote sensing, along with their characteristics:                                                       

1. OPTICAL SENSORS

   • PASSIVE OPTICAL SENSORS: These sensors detect and measure electromagnetic radiation emitted or reflected by objects without emitting any radiation of their own. They operate across the visible, near-infrared, and shortwave infrared portions of the electromagnetic spectrum.
   • ACTIVE OPTICAL SENSORS: Unlike passive sensors, active optical sensors emit their own electromagnetic radiation (e.g., lasers or microwaves) and measure the properties of the radiation reflected or scattered by objects. Lidar (Light Detection and Ranging) and laser-induced fluorescence sensors are examples of active optical sensors.

2. THERMAL INFRARED SENSORS

   • These sensors detect and measure the thermal radiation emitted by objects in the thermal infrared portion of the electromagnetic spectrum (wavelengths typically between 3 and 14 micrometers). They are commonly used for applications such as monitoring land surface temperature, detecting fires, and assessing thermal properties of materials.

3. MICROWAVE SENSORS

   • Microwave sensors operate in the microwave portion of the electromagnetic spectrum (wavelengths ranging from approximately 1 millimeter to 1 meter). They are particularly useful for penetrating clouds, vegetation, and soil to observe features that may be obscured in other wavelengths. Radar (Radio Detection and Ranging) and radiometry sensors are examples of microwave sensors.
4. HYPERSPECTRAL SENSORS
   
   • Hyperspectral sensors capture information across hundreds or even thousands of narrow spectral bands, providing detailed spectral signatures for each pixel in an image. These sensors offer superior capabilities for identifying and classifying materials based on their spectral characteristics, enabling applications such as mineral exploration, vegetation mapping, and environmental monitoring.
5. MULTISPECTRAL SENSORS
   
   • Multispectral sensors capture data across a limited number of discrete spectral bands, typically covering the visible, near-infrared, and sometimes thermal infrared regions of the electromagnetic spectrum. While offering less spectral detail compared to hyperspectral sensors, multispectral sensors are still valuable for many remote sensing applications, including agriculture, forestry, and land cover mapping.

Characteristics of sensors can vary widely depending on factors such as spatial resolution, spectral resolution, temporal resolution, radiometric resolution, and coverage area. These characteristics determine the sensor's suitability for specific applications and influence the quality and usability of the data it produces. Additionally, advancements in sensor technology continue to drive improvements in resolution, sensitivity, and data collection capabilities, expanding the range of applications and enhancing the accuracy and utility of remote sensing data.

Remote Sensor Platforms and Satellite Orbits

Remote sensor platforms and satellite orbits are crucial components of remote sensing systems, influencing the coverage, resolution, revisit time, and data quality of observations.

REMOTE SENSOR PLATFORMS

1. SATELLITES

   • GEOSTATIONARY SATELLITES: Orbit Earth at the same speed as the Earth's rotation, allowing them to remain stationary relative to a specific point on the Earth's surface. Ideal for continuous monitoring of weather patterns, environmental phenomena, and communications.
   • POLAR ORBITING SATELLITES: Orbit Earth from pole to pole, providing global coverage with each orbit. Suited for various remote sensing applications, including land observation, environmental monitoring, and weather forecasting.
   • SUN SYNCHRONOUS ORBIT SATELLITES: Orbit Earth in a polar orbit, synchronized with the Sun's position, resulting in constant lighting conditions during each pass over the Earth's surface. Well-suited for imaging and monitoring applications due to consistent illumination and shadowing conditions.
2. AIRCRAFT
   
   • MANNED AIRCRAFT: Piloted aircraft equipped with remote sensing instruments, offering flexibility in mission planning and data collection. Used for research, surveillance, disaster response, and high-resolution imaging.
   • UNMANNED AERIAL VEHICLES(UAVs AND DRONES): Autonomous or remotely piloted aircraft equipped with sensors for aerial data collection. Used for various applications, including agriculture, infrastructure inspection, environmental monitoring, and mapping.

3. GROUND BASED PLATFORMS:

   • FIXED GROUND STATIONS: Permanent installations equipped with sensors for continuous monitoring of specific locations or phenomena. Used for weather monitoring, air quality monitoring, seismic surveillance, and environmental research.
   • MOBILE GROUND PLATFORMS: Portable or vehicle-mounted sensor systems used for on-the-go data collection. Commonly employed for field surveys, disaster response, and mobile mapping applications.

SATELLITE ORBITS

1. LOW EARTH ORBIT(LEO):

   • Satellites in LEO typically orbit at altitudes ranging from a few hundred kilometers to around 2,000 kilometers above the Earth's surface. Offer advantages such as high spatial resolution, short revisit times, and low latency data acquisition. Used for a wide range of applications, including land observation, environmental monitoring, and disaster response.

2. MEDIUM EARTH ORBIT(MEO):

   • Satellites in MEO orbit at altitudes ranging from around 2,000 to 35,786 kilometers above the Earth's surface. Offer a balance between coverage and resolution, making them suitable for global navigation satellite systems (GNSS) such as GPS, GLONASS, and Galileo.

3. GEOSTATIONARY ORBIT(GEO):

   • Satellites in GEO orbit at an altitude of approximately 35,786 kilometers above the Earth's equator. Offer continuous coverage of specific regions, making them ideal for weather monitoring, communications, and environmental surveillance.

4. POLAR ORBIT:

   • Polar orbiting satellites pass over the Earth's poles during each orbit, providing global coverage over time. Suited for remote sensing applications requiring global observation, such as environmental monitoring, climate research, and disaster management.

The selection of a remote sensor platform and satellite orbit depends on factors such as the desired spatial and temporal resolution, coverage area, revisit time, mission objectives, and budget considerations. Each platform and orbit offers unique capabilities and advantages, allowing for tailored solutions to address diverse remote sensing needs.

Types of Satellites and images

Satellites play a crucial role in remote sensing by capturing imagery and data from space, enabling a wide range of applications, including environmental monitoring, disaster response, agriculture, urban planning, and defence. 

Types of Satellites

1. OPTICAL IMAGING SATELLITES:

   • These satellites capture imagery using sensors that detect visible and near-infrared light. They provide high-resolution images suitable for various applications, including land cover mapping, urban planning, agriculture, and environmental monitoring.

2. SYNTHETIC APERTURE RADAR (SAR)SATELLITES:

   • SAR satellites use radar technology to capture images regardless of weather conditions or time of day. They are particularly useful for mapping terrain, monitoring changes in land cover, detecting surface deformation, and assessing flood extents.

3. HYPERSPECTRAL IMAGING SATELLITES:

   • Hyperspectral satellites capture imagery across hundreds or even thousands of narrow spectral bands, providing detailed spectral signatures for each pixel in an image. These satellites are used for precise identification and classification of materials, vegetation mapping, mineral exploration, and environmental monitoring.

4. THERMAL INFRARED IMAGING SATELLITES:

   • These satellites capture imagery in the thermal infrared portion of the electromagnetic spectrum, allowing for the measurement of surface temperatures and thermal properties of objects. They are used for applications such as monitoring urban heat islands, detecting wildfires, and assessing thermal efficiency in buildings.

5. METEOROLOGICAL SATELLITES:

   • Meteorological satellites monitor weather patterns and atmospheric conditions using various sensors, including visible and infrared imagers, microwave radiometers, and sounders. They provide data for weather forecasting, climate research, and disaster monitoring.

6. NAVIGATION SATELLITES:

   • Navigation satellites, such as the Global Positioning System (GPS), provide accurate positioning and timing information for navigation and location-based services worldwide.

Types of Satellite Images

1. PANCHROMATIC IMAGES:

   • Panchromatic images are grayscale images captured using a single broad wavelength band, typically in the visible spectrum. They offer high spatial resolution but limited spectral information.

2. MULTISPECTRAL IMAGES:

   • Multispectral images are composed of several bands within the visible, near-infrared, and sometimes thermal infrared regions of the electromagnetic spectrum. They provide both spatial and spectral information, allowing for the differentiation of surface features based on their spectral signatures.

3. HYPERSPECTRAL IMAGES:

   • Hyperspectral images contain numerous narrow spectral bands, capturing detailed spectral information across a wide range of wavelengths. They enable precise identification and classification of materials based on their unique spectral signatures.

4. RADAR IMAGES:

   • Radar images are captured using synthetic aperture radar (SAR) satellites, which emit microwave pulses and measure the backscattered signals. These images provide information on surface roughness, vegetation structure, land cover changes, and terrain elevation, regardless of weather or lighting conditions.

5. THERMAL INFRARED IMAGES:

   • Thermal infrared images depict surface temperatures and thermal properties of objects. They are useful for monitoring urban heat islands, detecting thermal anomalies, and assessing thermal efficiency in buildings.

Each type of satellite and image offers specific capabilities and advantages, allowing for the collection of diverse and valuable information for various remote sensing applications. Integrating data from multiple satellite sources and image types can enhance the comprehensiveness and accuracy of remote sensing analyses.