Orbiting Satellites and Sensor Systems | Geology Optional Notes for UPSC PDF Download

Remote Sensing Platforms

• Definition: Platforms serve as vehicles or carriers for remote sensing devices, facilitating observations.
• Types of Platforms:
  • Terrestrial Platforms:
    • Include tripods, booms, cranes, and towers as examples.
    • Booms elevate sensors 1 to 2 meters above the target, while towers can extend tens of meters high.
  • Airborne Platforms:
    • Consist of helicopters, light planes, high-flying aircraft, and aerial photography systems.
    • Helicopters and planes operate at lower altitudes, while high-flying aircraft cover greater areas.
  • Spaceborne Platforms:
    • Include satellites equipped with optical sensors and Synthetic Aperture Radar (SAR).
    • Satellites orbit at altitudes of 185-900 kilometers, providing wide coverage.
• Selection Criteria for Platforms:
  • Altitude determines ground coverage; higher altitudes capture larger areas.
  • Various parameters influence ground coverage and image resolution.
  • Different sensors on a single platform can acquire data at varying resolutions and coverage areas.

Sensors and Platforms for Remote Sensing

• Ground-based Platforms:
  • Static platforms are fixed on stationary structures like tripods or masts, while dynamic platforms are mounted on moving vehicles.
  • They operate at short (50-100m), medium (up to 250m), and long ranges (up to 1000m).
  • Short-range applications involve mapping buildings and small objects, medium-range sensors are used for 3D modeling with millimeter accuracy, and long-range sensors are utilized for topographic purposes.
  • Terrestrial platforms provide high spatial resolution images compared to aerial or satellite platforms.
  • Data collected from ground-based platforms are used for various applications like bridge monitoring, landslide mapping, architectural restoration, and more.
• Airborne Platforms:
  • Aerial remote sensing began with photographic cameras and has evolved to advanced sensor technology.
  • Airborne platforms include airplanes, helicopters, balloons, and rockets, operating at various altitudes.
  • Aircraft like stable wing planes and helicopters are commonly used for data collection.
  • Aircraft offer advantages such as flying at low altitudes for high sensor spatial resolution, flexibility to avoid weather disturbances, and ease of sensor maintenance and configuration changes.
  • They can adjust schedules for lighting conditions and revisit locations as needed.
  • Aerial photographs have been crucial in providing detailed Earth surface information for over a century.
• Metrology:
  • Metrology encompasses the theoretical and practical aspects of measurement.
  • It is vital in ensuring accurate measurements across various fields and industries.

Sensors and Space Programmes

• Spaceborne Platforms
  • Definition: Spaceborne platforms refer to man-made satellites and space shuttles used for various purposes.
  • Examples: Satellites are artificial objects orbiting the Earth for remote sensing, communication, and navigation.
  • Orbits: Satellites are placed in geostationary, polar, and sunsynchronous orbits for different functionalities.
  • Duration: Spaceborne platforms can be short-term (e.g., space shuttles) or long-term (e.g., Earth monitoring satellites like Landsat).
  • Applications: Satellites assist in weather prediction, crop monitoring, mineral exploration, and more.
  • Advantages of Spaceborne Remote Sensing:
    • Allows large area coverage
    • Offers frequent and repetitive coverage
    • Enables quantitative measurements using calibrated sensors
    • Supports semi-automated processing and analysis
    • Cost-effective in terms of coverage per unit area
    • Global accessibility without airspace restrictions

If you are uncertain about which platform to utilize, the choice typically depends on the application. Ground-based platforms are suitable for detailed 3D modeling of structures, while airborne platforms excel in topographic mapping. Spaceborne systems are ideal for various applications requiring global accessibility and large-scale coverage.

Satellite Remote Sensing

• Remote sensing data from satellites are crucial for mapping remote areas of the globe for scientific purposes.
• Satellite remote sensing is increasingly used for topographic and cartographic applications, blurring boundaries between specialized and general applications.
• Decision-making for data platform selection is influenced by the cost of data acquisition and the desired information to be derived.

Platforms Used in Remote Sensing

• Satellites
• Aircraft
• Drones
• Balloons

Advantages of Aerial Platforms

• High spatial resolution
• Quick data acquisition

Astronomical Satellites

• Astronomical satellites function as telescopes in space, providing clearer vision compared to Earth-based telescopes due to lack of atmospheric interference.

Types of Satellites

• Satellites are classified into six major types based on their uses: astronomical, communication, weather, remote sensing, navigation, and reconnaissance.

Applications of Astronomical Satellites

• Creating star maps
• Studying celestial phenomena like black holes and quasars
• Mapping planetary surfaces

Hubble Space Telescope

• The Hubble Space Telescope, launched in 1990, is a vital research tool named after astronomer Edwin Hubble.
• It helps in studying celestial objects with its high precision and versatility.

Sensors and Space Programmes

• Astronomy satellites operate from Earth's orbit, unlike space exploration satellites, which venture into deep space. For instance, the renowned Hubble Space Telescope serves as an astronomy satellite.

Communication Satellites

• Communication satellites facilitate telephonic conversations, television broadcasts, FAX services, and more over long distances. They are commonly positioned in geostationary orbits, forming a substantial portion of satellites in orbit today.
• These satellites play a vital role in our daily lives, enabling activities like watching TV, making long-distance calls, using fax machines, and listening to radio stations.
• Communication satellites function as radio relay stations in space, receiving and transmitting analog and digital signals for global communication needs.

Weather Satellites

• Weather satellites are crucial for monitoring and forecasting weather patterns worldwide, offering valuable insights for meteorologists.
• These satellites provide detailed data on various aspects, including radiation measurements, snow cover, ice movement, ocean depth, crop conditions, deforestation, and drought regions.

Earth Observation Satellites

• Earth observation satellites, also known as remote sensing satellites, observe and measure Earth's environment from afar, aiding in monitoring essential resources.
• Placed in sun-synchronous orbits, these satellites can reveal hidden features such as animal migration, mineral deposits, agricultural conditions, and more.

Complex Information Summary

Satellites and Their Applications

• Role of Satellites

  • Satellites orbiting in space can capture extensive images of various regions worldwide.
  • They are crucial for monitoring remote or harsh environments that are challenging to access by land.

• Navigation Satellites

  • Navigation satellites were developed in the late 1950s to aid in precise location tracking, especially for ships at sea.
  • Global Positioning System (GPS) is a notable system that enables accurate positioning on Earth's surface.
  • GPS technology has evolved for widespread commercial and private use, including applications in car navigation and mobile devices.

• Reconnaissance Satellites

  • Reconnaissance satellites are utilized for intelligence gathering on military activities of foreign nations.
  • They can detect missile launches, nuclear explosions, and intercept radio and radar transmissions.
  • There are different types of reconnaissance satellites, such as optical-imaging, radar-imaging, signals-intelligence, and relay satellites.

Orbits and Their Types

• Understanding Orbits

  • An orbit represents the path a satellite follows while revolving around the Earth.
  • The orbital plane is the specific plane in which a satellite moves, and the orbital period is the time taken to complete one orbit.

Sensors and Space Programmes

• Instantaneous Field of View (IFOV)

  • IFOV characterizes the sensor by defining the projection of the detector element on the ground, often referred to as the 'footprint' of the detector element.
  • It is also known as the resolution element, indicating the smallest unit of ground area that the sensor can detect.

• Spatial Resolution

  • Spatial resolution refers to how well a sensor can distinguish between objects on the ground. It is determined by the imaging optics projecting the detector element onto the ground from the satellite or aerial platform.
  • In simpler terms, spatial resolution describes the level of detail in an image captured by the sensor.

• Orbit

  • Orbit is defined by three factors:
  • 
    
    • The shape of the orbit can be circular or elliptical, depending on its eccentricity.
    • The altitude of the orbit remains constant for a circular orbit but changes continuously for an elliptical orbit.
    • The angle that an orbital plane makes with the equator determines the type of orbit.

• Types of Orbits

  • Orbits can be categorized as equatorial, inclined, or polar based on the angle between the orbital plane and the equator.
  • An equatorial orbit has an inclination of 0° or 180°, while a polar orbit has an inclination of 90°.
  • Orbits with inclinations between 0° and 90° are known as inclined orbits.

• Ascending and Descending Passes

  • When a satellite moves from south to north, it's called an ascending pass. Conversely, when it moves from north to south, it's a descending pass.
  • Polar orbiting satellites typically capture images during their descending passes, over the sunlit hemisphere, and return during ascending passes over the night-time hemisphere.

• Field of View (FOV)

  • FOV represents the total viewing angle of the camera, defining the swath or area of Earth's surface observed by the sensor during its orbital motion.
  • Swath width varies among sensors but is generally wider for spaceborne sensors compared to airborne ones, ranging from tens to hundreds of kilometers.
  • Polar orbiting satellites can image most of the Earth's surface due to the Earth's rotation beneath them, despite their fixed east-west position relative to Earth.

Satellite Orbits Overview

• Overview of Satellite Orbits
• 
  
  • Satellite orbits are determined by a combination of the satellite's orbital motion and the Earth's rotation.
  • This interaction causes a westward shift of the swath, ensuring coverage of different areas with each pass.
  • An orbit is designed to revisit the same location every few weeks, known as an orbital cycle.
• Altitude and Angle of Orbits
• 
  
  • The elevation of a satellite's orbit impacts the time taken to image the Earth.
  • Orbits closer to the poles have a larger angle, affecting the speed of the satellite.
  • Selection of orbit is based on factors like altitude, orientation, and purpose of the mission.
• Types of Orbits
• 
  
  • Closed Orbits: Circular or elliptical paths where a body orbits another body.
  • Examples include planets orbiting the Sun and moons orbiting planets.
  • All planets and moons in our solar system follow closed orbits.
  • Open Orbits: Paths that are not continuously closed, such as hyperbolic or parabolic trajectories.

Johannes Kepler and Satellite Orbits

• Johannes Kepler's Contributions
• 
  
  • Kepler developed mathematical laws describing satellite orbits in the 18th century.
  • Based on observations of planetary motions, he showed that satellites can follow elliptical orbits.
• Significance of Kepler's Work
• 
  
  • Kepler's laws of planetary motion revolutionized our understanding of celestial mechanics.
  • His discoveries paved the way for modern satellite trajectory planning and space exploration.

Sensors and Space Programmes

Open Orbits

• Open orbits like parabolas and hyperbolas form curves that do not close in on themselves, unlike circles.
• Different types of closed orbits are used for operating various satellites.

Types of Satellite Orbits

• Geosynchronous Orbit
• Sunsynchronous Orbit

Geosynchronous Orbit

A geosynchronous orbit is an equatorial orbit where satellites are placed at an altitude of around 36000 km above the Earth's surface. Satellites in this orbit move at the same speed and direction as the Earth, appearing to hover over the same point on the Earth's surface.

• Satellites in geosynchronous orbits have an orbital period that matches the Earth's sidereal rotation period.
• Geostationary orbit is a special case of a geosynchronous orbit where satellites remain stationary above the equator.
• Geostationary satellites appear motionless from the ground as they complete one orbit in approximately 24 hours, matching Earth's rotation period.

Geostationary Orbit

• Satellites in geostationary orbit remain vertically above a fixed point on the Earth's surface, providing a stationary view.
• This orbit requires a significant amount of energy for satellites to reach.

Usage of Geostationary Satellites

• Communication and weather purposes are the most common applications of geostationary orbit.
• Communications satellites are often placed in geostationary orbits for fixed communication links.
• Geostationary satellites are beneficial for monitoring local storms, tropical cyclones, and dynamic phenomena due to their continuous viewing capability.

Imagery and Mapping

• Geostationary satellites offer broad coverage but with coarse imagery resolution.
• Imaging and mapping satellites typically avoid geostationary orbits due to poor spatial resolution.

Geostationary Orbits and Sun-Synchronous Orbits

• Geostationary Orbits

  • Geostationary orbits offer several advantages:
  • Large spatial coverage: Five geostationary satellites are adequate to cover all non-polar regions of the Earth.
  • One satellite can cover almost one-third of the Earth's surface.
  • Permanent visibility of the satellite enables continuous telecommunications and a high rate of repetition for observations.
  • Only one ground segment is required for satellite monitoring.

• Sun-Synchronous Orbits

  • Polar orbits have an inclination of 90° with respect to the equatorial plane of the Earth.
  • A polar orbit allows a satellite to pass close to both poles of the Earth, covering polar regions from north to south.
  • Satellites in polar orbits, also known as polar satellites, complete one orbit in approximately 90 minutes.
  • These orbits are low altitude orbits ranging from 200 to 1000 km above the Earth's surface, offering detailed views of the planet.
  • Polar orbit satellites are vital for reconnaissance, Earth observation, measuring ozone concentrations in the stratosphere, measuring atmospheric temperatures, and weather forecasting.

Sensors and Space Programmes

• Uniform Illumination Conditions

  Ideally, satellite images should be captured under consistent lighting conditions to reflect changes in ground features rather than observation conditions. However, variations in latitude, time of day, and season lead to differences in illumination across images taken on different dates.

• Sunsynchronous Orbits

  Sunsynchronous orbits are designed to maintain a fixed orientation of a satellite relative to the Sun throughout the year, reducing variations in illumination caused by differences in time of day. Satellites in these orbits pass over the same Earth location at approximately the same local time daily.

  • Orbital Characteristics

    Sunsynchronous satellites orbit at altitudes ranging from 700 to 800 km with orbital periods between 90 and 110 minutes. They shift their orbits by about 1° per day to synchronize with the Sun's position.

  • Pass Patterns

    Satellites in sunsynchronous orbits pass from north to south poles on the sunlit side (descending node) and from south to north on the shadowed side (ascending node). This pass pattern aids in acquiring consistent images for comparison.

• Advantages of Sunsynchronous Orbits

  These orbits are beneficial for Earth imaging missions as they ensure shadows cast by objects on the Earth's surface are consistent, simplifying the comparison of images from different days to identify changes.

• Examples of Satellites

  Satellites such as Orbview, Quickbird, IKONOS, SPOT, Landsat, ERS, and RADARSAT operate in sunsynchronous orbits, facilitating various remote sensing and meteorological tasks.

• Dawn-to-Dusk Orbit

  In a dawn-to-dusk orbit, a satellite's orbital plane aligns with the division between the Sunlit and dark halves of the Earth. This configuration allows the satellite to remain in constant sunlight, enhancing observational capabilities.

Satellite Sensor Systems Overview

• Importance of Satellite Orbits:
  • Dawn-to-Dusk Orbits: Satellites like the Canadian Radarsat use orbits that ensure constant illumination of their solar panels by the Sun, enabling them to primarily rely on solar power.
  • Gravitational Forces: Due to the Earth's non-spherical shape, satellites experience gravitational forces causing their orbits to either progress or regress.
  • Applications of Specific Orbits: Orbits with constant sunlight are ideal for Earth observation, solar studies, weather forecasting, and reconnaissance purposes.
• Sensor Systems on Satellites:
  • Function of Sensor Systems: Sensors act as the "eyes" of satellites, capturing and recording data about the observed scenes.
  • Spectral Characteristics: Sensors typically operate in the visible, infrared, thermal, and microwave regions of the electromagnetic spectrum.
  • Spatial Resolution: Selection of sensor systems depends on whether high spatial resolution (terrestrial and airborne) or synoptic coverage (satellite-based) is needed.
  • Imaging vs. Non-imaging Sensors: Sensors are categorized as imaging (providing image outputs) or non-imaging (providing numerical data outputs).
  • Passive vs. Active Sensors: Passive sensors detect natural sources of EMR, while active sensors detect radiation from objects irradiated by artificial sources like RADAR.

Sensors and Space Programs

• LIDAR is an active sensor detector utilized in radar and microwave regions.
• All imaging sensor systems are broadly categorized based on technical components and detection capabilities.
• Imaging sensors are classified into types such as photography, human eye, multispectral scanners, thermal scanners, and more.
• The sensors operate across various wavelengths including infrared, visible, UV, and artificial radiation.
• Passive sensors like optical-infrared sensors record energy reflected by objects.
• Remote sensing systems have specific wavebands and atmospheric transmission characteristics.
• The sensors are further categorized based on the wavelength region and sensing mechanism.

Types of Remote Sensing Sensors

• Subtypes based on the wavelength region of sensing mechanism
• 
  
  • Examples include Radar and passive microwave scanners, Push-broom scanners, and more.
  • Specific sensor examples are MKF-6M, S065, Bhaskara-1, 2, Landsat RBV, Landsat MSS and TM, INSAT, VHRR, IRS-LISS, WiFS, SPOT HRV.

Active Sensors

• Thermal sensors
• Microwave sensors
• Optical-infrared sensors

Sensor Systems

• Thermal infrared scanner
• Scanning microwave radiometer
• LIDAR
• RADAR
• Scatterometer/Altimeter
• Landsat TM, TIMS, MODIS, ASTER, AVHRR
• Multispectral imaging sensor systems
• Thermal remote sensing systems
• Microwave radar sensing system
• Oceansat-1 MSMR (Multi-channel scanning microwave)
• Leica ALS70 Airborne LIDAR Sensor
• RADARSAT SAR

Seasat

Multispectral Imaging Sensor Systems

Multispectral imaging sensors encompass photographic and scanning systems. Photographic systems utilize cameras, while scanning systems involve scanners with filters for different wavelength regions. Sometimes, a combination of both systems is utilized.

In photographic systems, various parts of the spectrum are detected using different film-filter combinations, forming images directly on film. On the other hand, in scanning systems, the optical image is first converted into an electrical signal (video data) before processing for recording or transmission. However, photographic systems may suffer from distortion at the edges due to large lens openings.

Analog (Photographic) Systems

Photographic cameras, as analog (photographic) systems, are the oldest and most commonly used remote sensing sensors, particularly in aerial photography. These passive optical sensors use lenses to create well-defined images on the focal plane. Images are typically captured on photographic films sensitive to light within the wavelength range of 0.3 µm to 0.9 µm, covering ultraviolet, visible, and near-infrared regions.

The photographic film reacts with radiation in specific areas when exposed, producing black and white or color representations of light intensity variations across those areas. The field-of-view (FOV) defines the width on the ground captured in an image.

Focal plane, perpendicular to the lens or mirror axis and passing through the focal point, plays a crucial role in image formation and resolution.

Sensors and Space Programmes

• Detector Overview

  • A detector is a device that generates an output signal based on the amount of radiation it receives on its active area.
  • It converts electromagnetic energy into an electrical signal, hence known as electro-optical detectors.
  • Each detector is designed for a specific spectral region in which it can be utilized.

• Instantaneous Field-of-View (IFOV)

  • IFOV refers to the patch of the landscape visible to a detector at any given moment, playing a crucial role in determining spatial resolution.

• Box-Framing Systems

  • These systems measure radiation from an entire scene simultaneously and capture an image of the area.
  • Common examples include cameras, eyes, and vidicons.
  • The size of the framed scene depends on the system's apertures and optics that define the field of view.

• Photographic Films

  • Panchromatic and color films are typically sensitive to visible light, while films developed for remote sensing focus on reflected-infrared light for applications like vegetative mapping.

• Scanning Systems Overview

  • Scanning systems utilize detectors to record the brightness of small terrain sections within their IFOV to create images.
  • The systems convert recorded electrical signals into digital form for image production.

• Multispectral Scanning Systems

  • These systems can sense a wide range of wavelengths and narrow bands within the electromagnetic spectrum.
  • By using a narrow beam of light, distortions can be minimized, and resolution significantly improved, as per lens theory.
  • There are various scanning modes for these systems, including across-track, along-track, side-looking (radar), and spin scanning systems.

Across-Track Scanners

• Overview

  • Scanning system using a rotating faceted mirror aligned with the flight direction.
  • Scans terrain in parallel lines perpendicular to the platform's direction.
  • Detectors receive energy reflected from the ground via mirrors.

• Factors Affecting Sensor Signal Strength

  • Energy flux, sensor altitude, spectral bandwidth, IFOV, and dwell time impact signal strength.
  • Short dwell time in across-track scanners results in weaker signals due to less energy reception.

• Examples

  • Multispectral Scanner (MSS) and Thematic Mapper (TM) in Landsat satellites.

Landsat Multispectral Scanner (MSS)

• Description

  • Primary sensor on Landsats 1-3 and 4-5.
  • Four spectral bands capturing radiation from Earth's surface.
  • Bands include green, red, near-infrared, and thermal infrared.

• Operational Details

  • Oscillating mirror scanning a 185 km swath.
  • Spectrometer separates reflected radiation into spectral bands.
  • Detects six scan lines per mirror sweep.

• Usage of MSS Bands

  • Band 1 for green reflectance, Band 2 for chlorophyll absorption, Bands 3 and 4 for vegetation and water detection.

Sensors and Platforms

• Applications of Landsat Data

  • Band-specific uses for vegetation health, chlorophyll, and water detection.

• Definitions

  • Angular field of view: Recorded mirror sweep portion measured in degrees.
  • Ground resolution cell: Area covered by detector's IFOV.
  • Dwell time: Duration for detector IFOV to sweep a resolution cell, impacting signal strength.
  • Ground swath: Width of recorded terrain strip.

Sensors and Space Programmes

• Thematic Mapper:
  • Thematic Mapper (TM) is an upgraded version of the Multispectral Scanner (MSS) and was first utilized on Landsat-4 and 5.
  • It brought about enhancements in spatial, radiometric, and geometric aspects compared to the MSS systems.
  • The design of TM was more intricate than MSS, functioning as an across-track scanner with an oscillating scan mirror and detector arrays.
  • TM captures data across seven narrow spectral bands simultaneously, covering various ranges from visible to thermal infrared.
  • Data provided by TM has different resolutions, with 30m in visible, near-infrared, and middle infrared bands, and 120m in the thermal infrared region.
• Along-Track Scanning System:
  • Along-track scanners capture multiband images along a swath below the aircraft.
  • Detectors in a linear array record data, forming two-dimensional images as the aircraft progresses forward.
  • This system, also known as a push broom system, offers fine spatial and high spectral resolutions due to long dwell times.
  • An example of an along-track scanner is the SPOT-High Resolution Visible (HRV) camera and LISS (Linear Imaging Self Scanning) sensors used in Indian remote sensing.
• Linear Imaging Self Scanning Sensor (LISS):
  • LISS, developed by ISRO for Indian remote sensing satellites, is a multispectral camera system.
  • Each LISS camera system includes four imaging lens assemblies, each for a specific band, coupled with a linear CCD array.
  • Images focused by the optics onto the sensor array are stored and transmitted as video signals.
  • LISS operates in multiple bands within the visible and near-infrared wavelengths, with different versions like LISS-III and LISS-IV, each with unique specifications and capabilities.

Summary and Explanation of Remote Sensing Systems

LISS-IV Camera System

• LISS-IV operates in two modes: monochromatic (single band out of B1, B2, and B3) and multispectral.
• The ground swath for LISS-IV is 23.9 km for multispectral mode and 70 km for monochromatic mode.
• The camera can be tilted up to ±26° across the track, allowing for a revisit period of 5 days.

Side Looking or Oblique Scanning Systems (Radar)

Side looking scanning systems, such as radar, are active scanning systems that emit electromagnetic radiation (EMR) to illuminate terrain and capture returning radar pulses as images.

• Main Components of SLAR:
  • Antenna
  • Duplexer
  • Transmitter
  • Receiver
  • Pulse-generating device
  • Cathode ray tube
• Functionality:

  The radar antenna sends pulses to the ground and receives returning radar signals. The duplexer prevents interference between return and transmitted beams. The receiver records radar return timing and intensity, amplifies weak signals, and displays the radar image.

• Resolution Calculation:

  The resolution of radar systems is determined by the radar beam width and can be calculated using the formula: Resolution (m) = range (km) × wavelength (cm) / antenna aperture (m).

• Direction Explanation:

  The range direction involves transmitting microwave pulses to illuminate landscape strips, while the look direction is perpendicular to the aircraft's flight path. Range direction can be near or far range.

Sensors and Space Programmes

• Introduction to Sensors:
• 
  
  • Thermal remote sensing systems are a type of electro-optical scanning systems that focus on the thermal infrared part of the electromagnetic spectrum.
  • These systems do not measure the actual internal temperature of objects but rather capture the radiant temperature variations emitted by the objects.
  • They operate based on the energy emitted by objects, allowing them to function both during the day and at night.
• Functionality of Thermal Scanners:
• 
  
  • Thermal scanners utilize photo detectors that are cooled to temperatures approaching absolute zero to minimize their own thermal emissions.
  • They scan the terrain in an across-track mode using a scan mirror and focusing mirrors to capture emitted infrared energy.
  • The detected energy is converted into electrical signals by the detectors, with signal intensity proportional to the emitted infrared radiation.
• Components of Thermal Airborne Infrared Scanner:
• 
  
  • The system comprises an electric motor and a rotating shaft aligned parallel to the aircraft's flight direction.
  • A scan mirror, tilted at a 45-degree angle, sweeps the terrain perpendicular to the flight path, detecting emitted infrared energy.
  • The energy is then directed to focusing mirrors for detection by the detector, which converts it into electrical signals.
  • A recorder mirror, moving synchronously with the scan mirror, projects the modulated light source's image onto recording film.

Thermal Infrared Scanner Technology Overview

• Thermal infrared scanners use modulated light sources and optional film recorders to capture scan patterns on the ground.
• A high-density magnetic tape recorder stores the scanned data for analysis.
• Controlled radiant temperature sources and mirrors help in scanning the terrain.
• Signal detectors are used to capture ground resolution details effectively.

Types of Thermal Infrared Scanners

• Heat Capacity Mapping Mission (HCMM): Detected radiation in visible/near-infrared and thermal infrared bands.
• Landsat Series: Landsat-3 MSS and TM include thermal bands in the electromagnetic spectrum.
• Thermal Infrared Multispectral Scanner (TIMS): Developed by NASA with six thermal bands for data collection.
• MODIS (Moderate Resolution Imaging Spectroradiometer): Launched in 1999 with 36 spectral bands for detailed spatial resolution.

Advanced Thermal Infrared Scanners

• ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer): Scans terrain in visible/NIR, SWIR, and thermal infrared with varying spatial resolutions.
• AVHRR (Advanced Very High Resolution Radiometer): Utilized by NOAA for measuring cloud and seawater temperatures with high ground resolution.

Sensors and Space Programmes

• Side Looking Airborne Radar (SLAR)

  SLAR is a type of airborne radar that looks perpendicular to the direction of the vehicle. It provides a view of terrain or moving targets. The platform (aircraft or satellite) carrying an SLAR moves forward with the nadir directly below it.

• Thermal Sensors

  Thermal sensors measure surface temperature and thermal properties of objects. Thermal imagery finds applications in various fields such as:

  • Geology: Geologic mapping, locating underground coal mine fires
  • Soil Science: Soil mapping, moisture determination
  • Forestry: Studying vegetation evapotranspiration, mapping forest fires, urban land surface temperature analysis

• Microwave Imaging System

  The microwave part of the electromagnetic spectrum covers wavelengths from 1mm to 1m. Microwave remote sensing offers advantages like the ability to penetrate through atmospheric conditions like clouds, snow, and smoke. It can operate day and night.

  • Active Microwave Remote Sensing

    Active systems include imaging sensors and non-imaging sensors. Imaging radars like SLAR can be real aperture or synthetic aperture systems.

  • Passive Microwave Remote Sensing

    Non-imaging radars are scatterometers or altimeters. Passive microwave sensors (radiometers) measure natural emitted energy from the Earth's surface. This energy is collected by suitable antennas and represented as an equivalent temperature.