Properties of satellite images Remote sensing includes the set of sensors anddata processing procedures that are deployed to develop information concerningdifferent properties of the Earths’ surface (i.e. the land, atmosphere, andoceans) which can be achieved with no direct physical contact. Information isobtained from capturing the reflected waves from the earth surface, and itsdifference as a function of wavelength, phase, location, and time.
A variety ofsensors are generally deployed both passive (i.e. which depends on reflectedsolar radiation or emitted radiation) and active (i.e. which generates its ownsource of Electromagnetic (EM) waves).These sensors work all over the EM rangefrom visible to microwave wavelengths.
There are different platforms on whichthese sensors are attached such as earth-orbiting satellites, aircraft are byfar the most common. Normally, remote sensing images are classified accordingto four different characteristics based on the type of sensors technology. Thefirst characteristic is the spatial resolution where the higher is the resolutionthe clearer objects on earth appears in the image (the known highest resolutionis approximately few centimeters while the lowest can reach 9 kilometers). Thelower is the resolution the more area is captured. The second characteristic ofthe remote sensing images is the spectral resolution. It describes the capacityof the sensors to differentiate between wavelengths in the EM spectrum.
Thefiner the resolution more bands are recorded which means that it is possible toobtain more details about the reaction of sensed objects to the light. Thereare two technologies hyperspectral and multispectral where the first can have aspectral resolution of 1 nm with hundreds of bands compared to rougher spectralresolution for the multispectral that can reach 100 nm with few bands. Thethird property is the temporal one which defines the period of time needed by asatellite to revisit the same spot on earth. The less time needed to revisit aspecific area the higher is the temporal resolution. The fourth and lastcharacteristic is the radiometric which specifies how well the differences inbrightness in an image can be perceived. This is measured through the number ofthe levels of the gray value.
The maximum value related to radiometricresolution is defined by the number of bits where eight bits representation has256 grey values, a sixteen bits representation has 65,536 grey values. Thefiner or the higher the radiometric property is the better is the recorded reflectedwaves, but the volume of measured data will be larger.