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Introduction to Remote Sensing 1: The EM Spectrum, Atmospheric Interactions and Sensors


Remote sensing is the process of collecting information about an object or area from a distance, typically using specialized instruments on an aircraft or satellites through measurement of reflected or emitted electromagnetic waves. Remote sensing is unique in comparison to other forms of data collection as it allows scientists and researchers to study the Earth's surface and atmosphere, as well as other planets and celestial bodies, without the need for physical contact or direct observation. In today’s post we’ll explore the basic remote sensing techniques employed by various sensors.


What is the Electromagnetic Spectrum?

The electromagnetic spectrum refers to the range of all types of electromagnetic radiation, which includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. These different types of radiation are characterized by their wavelength and frequency, with radio waves having the longest wavelengths and the lowest frequencies, while gamma rays have the shortest wavelengths and the highest frequencies. When it comes to remote sensing, understanding of the electromagnetic spectrum is key as different types of electromagnetic radiation interact with matter in different ways.


Atmospheric Scattering and Windows

When objects such as particles or gaseous molecules present in the atmosphere cause EM waves to be redirected from their original path this is known as atmospheric scattering. Atmospheric scattering is a factor that remote sensors have to take into account when considering what wavelength of the EM spectrum to employ. In general the 2 fundamental types of atmospheric scattering are Rayleigh and Mie Scattering:

  • Named after the 19th-century British physicist Lord Rayleigh, Rayleigh Scattering occurs when the size of the molecules in the atmosphere is much smaller than the wavelength of the electromagnetic radiation. This type of scattering is responsible for the blue color of the sky as blue light is the shortest wavelength a human eye can observe and the shorter a wave the more it scatters. Rayleigh scattering is also the reason why the sky appears orange or reddish during sunrise and sunset as during these times the Sun's rays have to travel through a greater distance of the Earth's atmosphere causing the shorter wavelengths to be scattered. Rayleigh Scattering predominantly occurs within the upper atmosphere and tends to distort remote sensing images by diminishing contrast and causing shorter wavelengths to be overestimated

  • Mie Scattering occurs when the wavelength of photons is equal to or smaller than the size of the particle it hits. These larger particles can be solid or liquid and may include aerosols, gas mixtures, water vapor, or dust. This type of scattering is most likely to occur in the lower atmosphere, where there are more particles that can absorb or deflect the radiation. Although Mie scattering can universally affect all types of EM radiation, it’s especially pronounced at longer wavelengths in the infrared portion of the EM spectrum.

In addition to accounting for these forms of scattering, in order to ensure that collected data is accurate and representative of the objects or areas being studied, earth scientists make use of atmospheric windows. Atmospheric windows refer to specific regions of the EM spectrum where the Earth's atmosphere is transparent and allows EM radiation to pass through with minimal absorption or scattering. There are several atmospheric windows in the electromagnetic spectrum, with the most commonly utilized being the visible light window(0.3 to 0.9 μm), the infrared window(8 to 13 μm), and the microwave window(> 1 mm).


Active vs Passive Sensors:

Active Sensors are remote sensing instruments that emit their own energy in order to collect data on an object or area. This energy can be in the form of radio waves, lasers, or other types of electromagnetic radiation. The energy that is emitted by active sensors is typically reflected back to the sensor by the object or area being studied, allowing the sensor to gather information about the object's characteristics, such as its shape, size, and surface reflectance. Examples of active sensors include:

  • Lidar (Light Detection and Ranging) is a type of active sensor that uses lasers to emit visible or near-infrared wavelength pulses and measure the time it takes for the light to be reflected back to the sensor. This allows it to gather information about the distance, shape, and surface reflectance of objects. Lidar sensors are often used for applications such as mapping the Earth's surface, measuring the distance to objects, and detecting the presence of objects through clouds or darkness.

  • Radar (Radio Detection and Ranging) is another type of active sensor that uses microwaves to emit pulses of energy and measure the time it takes for the waves to be reflected back to the sensor. This allows it to gather information about the distance and shape of objects, as well as the surface reflectance and texture of the objects. Radar sensors are often used for applications such as mapping the Earth's surface, measuring the distance to objects, and detecting the presence of objects through clouds or darkness. That said, resolution of RADAR images is still limited by the size of the antenna used.

  • Radar Altimetry is a specialized application of radar that uses the Doppler effect to measure the altitude of an aircraft or spacecraft above the terrain beneath it. This is done by transmitting radio waves down to the ground and measuring the time it takes them to be reflected back up to the aircraft. The altitude above the ground is then calculated from the radio waves' travel time and the speed of light. Radar altimetry is often used for applications such as monitoring the ocean and accurately mapping ice sheets and sea ice.

  • Bathymetry and Side Scan Sonar use sound waves to map the topography of the seafloor and observe underwater objects. Bathymetry sensors emit bursts of sound and measure the time it takes for the reflected pulse to come back in order to find the depth of the water. Side scan sonars map large areas of the ocean floor very accurately by sending out acoustic beams left and right and processing their reflections to produce a very accurate image.

Passive Sensors, on the other hand, do not emit their own energy and instead rely on energy that is naturally emitted or reflected by the object or area being studied. Passive sensors can detect a wide range of energy sources, including visible light, infrared radiation, and microwave radiation. They are often used to measure the temperature of the Earth's surface or to detect the presence of certain gasses in the atmosphere. Examples of passive sensors include:

  • A Video Camera is a type of sensor that captures and records visual data in the form of a video feed. Some video cameras are sensitive to the visible spectrum, while others are capable of recording data in the near infrared range, thanks to advances such as thermal infrared cameras. Video cameras are often used to provide low-cost, qualitative data and to provide additional visual information about an area that has been captured by another type of sensor, such as a laser scanner or radar.

  • A Multispectral Scanner (MSS) is a type of sensor that collects data across a range of different wavelength ranges. It works by collecting observations in a point-by-point, line-by-line manner, which is different from aerial cameras, which record full images in a single exposure. Multispectral scanners measure reflected sunlight in the visible and infrared spectrum, allowing them to gather information about the energy reflected by a specific area. They are often used to map land cover, vegetation, surface mineralogy, and surface water, and come in two types: whiskbroom scanners and pushbroom sensors. Multispectral scanners can be used on both air and space platforms, and the portion of the Earth's surface viewed by the satellite is known as the swath. Multispectral scanners are the second most commonly used sensor after aerial cameras.

  • An Imaging Spectrometer or hyperspectral imager is a type of sensor that measures several (around 64-256) extremely narrow (5-10 nm) spectral bands. This allows it to produce a continuous reflectance curve for each pixel, rather than the limited number of values provided by a multispectral scanner. Spectral curves can depend on the chemical composition and microscopic structure of a material, allowing imaging spectrometers to determine the mineral composition of the Earth's surface, the chlorophyll content of surface water, or the concentration of total suspended matter in surface water.

  • A Thermal Scanner is a type of sensor that measures thermal data in the 8-14 micrometer range. Thermal scanners are often used in meteorological remote sensing systems to measure cloud, land, and sea surface temperatures, as well as to detect the effects of drought, the temperature of cooling water discharged from thermal power plants, and underground coal fires.

  • A Microwave Radiometer is a device that measures the intensity of radiant energy. It does this by transmitting laser light at a visible/near-infrared wavelength as a series of pulses and measuring the time it takes for the reflected pulses to return. Microwave radiometers can be used as profilers and scanners on air and space platforms, and can be used for determining altitudes, measuring speeds, analyzing particles in the air, assessing canopy conditions, and measuring depths in shallow waters. They are known for their low resolution, and often use an optical filter.

Both active and passive sensors have their own advantages and disadvantages, and the choice between the two types of sensors depends on the specific application and the type of data that is needed. Active sensors are typically more expensive and require more power to operate, but they can collect data in a wide range of conditions, including in darkness or through clouds. Passive sensors are typically less expensive and require less power, but they are limited to collecting data when the object or area being studied is emitting or reflecting energy.


Takeaways

  • Remote sensing is the process of collecting information about an object or area from a distance through measurement of reflected or emitted electromagnetic waves.

  • Astronomers have to take into account distortion due to atmospheric interactions such as Rayleigh and Mie Scattering.

  • Active sensors emit their own EM energy and measure the backscatter

  • Passive sensors rely upon natural EM energy such as that emitted by the Sun

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