Lecture- 03 Fundamentals of Remote Sensing Electromagnetic Radiation The first requirement for remote sensing is to have an energy source to illuminate the target (unless the sensed energy is being emitted by the target). This energy is in the form of electromagnetic radiation. Two characteristics of electromagnetic radiation are particularly important for understanding remote sensing. These are the wavelength and frequency. The Electromagnetic Spectrum and Bands What is the information being collected? It is measurements of electromagnetic radiation. The electromagnetic spectrum is a continuum of energy from short wave high frequency cosmic waves to longer wavelength low frequency radio waves. Our eyes are sensitive to the visible part of the electromagnetic spectrum. Within the visible spectrum our eyes can see the different colours which are variations in the wavelengths. Blue, green, and red are the primary colors or wavelengths of the visible spectrum. They are defined as such because no single primary color can be created from the other two, but all other colors can be formed by combining blue, green, and red in various proportions. Although we see sunlight as a uniform or homogeneous color, it is actually composed of various wavelengths of radiation in primarily the ultraviolet, visible and infrared portions of the spectrum. The visible portion of this radiation can be shown in its Violet: Blue: Green: Yellow: Orange: Red: 0.4 - 0.446 μm 0.446 - 0.500 μm 0.500 - 0.578 μm 0.578 - 0.592 μm 0.592 - 0.620 μm 0.620 - 0.7 μm 6 Active Sensor Active sensors, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Passive Sensor Remote sensing systems which measure energy that is naturally available are called passive sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. Example of Active Sensor & Passive sensors Interactions with the Atmosphere Before radiation used for remote sensing reaches the Earth's surface it has to travel through some distance of the Earth's atmosphere. Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are caused by the mechanisms of Scattering and Absorption. Scattering Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. How much scattering takes place depends on several factors including the wavelength of the radiation, the abundance of particles or gases, and the distance the radiation travels through the atmosphere. 11 There are three (3) types of scattering which take place. Rayleigh Scattering Mie Scattering Nonselective Scattering Rayleigh Scattering occurs when particles are very small compared to the wavelength of the radiation. These could be particles such as small specks of dust or nitrogen and oxygen molecules. Rayleigh scattering causes shorter wavelengths of energy to be scattered much more than longer wavelengths. The fact that the sky appears "blue" during the day is because of this phenomenon. 16 Mie Scattering Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation. Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to affect longer wavelengths than those affected by Rayleigh scattering. Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast. 17 Nonselective Scattering Nonselective Scattering occurs when the particles are much larger than the wavelength of the radiation. Water droplets and large dust particles can cause this type of scattering. This type of scattering causes fog and clouds to appear white to our eyes because blue, green, and red light are all scattered in approximately equal quantities (blue+green+red light = white light). Nonselective Scattering by a cloud 18 Absorption Absorption is the other main mechanism at work when electromagnetic radiation interacts with the atmosphere. In contrast to scattering, this phenomenon causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapour are the three main atmospheric constituents which absorb radiation. 19 Example Spectral Signature Every natural and artificial object reflects and emits EMR over a range of wavelengths in its own chemical composition and physical state. The distinctive reflectance and emission properties of objects are called spectral signature. Within some limited wavelength region, a particular object will usually exhibit a diagnostic spectral response patterns that differs from other objects. Spectral Signature It is hoped that each material on the earth would have a distinctive spectral response patterns that would allow it to be reliably detected by visual and digital means. Finding distinctive spectral response patterns is the key to most procedures for remote sensing image interpretation. Pixel and DN values A photograph could also be represented and displayed in a digital format by subdividing the image into small equal-sized and shaped areas, called picture elements or pixels, and representing the brightness of each area with a numeric value or digital number. Resolutions There are four types of resolutions in Remote Sensing Spatial resolution – ‘Area’ aspects Spectral resolution -‘ Band’ aspect Radiometric resolution- ‘Radiance’ aspect Temporal resolution- ‘Frequency’ aspect Spatial Resolution The earth surface area covered by a pixel of an image is known as spatial resolution. Large area covered by a pixel means low spatial resolution and vice versa. Spatial resolution Pixel: smallest unit of an image Pixel size Spatial coverage Field of view (FOV), or Area of coverage, such as MODIS: 2300km Spatial Resolution 30 meter, Spatial Resolution 1 meter, Spatial Resolution Spatial Resolution Spectral Resolution Is the ability to resolve spectral features and bands into their separate components. More number of bands in a specified bandwidth means higher spectral resolution and vice versa Spectral resolution describes the ability of a sensor to define fine wavelength intervals The finer the spectral resolution, the narrower the wavelength range for a particular channel or band Radiometric Resolution The radiometric resolution of an imaging system describes its ability to discriminate very slight differences in energy The finer the radiometric resolution of a sensor, the more sensitive it is to detecting small differences in reflected or emitted energy. The maximum number of brightness levels available depends on the number of bits used in representing the energy recorded. Thus, if a sensor used 8 bits to record the data, there would be 28=256 digital values available, ranging from 0 to 255. 2-Bit Image 8-Bit Image Radiometric Resolution 2-bit range 0 4 6-bit range 0 63 8-bit range 0 255 10-bit range 0 1023 Temporal resolution • Temporal resolution is the revisit period, and is the length of time for a satellite to complete one entire orbit cycle, i.e. start and back to the exact same area at the same viewing angle. For example, Landsat needs 16 days, MODIS needs one day, NEXRAD needs 6 minutes for rain mode and 10 minutes for clear sky mode. July 2 July 18 August 3 16 days Time 11 days July 1 July 12 July 23 August 3 Thanks