What is remote sensing?
In remote sensing, three elements are essential. They are:
In the process of Remote Sensing involves an interaction between the incoming radiation and interest of target. This is done by using imaging and non-imaging system; however the following steps are involved in the process
a. Energy Sources:
The first and most important requirement for a Remote Sensing system is an ideal energy source or illumination which provides electromagnetic radiation to the Target interest.
b. Atmosphere and Radiation:
As the energy traveling from its source to Earth surface, it will come in contact with atmosphere when it passes through. This is also happening when the energy from target reflected bake to sensor.
c. Interaction with the Target and Recording of the Reflected Energy
Once the energy is passed through the atmosphere, it interacts with the target object and depending upon the physical and chemical properties of the Target the energy is reflected or emitted back the Sensor collect and record the Electromagnetic radiation.
d. Transmission and Ground level Processing
After the energy sensed it has to be transmitted in the form of electronic signals to the ground stations for processing and generate the output as image (Hard copy/Soft copy). The processing involves various steps that we will discuss in later.
e. Interpretation, Analysis and Application.
The processed image is interpreted visually and digitally using various interpretation Techniques to extract the information.
The final step is that we are applying the extracted information on various fields of our studies. It may reveal some new information about which the target. The data we gained through Remote Sensing may not be able to collect it through other conventional methods. The applications are infinite.
The five headings are the major elements in Remote Sensing from beginning to end. The following chapters will be covering these main headings.
In a more restricted sense, remote sensing usually refers to the technology of acquiring information about the earth‘s surface (land and ocean) and atmosphere using sensors onboard airborne(aircraft, balloons) or spaceborne (satellites, space shuttles) platforms.
Satellite Remote Sensing
You will see many remote sensing images around Asia acquired by earth observation satellites. These remote sensing satellites are equipped with sensors looking down to the earth. They are the “eyes in the sky” constantly observing the earth as they go round in predictable orbits.
Effects of Atmosphere
In satellite remote sensing of the earth, the sensors are looking through a layer ofatmosphere separating the sensors from the Earth’s surface being observed. Hence, it is essential to understand the effects of atmosphere on the electromagnetic radiation travelling from the Earth to the sensor through the atmosphere. The atmospheric constituents cause wavelength dependentabsorption and scattering of radiation. These effects degrade the quality of images. Some of the atmospheric effects can be corrected before the images are subjected to further analysis and interpretation.
A consequence of atmospheric absorption is that certain wavelength bands in the electromagnetic spectrum are strongly absorbed and effectively blocked by the atmosphere. The wavelength regions in the electromagnetic spectrum usable for remote sensing are determined by their ability to penetrate atmosphere. These regions are known as theatmospheric transmission windows. Remote sensing systems are often designed to operate within one or more of the atmospheric windows. These windows exist in the microwave region, some wavelength bands in the infrared, the entire visible region and part of the near ultraviolet regions. Although the atmosphere is practically transparent to x-rays and gamma rays, these radiations are not normally used in remote sensing of the earth.
Optical and Infrared Remote Sensing
In Optical Remote Sensing, optical sensors detect solar radiation reflected or scattered from the earth, forming images resembling photographs taken by a camera high up in space. The wavelength region usually extends from the visible and near infrared (commonly abbreviated as VNIR) to the short-wave infrared (SWIR).
|Different materials such as water, soil, vegetation, buildings and roads reflect visible and infrared light in different ways. They have different colours and brightness when seen under the sun. The interpretation of optical images require the knowledge of the spectral reflectance signatures of the various materials (natural or man-made) covering the surface of the earth.|
|There are also infrared sensors measuring the thermal infrared radiation emitted from the earth, from which the land or sea surface temperature can be derived.|
Microwave Remote Sensing
There are some remote sensing satellites which carry passive or activemicrowave sensors. The active sensors emit pulses of microwave radiation to illuminate the areas to be imaged. Images of the earth surface are formed by measuring the microwave energy scattered by the ground or sea back to the sensors. These satellites carry their own “flashlight” emitting microwaves to illuminate their targets. The images can thus be acquired day and night. Microwaves have an additional advantage as they can penetrate clouds. Images can be acquired even when there are clouds covering the earth surface.A microwave imaging system which can produce high resolution image of the Earth is the synthetic aperture radar (SAR). The intensity in a SAR image depends on the amount of microwave backscattered by the target and received by the SAR antenna. Since the physical mechanisms responsible for this backscatter is different for microwave, compared to visible/infrared radiation, the interpretation of SAR images requires the knowledge of how microwaves interact with the targets.
Remote Sensing Images
Remote sensing images are normally in the form of digital images. In order to extract useful information from the images, image processing techniques may be employed to enhance the image to help visual interpretation, and tocorrect or restore the image if the image has been subjected to geometric distortion, blurring or degradation by other factors. There are many image analysis techniques available and the methods used depend on the requirements of the specific problem concerned. In many cases, imagesegmentation and classification algorithms are used to delineate different areas in an image into thematic classes. The resulting product is a thematic map of the study area. This thematic map can be combined with other databases of the test area for further analysis and utilization.