Principles of Passive Remote Sensing
The principles of passive remote sensing include an understanding of electromagnetic radiation, the various types of sensors and platforms used to detect that radiation, interpretation of the data and its applications. We will look at each one of these principles briefly.
1. Electromagnetic Radiation - In passive remote sensing, passive sensors detect electromagnetic radiation emitted from an object. It is therefore essential to understand the nature of electromagnetic radiation and its interaction with the atmosphere. The propagation of electromagnetic energy is in the form of sine waves which are characterized by electric and magnetic fields. It can be in the form of either waves or photons. In passive remote sensing, we look for the wavelengths of the electromagnetic radiation emitted. Electromagnetic radiation can be in the form of ultraviolet radiation, x-rays, visible light and so on. When this radiation enters our atmosphere from the Sun, it is either reflected, scattered, absorbed or scattered. Passive remote sensing methods use either the Sun as a source of energy or artificial sources emitting radiation. Passive remote sensing systems are greatly affected by the atmospheric conditions from the varying amount of radiation received from the Sun.
2. Passive Sensors and Platforms - There are numerous passive sensors and platforms used for a variety of applications. A passive sensor measures the electromagnetic radiation emitted and stores it. The platform is a something the sensors is attached to. The following are of some of the passive sensors, the platforms used and their applications:
Gamma-ray spectrometer - A gamma-ray spectrometer is a passive sensor that detects gamma rays. The sources for the radiation is are generally upper-soil layers as well as rock layers. The radiation is caused by radioactive decay. Fundamentally, the gamma-ray spectrometer is used to explore mineral deposits.
Aerial cameras - An aerial camera is another type of passive remote sensor. This is used in aerial photography. Aircraft serve as a platform as well as many low-earth orbiting satellites deploy many aerial cameras. The main application of aerial cameras is for topographic mapping.
Thermal infrared video cameras - Although a video camera is a passive sensor, thermal infrared video cameras are equipped to detect radiation in the near-infrared range. Applications can be to provide additional qualitative data about a region. These devices are sometimes combined with active sensors, such as radar, to provide additional information. Aircraft as well as satellites can serve as platforms.
Multispectral scanner - A multispectral scanner records information in the visible and infrared spectrum. The scanner scans the Earth's surface for various wavelength bands. Satellites act as platforms for such passive sensors. One of the applications of the multispectral scanner is for geological purposes.
Imaging Spectrometer - An imaging spectrometer is principally similar to the multispectral scanner. It scans very narrow wavelength bands of the spectrum. Satellites are used as platforms. Applications can range from determining the mineral composition of the Earth's surface to concentrations of suspended matter in surface water.