REMOTE SENSING Fundamentals of Remote Sensing

REMOTE SENSING Fundamentals of Remote Sensing
Professor Ke-Sheng Cheng Department of Bioenvironmental Systems Engineering National Taiwan University

Outline Definition of remote sensing Principles of EM radiation
Definition of radiometric terms The concept of point source radiation An overall account of EM radiation reaching the sensor 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Definition of remote sensing
Remote sensing is generally known as the technology of deriving information about the target object without making physical contact with the object. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Although these definitions differ in specifying the kinds of energy that are recorded by remote sensing instruments and the formats of the acquired data or information, they all share a common concept of recording (or measuring) energy representative of the target object without making physical contact. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

In addition to the data acquisition aspect, some definitions also emphasize the aspect of data analysis and information extraction and interpretation, and the term of remote sensing system is more preferable. For the purpose of earth monitoring, most applications involve analyzing electromagnetic (EM) energy measured and presented in image format. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Basic Processes of Remote Sensing
Data acquisition Energy source Sensor and its platform Data products (digital images, photos) Data analysis Interpreter and analysis A signal acquisition/processing system 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Elements of a remote sensing system
(Canada Center for Remote Sensing, CCRS) 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Principles of EM radiation
The major energy source for earth remote sensing is the electromagnetic energy radiated from the Sun’s surface. The Sun produces a wide and continuous spectrum of electromagnetic energy. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

in which c is the speed of light ( m/sec) and f is the frequency.
The propagation property of electromagnetic energy can be described by the general equation of wave propagation in which c is the speed of light ( m/sec) and f is the frequency. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Stefan-Boltzmann law The total energy M (W/m2) that can be emitted by a blackbody of absolute temperature T (K) per unit surface area and per unit time is described by where W/m2/K4 is the Stefan-Boltzmann constant. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The Stefan-Boltzmann law states that the magnitude of the total energy a blackbody can emit is proportional to the fourth power of its absolute temperature. Thus, any object with absolute temperature above zero is a source of electromagnetic radiation. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

However, the spectral composition and the magnitude of spectral radiant emittance vary with the absolute temperature of the object. Such phenomenon can be explained by the Planck’s law. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Planck’s law 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Figure 1.1 demonstrates the dependence of on the temperature T and wavelength .
The Sun emits radiation energy in a manner similar to a blackbody with 6000K surface temperature, whereas the earth behaves like a blackbody of 300K surface temperature. The peak spectral radiant emittance of the Sun occurs near the 0.4 – 0.7m wavelength region. At the surface temperature of 300K, the peak spectral radiant emittance of the earth occurs at a wavelength near 10m. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig Variation of the spectral composition and the magnitude of radiant energy emitted by blackbodies of various absolute temperatures. The wavelength of peak radiant emittance is inversely proportional to the blackbody temperature. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The total emitted energy M is obtained by integrating over the full range of spectral wavelength.
Although the radiant behavior of a blackbody can be described by the Planck’s law, real materials do not behave as a blackbody. Instead, all real materials emit only a fraction of the energy emitted from a blackbody at the equivalent temperature. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Blackbody A blackbody is an idealized surface that has the property that all incident electromagnetic flux is perfectly absorbed and then re-radiated. A blackbody at a given temperature provides the maximum radiant exitance into a hemisphere at any wavelength that any body in thermodynamics equilibrium at that temperature can provide. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Emissivity 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Wien’s displacement law

It can be observed that the spectral radiant emittance curves in Figure 1.1 tend to become parallel lines in longer wavelength regions. Such phenomenon is known as the Rayleigh-Jean’s approximation and can be explained as follows. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Rayleigh-Jean’s approximation

Planck’s law 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Spectral band emittance

Values of f(xi) for xi varies from 0 to 20 are tabulated in Table 1.2.
Using the data in Table 1.2, the energy emitted by a blackbody of absolute temperature 300K over the 6m ~ 30m wavelength range is approximately 386 Joules/sec/m2. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Definition of radiometric terms
The behavior of EM radiation, including its emission by a blackbody, propagation through space, and interaction with matters, can be described by two different models: the wave model and the particle model. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The wave model is useful for characterizing the behavior of generation and propagation of EM radiation. However, when EM radiation interacts with matters, the particle model is more useful in describing the behavior of EM radiation. Electromagnetic radiation may come in contact with or be emitted from an object. Properties of the radiation onto or away from the surface of an object are of interest in remote sensing applications and technical terminologies are used to describe these properties. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The particle model considers EM radiation as energy carried by a stream of discrete particles called photons or quanta. The energy carried by a photon or quantum is inversely proportional to the wavelength associated with that particle, i.e. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiant energy Consider a beam of photons traveling through space. The beam is composed of photons of different wavelengths or frequencies. The total quantity of radiant energy carried in a beam can thus be calculated as where nj represents the number of photons of wavelength j. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiant flux The quantity of radiant energy arriving at or leaving from a surface per unit time is defined as radiant flux, measured in Joules/second or Watts, and can be expressed as 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiant flux density - irradiance, exitance, emittance
Radiant flux received by or exited from a unit area of plane surface is defined as the radiant flux density, measured in Watts/m2. Specially, the radiant flux density incident upon a surface is termed the irradiance E and the radiant flux density leaving a surface is called radiant exitance (or radiant emittance, if the radiant flux is emitted by the surface) M. Thus 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

As shown in Figure 1.2, if the normal vector of a surface element (dA1) is not parallel to the incident radiant flux, the irradiance onto the surface is calculated as 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig. 1. 2 Irradiance onto a surface element
Fig Irradiance onto a surface element. Surface element dA0 is perpendicular to the incident flux while surface element dA1 is not perpendicular to the incident flux. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiant intensity 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The unit of solid angle is steradian (sr)
The unit of solid angle is steradian (sr). If the surface area equals the square of the radius, the solid angle is one steradian. Thus, a sphere corresponds to 4 steradians. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig. 1.3 Radiant intensity as a function of the zenith and azimuth angles.

Radiance The radiance is defined as the radiant flux per unit projected area (at the specified location (x,y) on the plane of interest) per unit solid angle (in the direction specified relative to the reference plane), as illustrated in Figure 1.4. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig. 1.4 Illustration of the at-sensor radiance from a source surface.

It is measured in Watts/m2/sr and can be expresses as
where  represents the angle between the normal of the surface and the ray direction. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiance from the source surface may vary with the specified direction characterized by the azimuth and zenith angles. In addition, viewing from an observer or a sensor, the radiance may vary with location of the source object. Thus, the radiance is the most precise term for radiometric measurement and is a function of both the object-to-sensor angular direction and the spatial location of the object. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiant flux, irradiance, and radiance are different measures of radiant energy.
In remote sensing, radiant flux is the sensible item and we measure the spectral radiant flux coming from the target. Radiance is the concentration of radiant flux with respect to position and direction as a function of position and direction. Our visual perception of brightness is directly proportional to radiance. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Exoatmospheric solar irradiance
The solar irradiance at the top of the Earth’s atmosphere is called the exoatmospheric solar irradiance or the solar constant. Based on the results of intensive investigations during the period , Forgan (1977) suggests the value 137521 W/m2 and Frohlich (1977) suggests the value 137320 W/m2 for the solar constant. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Slater (1980) shows that for many engineering calculations the value of 6000 K is a suitable general-purpose blackbody temperature of the Sun. We can also calculate the wavelength-dependent solar irradiance and radiance at the top of the earth atmosphere. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Calculation of wavelength-specific exoatmospheric irradiance
At the absolute temperature of 5800 K, the maximum radiant emittance of the Sun occurs at wavelength Solar irradiance of wavelength max=0.5 m at the top of the Earth’s atmosphere is Exoatmospheric irradiance 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig Comparison of the exoatmospheric solar irradiance and the radiant exitance of the Earth at 300K. The Earth emittance is far greater than the solar irradiance at the thermal infrared range (8－14 m). 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig Variation of the spectral composition and the magnitude of radiant energy emitted by blackbodies of various absolute temperatures. The wavelength of peak radiant emittance is inversely proportional to the blackbody temperature. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig Schematic illustration of the Earth-Sun distance and a solid angle subtended by a unit area at the top of the Earth’s atmosphere. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

When electromagnetic radiation is incident on the surface of a material body, absorption, transmission and reflection by the material may occur. Thus, three fundamental properties – transmittance (or transmission), absorptance, and reflectance, of the material are respectively defined as 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The concept of point source radiation
A point source describes a source of radiation which can physically be represented by a point in space. Whereas in reality this is not the case, it provides a convenient and mathematically workable way for calculation of EM radiation from extended target objects. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

As long as the distance between an extended source (which is a plane of fixed size) and the plane of interest is at least 10 times the largest dimension of the extended source, the irradiance received at the plane of interest can be well approximated by assuming a point source of radiation (Grum and Becherer, 1979; Slater, 1980). 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

As illustrated in Figure 1
As illustrated in Figure 1.7, the irradiance from a point source upon a spherical surface which is at a distance r from the point source and in a given direction can be expressed as 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig. 1.7 Illustration of the irradiance onto a surface element from a point source.

Inverse law of irradiance
The above equation states that the irradiance arriving at a plane of interest from a point source is inversely proportional to the square of the distance between the point source and the plane of interest. It is known as the inverse square law of irradiance from a point source. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

An overall account of EM radiation reaching the sensor
The EM radiations received and detected by satellite or airborne sensors may come from different propagation paths. Each path is characterized by its distinct interaction between the radiation and the earth environment (surface features and the atmosphere) and the origin of EM radiation. An overall account of EM radiation reaching the sensor is illustrated in Figure 1.8. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Fig Schematic illustration of energy paths of radiances reaching the satellite or airborne sensor. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Solar energy paths (reflected energy paths)
Path A-A is the propagation of solar irradiance arriving at and reflected by the target on Earth surface before reaching the sensor (primary solar radiance). Path B-B represents the solar irradiance scattered by the atmosphere toward the target (downwelled solar radiance) and then reflected back to the atmosphere. Path C-C is the solar irradiance scattered upward to the sensor by the atmosphere (upwelled solar radiance). Path D-D represents the solar irradiance arriving at and reflected by background objects toward the target, and then reflected from the target back through the atmosphere to the sensor (multiple bounced or background solar radiance). 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Thermal energy paths (emitted energy paths)
The atmosphere and earth surface themselves also emit energy which may travel through different paths and finally reach the sensor. Path E represents propagation of thermal energy emitted by the target on Earth surface in the target-sensor direction (primary thermal radiance). Radiation of path F is the thermal radiation emitted by the atmosphere toward the sensor (thermal upwelled radiance). Path G-G is the atmospheric thermal radiation onto (downwelled thermal radiance) and reflected by the target toward the sensor. Thermal radiation of neighboring objects onto and reflected by the target before reaching the sensor is characterized by path H-H (multiple bounced thermal radiance). 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

For most remote sensing applications, the background solar and thermal radiances are negligible.
In addition, the upwelled radiances (also known as the path radiances) do not reach the Earth surface and are not dependent on physical properties (e.g. surface roughness and temperature) of the target on Earth surface. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

For applications aiming at earth surface monitoring, path radiances are noises and their removal is desired. Whereas path radiances are noises for earth surface monitoring, they can provide useful information for applications in atmospheric studies such as dust storm monitoring. In practical remote sensing applications, different sensors are designed and utilized to detect radiant energy of solar energy paths and thermal energy paths separately. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Visible Bands Violet: 0.4 - 0.446 m Blue: 0.446 - 0.500 m
Green: m Yellow: m Orange: m Red: m 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The light which our eyes can detect is part of the visible spectrum
The light which our eyes can detect is part of the visible spectrum. It is important to recognize how small the visible portion is relative to the rest of the spectrum. There is a lot of radiation around us which is "invisible" to our eyes, but can be detected by other remote sensing instruments and used to our advantage. The visible wavelengths cover a range from approximately 0.4 to 0.7 m. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The Infrared Spectral Bands
The next portion of the spectrum of interest is the infrared (IR) region which covers the wavelength range from approximately 0.7 m to 100 m. The infrared region can be divided into two categories based on their radiation properties - the reflected IR, and the emitted or thermal IR. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Radiation in the reflected IR region is used for remote sensing purposes in ways very similar to radiation in the visible portion. The reflected IR covers wavelengths from approximately 0.7 m to 3.0 m. The thermal IR region is quite different than the visible and reflected IR portions, as this energy is essentially the radiation that is emitted from the Earth's surface in the form of heat. The thermal IR covers wavelengths from approximately 3.0 m to 100 m. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

Thermal infrared energy refers to radiation emitted by earth surface features (active); reflected infrared energy refers to energy reflected by earth surface features (passive). The general dividing line between reflected and emitted IR wavelength is approximately 3m. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The wavelength region between 2.5m and 6m contains some of each.
In remote sensing, the wavelength of 2.5 m is used as the upper limit for reflected solar energy while the wavelength of 6m is used as the lower limit for self-emitted thermal energy. The wavelength region between 2.5m and 6m contains some of each. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The term “reflected IR” does not mean infrared energy of 0. 7 – 3
The term “reflected IR” does not mean infrared energy of 0.7 – 3.0m can not be emitted by an object. Spectral wavelength definitions of thermal IR and reflected IR are based on energy observed on Earth. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

The Microwave Spectral Band
The portion of the spectrum of more recent interest to remote sensing is the microwave region from about 1 mm to 1 m. This covers the longest wavelengths used for remote sensing. The shorter wavelengths have properties similar to the thermal infrared region while the longer wavelengths approach the wavelengths used for radio broadcasts. 7/18/2018 Lab for Remote Sensing Hydrology and Spatial Modeling, Department of Bioenvironmental Systems Engineering, NTU

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REMOTE SENSING Fundamentals of Remote Sensing

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