File Name: difference between passive and active sensors .zip
Radars and lasers are breathtaking attributes of sci-fi movies no longer as it was just a few decades ago.
- Passive vs Active Sensors in Remote Sensing
- Introduction to Remote Sensing
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Passive vs Active Sensors in Remote Sensing
There are two main types of sensors used in remote sensing technology, active sensors and passive sensors. Difference between these two is quite small, however, leaves great impact on their functionality. As we learnt in primary school, sun constantly emits light energy and is only source of natural light for earth. Moreover, this energy is not observed by any component in solar system. Instead it is reflected to somewhere else. Just like the moon, it shines despite of not possessing its own light source.
Introduction to Remote Sensing
Another possibility to distinguish between earth observation satellites is to compare the sensors used. In general, there are passive sensors which measure the reflected sunlight or thermal radiation, and active sensors which make use of their own source of radiation. Active sensors for example Radar and laser scanners emit artificial radiation to monitor the earth surface or atmospheric features. Radars are imaging instruments while radar altimeters and scatterometers are non imaging. Radar is the abbreviation for Radio Detection and Ranging , a method for the detection and ranging of earth surface features. Radar satellites use short pulses of electromagnetic radiation in the microwave spectral range, therefore they do not depend on daylight and are hardly affected by clouds, dust, fog, wind and bad weather conditions. They measure the radar pulses reflected from the ground, analyse the signal intensity in order to retrieve information on the structure of the earth surface, and detect the elapsed time between pulse emission and return.
Platforms refer to the structures or vehicles on which remote sensing instruments are mounted. The platform on which a particular sensor is housed determines a number of attributes, which may dictate the use of particular sensors. These attributes include: distance the sensor is from the object of interest, periodicity of image acquisition, timing of image acquisition, and location and extent of coverage. There are three broad categories of remote sensing platforms: ground based, airborne, and satellite. Some of the more common ones are hand held devices, tripods, towers and cranes. Instruments that are ground-based are often used to measure the quantity and quality of light coming from the sun or for close range characterization of objects. For example, to study properties of a single plant or a small patch of grass, it would make sense to use a ground based instrument.
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Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus is in contrast to on-site observation. The term is applied especially to acquiring information about the Earth. Remote sensing is used in numerous fields, including geography, land surveying and most Earth science disciplines for example, hydrology, ecology , meteorology, oceanography, glaciology, geology ; it also has military, intelligence, commercial, economic, planning, and humanitarian applications, among others.
Satellites are outfitted with sensors as a means of monitoring. The sensors we are concerned about in this blog are those that monitor the earth through imagery collection. There are several types of imaging sensors, but they all fall into one of two categories: passive or active. Passive sensors detect reflected electromagnetic radiation from a source such as the sun. The camera on your phone is a passive sensor, receiving the reflected spectrum from the sun as it reflects off your body and clothes, thus capturing your likeness in a photo.
So far, throughout this chapter, we have made various references to the sun as a source of energy or radiation. The sun provides a very convenient source of energy for remote sensing. The sun's energy is either reflected , as it is for visible wavelengths, or absorbed and then re-emitted , as it is for thermal infrared wavelengths.
If the sun disappeared, which type of sensor would miss it most?