Introduction
Remote sensing is the part of geospatial technologies that uses the detection of electromagnetic radiation to determine characteristics without direct contact. Ultrasonics can also be used which are sound waves and not part of the electromagnetic spectrum. Remote sensing can be passive detection of electromagnetic radiation such as visible light (the colors which the human eye can detect) and the active detection which involves sending electromagnetic energy from a transmitter and detecting the returned signal. For example, every time a smart phone is used to take a picture this is a form of remote sensing, since the detector within the camera is recording the electromagnetic visible light in three different color bins, this is a passive process if the flash is not used. When the flash is used on the smart phone it becomes an active process, since a signal, the flash, is sent out of the device and the device receives the reflected information. Another example of active remote sensing is weather radar, a pulse of energy is sent from the radar station and the reflected information is recorded by an antenna. X-raying a person, would be another form of active remote sensing, since the X-ray machine sends out a beam of energy and what passes through the individual is recorded. For this lesson remote sensing will be of the surface of the earth using aerial vehicles, a more complete coverage of the subject of remote sensing can be found in the remote sensing course.
Remote sensing is always dealing with the reception of information, which can be classified according to the intensity and frequency of the received signal as well as the time from sending of an energy pulse to receiving the returned information. Today the detection of the signal is done by solid state devices, in the early days of this field of study, it was mostly by direct photography. Remote Sensing is a critical component of GEOINT[1]. Remote Sensing uses the concepts of Physics from the fields of Optics and Electromagnetic Theory. Detectors, many times are limited by current engineering principles and the characteristics of optics.
A simple digital camera, like in your phone, is actually a remote sensing device, it is receiving reflected light and is usually classified into one of three bands, red, green or blue. The receiving device has cells, that receive photons[2]. The device can be thought of as having blue cells, red cells and green cells. Together these three cells make up a single picture element (pixel). The more photons in each cell the more dominated that color of light. For example, if only red photons are received then the pixel will be red. If there is a mixing of different colored photons, different colors of light will be created. Depending on the intensity of each of the three colors of light received will determine the color of the object displayed on the screen. There are a few cameras that uses four or five bands instead of the standard three in visible light. A digital camera in an aircraft or satellite works in the same fashion. One big difference in the detector is that it might be receiving light in multiple bands and it may receive information outside of the visible spectrum such as the near infrared.
Software can be used to determine optical characteristics of a feature, such as grass, which is made up of a little red, lots of green and some blue. Once the ratios of representative colors is known, a spectral signature can be created. From this information it can be determined how much grass is in the image. By using multiple bands (more than three) the ability to accurately determine features is enhanced. Change detection can be done by imaging the same area periodically.
Not all research is done in the visible or near visible wavelength. Thermal imagery is used to determine heat loses, radio/microwaves are used in the detection of elevation, some electromagnetic wavelengths actually penetrate the surface of the earth.
[1] GEOINT is Geospatial Intelligence.
[2] Photons are chargeless, massless particles of light, photons have different energies depending on the color they represent, blue photons contain more energy than red ones.
Remote sensing is always dealing with the reception of information, which can be classified according to the intensity and frequency of the received signal as well as the time from sending of an energy pulse to receiving the returned information. Today the detection of the signal is done by solid state devices, in the early days of this field of study, it was mostly by direct photography. Remote Sensing is a critical component of GEOINT[1]. Remote Sensing uses the concepts of Physics from the fields of Optics and Electromagnetic Theory. Detectors, many times are limited by current engineering principles and the characteristics of optics.
A simple digital camera, like in your phone, is actually a remote sensing device, it is receiving reflected light and is usually classified into one of three bands, red, green or blue. The receiving device has cells, that receive photons[2]. The device can be thought of as having blue cells, red cells and green cells. Together these three cells make up a single picture element (pixel). The more photons in each cell the more dominated that color of light. For example, if only red photons are received then the pixel will be red. If there is a mixing of different colored photons, different colors of light will be created. Depending on the intensity of each of the three colors of light received will determine the color of the object displayed on the screen. There are a few cameras that uses four or five bands instead of the standard three in visible light. A digital camera in an aircraft or satellite works in the same fashion. One big difference in the detector is that it might be receiving light in multiple bands and it may receive information outside of the visible spectrum such as the near infrared.
Software can be used to determine optical characteristics of a feature, such as grass, which is made up of a little red, lots of green and some blue. Once the ratios of representative colors is known, a spectral signature can be created. From this information it can be determined how much grass is in the image. By using multiple bands (more than three) the ability to accurately determine features is enhanced. Change detection can be done by imaging the same area periodically.
Not all research is done in the visible or near visible wavelength. Thermal imagery is used to determine heat loses, radio/microwaves are used in the detection of elevation, some electromagnetic wavelengths actually penetrate the surface of the earth.
[1] GEOINT is Geospatial Intelligence.
[2] Photons are chargeless, massless particles of light, photons have different energies depending on the color they represent, blue photons contain more energy than red ones.