Remote Sensing Overview

 

Remote sensing is gathering information about something without being in direct contact with it. It is the exact opposite of in-situ sensing,  This is done by sensing and recording reflected or emitted energy and processing, analyzing, and applying that information.  Your eyes are remote sensing devices, since they obtain information about your surroundings without actually touching them. Cameras are also remote sensing devices, and we will discuss them in more later.  

Sensors collect and store data about the spectral reflectance of natural features and objects, both of which reflect radiation. This radiation can be quantified on an electromagnetic spectrum.

The electromagnetic spectrum is a continuum of electromagnetic energy arranged according to its frequency and wavelength. As the electromagnetic waves are radiated through space, their energy interacts with matter and one of three reactions occurs. The radiation will either be:

  1. reflected off the object

  2. absorbed by the object

  3. transmitted through the object

Aircraft and satellites are the common platforms for remote sensing observations.

The term remote sensing is commonly restricted to methods that employ electromagnetic energy such as heat, light and radio-waves, as the means of detecting and measuring target characteristics.

This definition of remote sensing excludes electrical, gravity and magnetic surveys that measure force fields rather than electromagnetic radiation.

Aerial photography is the original form of remote sensing and has played a major role in the discovery of many oil and mineral deposits around the world.

The success of aerial photography, using the visible spectrum of the electromagnetic radiation, suggested that it might be possible to obtain comparable results by using the other wavelength regions.

In the 1960s, technological development enabled the acquisition of images at other wavelengths, including thermal infrared (IR) and microwave.

The development and deployment of manned and unmanned satellites provided an orbital vantage point for acquiring images of the earth.

 

Elements of Remote Sensing

In much of remote sensing, the process involves an interaction between incident radiation and the targets of interest. This is exemplified by the use of imaging systems where the following seven elements are involved.

Note, however that remote sensing also involves the sensing of emitted energy and the use of non-imaging sensors.

1. Energy Source or Illumination (A)

The first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest.

2. Radiation and the Atmosphere (B)

As the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.

 

3. Interaction with the Target (C)

Once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation.

4. Recording of Energy by the Sensor (D)

After the energy has been scattered by, or emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation.

5. Transmission, Reception, and Processing (E)

The energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital).

6. Interpretation and Analysis (F)

The processed image is interpreted, visually and/or digitally or electronically, to extract information about the target which was illuminated.

7. Application (G)

The final element of the remote sensing process is achieved when we apply the information we have been able to extract from the imagery about the target in order to better understand it, reveal some new information, or assist in solving a particular problem.

Remote Sensing Devices

In this class we will look at four main types of remote sensing systems:

  1. Photographic - these involve cameras that image in the visible and reflected infrared

  2. Optical - for this class, we are restricting what we call optical to the visible (0.4 - 0.7 m) and the reflected infrared (0.7 - 3.0 m) This and all subsequent discussion will be about digital (non-photographic) imaging systems.

  3. Thermal - remote sensing in these wavelengths differs from the shorter IR, since it is emitted energy rather than reflected.

  4. Active microwave (radar) - this type of remote sensing differs from other types that we will discuss, since active systems supply their own energy source (no sunlight required!)


   


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S. Farooq

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