Introduction to Remote Sensing

Introduction to Remote Sensing
History EMR EMS Radiation Characteristics Spectral Signatures

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Remote Sensing A technique of obtaining information about objects through the analysis of data collected by special instruments that are not in physical contact with the objects of investigation. Reconnaissance from a distance.

History 1839 - first photograph 1858 - first photo from a balloon
first plane 1909 first photo from a plane B/W infrared film WW I and WW II space See your lecture notes for more details

Electromagnetic Radiation (EMR)
wavelength frequency Electromagnetic Radiation (EMR) - travels at the speed of light (c) (186,000 miles/second or 300,000 km/second) - behaves as waves, both electric and magnetic. - The nature of EMR is characterized by wavelength and frequency (the number of wave crests (cycles) that pass a fixed point per second). They are related to the speed of light in the following manner; c = v \

Electromagnetic Radiation
a) Energy exists in many different forms including electrical energy, heat energy and light energy. b) Light energy can be thought of as a wave. This waves moves through space causing fluctuations in the electrical and magnetic ‘fields’ that surround us. Like the waves found in water, light wave have peaks and troughs.

EMR Remote sensing is concerned with the measurement of EMR returned by the earth’s natural and cultural features that first receive energy from the sun or an artificial source such as a radar transmitter.

EMR Because different objects return different types and amounts of EMR, the objective in remote sensing is to detect these differences with the appropriate instruments. This, in turn, makes it possible for us to identify and assess a broad range of surficial features and their conditions.

Electromagnetic Spectrum
Ranges From: Gamma rays (short wavelength, high frequency and high energy content) To: Passive radio waves (long wavelength, low frequencies, and low energy content). Electromagnetic Spectrum (EMS) - the entire range of EMR is the EMS. - for the sun, the EMS stretches from gamma rays (short wavelength, high frequencies, and high energy content) to passive radio waves (long wavelength, low frequencies, and low energy content).

EMS A spectral band is composed of some defined group of continuous spectral lines, where a line represents a single wavelength or frequency. The boundaries between most of the bands are arbitrarily defined because each portion overlaps adjacent portions. - largely for convenience the continuum is divided into several named divisions called spectral bands, which share similar characteristics. - A spectral band is composed of some defined group of continuous spectral lines, where a line represents a single wavelength or frequency. The boundaries between most of the bands are arbitrarily defined because each portion overlaps adjacent portions. The boundaries of the visible band are the most precise because they are defined by the wavelength limits of human vision.

EMS centimeter = .01 meters millimeter = .001 meters
micrometer = ,000,1 meters nanometer = ,000,000,1 meters angstrom = ,000,000,01 meters

The EM Spectrum Different wavelengths of light can be grouped together into different types Visible light contains light from 0.4 to 0.7 micrometers Infrared light from 0.1 micrometers to 1 millimeter These peaks and troughs can be used to characterize light. i) The distance between successive peaks is called the wavelength often denoted using the Greek letter lambda, . ii) Light can also be characterize by how many times the peaks pass a particular point per second. This is the frequency of light, often denoted using the Greek letter mu, . iii) Visible light has wavelengths between 400 and 700 nanometers (10 -9), that is 0.000,000,400 and 0.000, meters. iii) infrared radiation, microwaves and radio waves all have lower energy than visible light: ultraviolet radiation, X-rays and  -rays have higher energy. in the same way that visible light can be conveniently broken down into three colors according to frequency (red, green, blue), the infrared region can be broken down into regions: near infrared, mid and thermal infrared · near infra red are infra red frequencies that are near light frequencies (e.g. approx. 0.7 to 2.5 m wavelengths) · mid infra red lies between near and thermal IR (e.g. approx. 2.5 to 10 m wavelength). Water absorbs mid IR radiation strongly · thermal IR has wavelength longer than 10 m

Radiation

R/S Spectral Regions Ultraviolet (UV) Visible Infrared (IR) Microwave
Remote sensing systems have been designed to detect EMR selectively within one or more of the divisions, but no single instrument is capable of detecting radiation within all of the divisions. As a group these sensors can extract information from a spectral range that is several million times wider than the visible band.

R/S Spectral Regions

R/S Spectral Regions Traditionally, the most common used region of the EMS in remote sensing has been the visible band. Its wavelength span is from 0.4 to 0.7 micrometers, limits established by the sensitivity of the human eye.

R/S Spectral Regions The bulk of the sun’s radiation is concentrated here ( use table 1-2 p.6 ) and it able to pass relatively unimpeded through the earth’s atmosphere to the earth’s surface.

Visible Light Composed of colors (different wavelengths)
These familiar colors range from violet (shortest wavelength) through indigo, blue, green, yellow, orange and red (ROYGBIV). White light refers to the evenly distributed wavelengths of the visible band that have not been separated into their spectral components. When a beam of white light is passed through a prism , it is separated into different wavelength bands that are made noticeable by their different colors. These familiar colors range from violet (shortest wavelength) through indigo, blue, green, yellow, orange and red.

Color The visible spectrum is also viewed as being composed of three equal-wavelength segments that represent the additive primary colors; Blue (0.4 to 0.5 micrometers) Green (0.5 to 0.6 micrometers) Red ( 0.6 to 0.7 micrometers)

Primary Colors A primary color is one that cannot be made from any other color. All colors perceived by the human optical system can be produced by combining the proper proportions of light representing the three primaries. This principle forms the basis for the operation of the color TV. Most of the colors we see are the result of the preferential reflection and absorption of wavelengths that make up white light.

Color The chlorophyll of healthy grass selectively absorbs more of the blue and red wavelengths of white light and reflects relatively more of the green wavelengths to our eyes.

Infrared (IR) Band The infrared (IR) band has wavelengths between red visible light (0.7 micrometers) and microwaves at 1,000 micrometers. Infrared means “below the red.” In remote sensing the IR band is usually divided into two components that are based on basic property differences; Reflected IR band Emitted/Thermal IR band

Reflected IR The reflected IR band represents reflected solar radiation which behaves like visible light. Its wavelength span is from 0.7 to about 3 micrometers.

Thermal IR (Heat) The dominant type of energy in the thermal IR band is heat energy, which is continuously emitted by the atmosphere and all objects on the earth’s surface. Its wavelength span is from about 3 micrometers to 1,000 micrometers or 0.1 centimeters. Because of atmospheric attenuation, the thermal IR region beyond about 14 micrometers is generally not available for remote sensing studies.

Microwave Band The microwave band falls between the IR and radio bands and has a wavelength range extending from approximately 0.1 centimeters to 1 meter.

Microwave Band At the proper wavelengths microwave radiation can pass through; - clouds - precipitation - tree canopies - dry surficial deposits such as; - sand and - fine-grained alluvium

Microwave Sensors Passive Microwave - detect natural microwave radiation that is emitted from the earth’s surface. RADAR - propagates artificial microwave radiation to the surface and detects the reflected component.

Solar and Terrestrial Radiation
Most remote sensing systems are designed to detect; solar radiation which passes through the atmosphere and is reflected in varying degrees by the earth’s surface features. terrestrial radiation which is continuously emitted by these same features. Solar radiation is detectable only during daylight, terrestrial radiation can be detected both day and night. The wavelength span and intensity of electromagnetic energy radiated by the sun and earth are primarily a function of their surface temperatures.

The sun’s visible surface has a temperature of about 6000oK (11,000oF). At this very high temperature, the radiated energy covers a broad range of wavelengths, extending from very short gamma rays to very long radio waves. Maximum radiation occurs at a wavelength of 0.48 micrometers in the visible band. About half the solar radiation passes through the earth’s atmosphere and is absorbed in varying degrees by surface features of the earth. Most of this absorbed radiation is transformed into low-temperature heat (warming the surface), which is continuously emitted back into the atmosphere at longer thermal IR wavelengths. The earth’s land and water surface has an ambient temperature of about 300oK (80oF)

99% of the sun’s radiation falls between wavelengths of 0.2 and 5.6 micrometers. 80% is contained in wavelengths between 0.4 and 1.5 micrometers (visible and reflected IR), to which the atmosphere is quite transparent. Maximum radiation occurs at a wavelength of 0.48 micrometers in the visible band.

About half the solar radiation passes through the earth’s atmosphere and is absorbed in varying degrees by surface features of the earth. Most of this absorbed radiation is transformed into low-temperature heat (warming the surface), which is continuously emitted back into the atmosphere at longer thermal IR wavelengths. The earth’s land and water surface has an ambient temperature of about 300oK (80oF) At this relatively cool temperature, the earth radiates about 160,000 times less energy than the sun. Essentially all the energy is radiated at invisible thermal IR wavelengths between about 4 and 25 micrometers, with a maximum occurring at a wavelength of 9.7 micrometers.

Because the wavelengths covering most of the earth’s energy output are several times longer than those covering most of the solar output, terrestrial radiation is frequently called longwave radiation and solar radiation is termed shortwave radiation. At this relatively cool temperature, the earth radiates about 160,000 times less energy than the sun. Essentially all the energy is radiated at invisible thermal IR wavelengths between about 4 and 25 micrometers, with a maximum occurring at a wavelength of 9.7 micrometers.

Longwave radiation is also emitted by; - the atmosphere’s gasses and clouds and - from artificially heated objects on the earth’s surface such as - from buildings - steam lines - certain industrial effluents. At this relatively cool temperature, the earth radiates about 160,000 times less energy than the sun. Essentially all the energy is radiated at invisible thermal IR wavelengths between about 4 and 25 micrometers, with a maximum occurring at a wavelength of 9.7 micrometers.

Radiation-Matter Interactions
EMR manifests itself only through its interactions with matter which can be in the form of; a gas a liquid a solid

When EMR strikes matter, EMR may be; transmitted reflected scattered absorbed EMR that impinges upon matter is called incident radiation. For the earth the strongest source of incident radiation is the sun. Such radiation is called insolation, a shortening of incoming solar radiation. The full moon in the second strongest source, but its radiant energy measures only about one-millionth that of the sun.

The amount on interaction depends upon; the composition and physical properties of the medium. the wavelength or frequency of the incident radiation. the angle at which the incident radiation strikes a surface.

Transmission Transmission is the process by which incident radiation passes through matter without measurable attenuation. The substance is thus transparent to the radiation.

Transmission Transmission through material media of different densities (such as air to water) causes the radiation to be refracted or deflected from a straight-line path with an accompanying change in its velocity and wavelength; frequency always remains constant.

Reflection Reflection (also called specular reflection) is the process where incident radiation “bounces off” the surface of the substance in a single, predictable direction.

Reflection The angle of reflection is always equal and opposite to the angle of incidence. Reflection is caused by surfaces that are smooth relative to the wavelength of the incident radiation. These smooth mirror-like surfaces are called specular reflectors. Specular reflection causes no change to either EMR velocity or wavelength.

Scattering Scattering (also called diffuse reflection) occurs when incident radiation is dispersed or spread out unpredictable in many different directions, including the direction from which it originated.

Scattering In the real world, scattering is much more common than reflection. The scattering process occurs with surfaces that are rough relative to the wavelengths of incident radiation. Such surfaces are called diffuse reflectors. EMR velocity and wavelength are not affected by the scattering process.

Absorption Absorption is the process by which incident radiation is taken in by the medium. For this to occur, the substance must be opaque to the incident radiation. A portion of the absorbed radiation is converted to internal heat energy, which is subsequently emitted or reradiated at longer thermal IR wavelengths.

EMR - Atmosphere Interactions
Areas of the spectrum where specific wavelengths can pass relatively unimpeded through the atmosphere are called transmission bands or atmospheric windows. Although EMR travels through empty space without modification, a series of interactions occur as solar and terrestrial radiation interact with the earth’s atmosphere. This interference is wavelength selective, meaning that EMR at certain wavelengths passes freely through the atmosphere, whereas it is restricted at other wavelengths. Areas of the spectrum where specific wavelengths can pass relatively unimpeded through the atmosphere are called transmission bands or atmospheric windows.

Absorption bands define those areas where specific wavelengths are totally or partially blocked.

To observe the earth’s surface different remote sensing instruments have been designed to operate within the windows where the atmosphere will transmit sufficient radiation for detection.

EMR interacts with the atmosphere in the following ways; it may be absorbed and re-radiated at longer wavelengths, which causes the air temperature to rise. it may be reflected and scattered without change to either its velocity or wavelength. it may be transmitted in a straight-line path directly through the atmosphere.

Radiation Balance - Incoming equals outgoing.

Atmospheric Absorption and Transmission
Significant absorbers of EMR in the atmosphere; oxygen nitrogen ozone carbon dioxide water vapor

Atmospheric Absorption and Transmission
The atmosphere’s gasses are selective absorbers with reference to wavelength. Gamma and X-ray radiation are completely absorbed in the upper atmosphere by oxygen and nitrogen.

Atmospheric Scattering
EMR within certain sections of the UV, visible and reflected IR bands is scattered by the atmosphere. Important scattering agents include; gas molecules suspended particulates clouds UV radiation, at wavelengths less than 0.2 micrometers, is absorbed by molecules of oxygen. This added energy is enough to cause the affected diatomic oxygen to split apart, leaving single atoms of oxygen. Reactions occur to combine monatomic and diatomic oxygen into triatomic oxygen (ozone). The resulting ozone effectively absorbs UV radiation with wavelengths between 0.2 and 0.3 micrometers in the ozone layer in the stratosphere. Carbon dioxide effectively absorbs between 14 and 20 micrometers and ozone in the 9 to 10 micrometer span. This absorbed radiation heats the lower atmosphere, especially in humid areas where there is abundant water vapor. Oxygen and water vapor cause three narrow absorption bands at the beginning of the microwave region between 0.1 and 0.6 cm. Most atmospheric windows become less transparent when the air is moist.

In addition, clouds absorb most of the longwave radiation emitted by the earth’s surface, essentially closing the thermal IR windows. This is why cloudy nights tend to be warmer than clear nights. Only microwave radiation with wavelengths longer than about 0.9 cm is capable of penetrating clouds. UV radiation, at wavelengths less than 0.2 micrometers, is absorbed by molecules of oxygen. This added energy is enough to cause the affected diatomic oxygen to split apart, leaving single atoms of oxygen. Reactions occur to combine monatomic and diatomic oxygen into triatomic oxygen (ozone). The resulting ozone effectively absorbs UV radiation with wavelengths between 0.2 and 0.3 micrometers in the ozone layer in the stratosphere. Carbon dioxide effectively absorbs between 14 and 20 micrometers and ozone in the 9 to 10 micrometer span. This absorbed radiation heats the lower atmosphere, especially in humid areas where there is abundant water vapor. Oxygen and water vapor cause three narrow absorption bands at the beginning of the microwave region between 0.1 and 0.6 cm. Most atmospheric windows become less transparent when the air is moist.

Important scattering agents include; gas molecules suspended particulates clouds

There are three types of atmospheric scattering important to remote sensing; Rayleigh or molecular scattering Mie or non-molecular scattering Non-selective scattering

Rayleigh or Molecular Scattering
primarily caused by oxygen and nitrogen molecules whose diameters are, at least, 0.1 times smaller than the affected wavelengths. Rayleigh scattering is highly selective being inversely proportional to the fourth power of the wavelength.

Mie or non-molecular scattering
occurs when there are sufficient particles in the atmosphere that have diameters from about 0.1 to about 10 times larger than the wavelengths under consideration.

Mie or non-molecular scattering
Important Mie scattering agents include; water vapor smoke dust volcanic materials salt from evaporated sea spray Mie scattering influences longer wavelengths than Rayleigh scattering.

Non-selective scattering
is found in the lower atmosphere when there are sufficient numbers of suspended aerosols having diameters at least 10 times larger than the wavelengths under consideration.

Important nonscattering agents include; larger Mie particles water droplets ice crystals

depends upon wavelength. Within the visible band, colorless water droplets and ice crystals scatter all wavelengths equally well, causing, for example, the sunlit surfaces of clouds to appear brilliant white.

Skylight and Haze The clear sky is a source of illumination because its gasses preferentially scatter the shorter wavelengths of sunlight. This diffuse radiation is called sunlight or sky radiation. Daylight illumination at the earth’s surface consists of both direct sunlight and diffuse skylight. By comparison, only direct sunlight reaches the moon’s surface because the moon has no atmosphere and so no scattering agents to cause skylight. The moon’s sky is black. Within the visible spectrum, skylight is compose primarily of blue wavelengths. It is skylight that prevents absolute darkness in shadows where direct sunlight is absent.

Skylight and Haze To our eyes sky radiation is manifested as haze which causes a reduction in visibility and also causes distant landscapes to take on a soft, blue-gray appearance. Atmospheric haze has important ramifications in remote sensing.

Skylight and Haze In the short wavelength region, radiation reaching an airborne or spaceborne sensor consists of two components; radiation that is scattered by the earth’s surface and then reaches the sensor without being affected by the intervening atmosphere. radiation that is scattered by the atmosphere, either before or after it reaches the earth’s surface.

Skylight and Haze The radiation scattered by the atmosphere contains no information about the earth’s surface, and it acts as a masking agent when a remote sensing system records these wavelengths. The most strongly affected wavelengths are UV and blue.

Skylight and Haze The net effect of this extra illumination, or non-image forming “haze light”, is a loss of detail and a reduction in scene contrast. Haze is visualized as a fog-like veil in black and white photos and as an overall blueish tint in a color photo. Haze can be reduced or eliminated by using special lens filters that do not transmit blue and/or UV radiation to the film.

EMR - Surface Interactions
The natural and cultural features of the earth’s surface interact differently with solar radiation. Albedo or Spectral Reflectance is the percentage radiation reflected by an object.

EMR - Surface Interactions

Spectral Signatures Every natural and synthetic object reflects and emits EMR over a range of wavelengths in its own characteristic manner according , in large measure, to its chemical composition and physical state.

Spectral Signatures Spectral signatures are the distinctive reflectance and emittance properties of objects. Within some limited spectral region, a particular object will usually exhibit a unique spectral response pattern that differs from that of other objects.

Spectral Signatures Remote sensing depends upon operation in wavelength regions of the spectrum where these detectable differences in reflected and emitted radiation occur.

Spectral Signatures The diagnostic response patterns of that make it possible to discriminate objects (spectral signatures) often lie beyond the narrow confines of the visible spectrum where no detectable differences occur.

Spectral Signatures Detectors translate the sensed radiation into electrical energy which is used to drive invisible-to-visible translation devices. In the field, quantitative measurements of reflected and/or emitted radiation emanating from surface features are often made with portable radiometers. The principle types of radiometers are; - single band radiometers which measure radiation intensity integrated through one broad wavelength band. - multiband/multispectral radiometers which measure radiation intensity in more than one wavelength band. Radiometer measurements provide useful reference data for identifying spectral regions in which various features can be best differentiated for a particular application. This, in turn, allows for the improved interpretation of remote sensing images and for the selection of spectral bands for future remote sensing imaging devices.

Spectral Signatures Radiometer measurements are used to prepare spectral signature curves which are line plots showing the radiation intensity for various features as a function of wavelength. Here are typical spectral signature curves for three common materials; vegetation, soil and water.

Spectral Signatures Note how distinctive the curves are for each material. Most importantly, a wide separation between the spectral signature curves relates to large intensity differences. These differences are rendered as distinct tonal or color variations in the same wavelength range. For example, the largest difference in radiation intensity between vegetation, soil and water occurs near a wavelength of 1 micrometer. Therefore, we could best discriminate the three material with a remote sensing system that detects reflected IR radiation.

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Introduction to Remote Sensing

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