Remote sensing Flashcards
Discuss the technical features of satellites used for remote sensing of the earth’s surface.
Spatial:
orbit geometry
revisit time
spatial resolution
coverage area
Spectral:
width of bands
Numbers of bands
positions of bands
Radiometric:
Radiometric resolution
Gain setting
Signal to noise ratio
Sketch and explain the spectral reflectance curves for vegetation, soil, and water for wavelength ranging from 0.4 - 2.6 um (Remote Sensing Basics).
- The reflectance characteristics of the earth features may be quantified by measuring the fraction of incident energy reflected.
*This fraction is dependent on wavelength and is
called spectral reflectance:
𝜌𝐴(𝜆) =𝐸𝑅(𝜆)/ 𝐸𝐼(𝜆) - A graph of the spectral reflectance of an object as a function of wavelength is termed spectral reflectance curve. The spectral reflectance curve of a surface gives an insight into the properties and the state of it
*For example, chlorophyll absorbs energy in the wavelength band 0.45 µm – 0.67 µm (chlorophyll absorption bands) → red and blue bands are strongly absorbed, so leafs
appear green.
*In addition to being subject to temporal and spatial effects, spectral
response is affected by the state of the atmosphere
Definition of remote sensing
*Remote sensing is the collection of information about an object without making physical contact with it
* It can be thought as a reading process, using various sensors. We remotely collect data that may be analyzed to obtain information about the objects, areas or phenomena being investigated
- The remotely collection can be done in many forms, e.g.,
- Active systems: radiation is actively emitted.
- Passive systems: use a natural emitter (the sun)
- Radar: an emitter emits electromagnetic radiation that is absorbed and reflected by, e.g., raindrops (precipitation radar) and captured by a sensor.
- Lidar: technology that can be used, e.g., to measure distance by emitting and capturing laser radiation.
Airborne platforms such as unmanned air vehicles, airplanes and satellites carry sensors to measure electromagnetic energy. The sensors capture radiation reflected or emitted by the various earth surface features.
Electromagnetic remote sensing
- The main processes for remote sensing are data acquisition and data analysis.
- The elements of data acquisition are: - energy sources (electromagnetic wave),
- propagation of energy through the atmosphere,
- energy interactions with earth surface features,
- retransmission of energy through the atmosphere,
- airborne/space borne sensors.
*Sensors are used to record variations in the way earth features
reflect and emit electromagnetic energy.
*The data analysis process involves
- examining the data using viewing and interpretation devices to analyze pictorial data and/or computer to analyze digital sensor data.
- Reference data about the resources studied are used, when available, to
assist data analysis.
- The information is compiled in form of hard copy maps and tables or
computer files to be combined in a GIS.
- Finally, the information is presented to users who apply it in various ways, e.g., in a decision–making process
energy sources and radiation principles
*Visible light is a specific form of electromagnetic radiation. *Electromagnetic energy as travelling
in harmonic, sinusoidal fashion at the velocity of light c.
* The distance from one wave peak to the next is the wavelength 𝝀,
*the number of peaks passing
a fixed point in space per unit of time is the wave’s frequency v
*c= v x 𝝀 (speed of light: 3x10^8 m/s)
* in remote sensing –> wavelength µm
electromagnetic spectrum and energy content of electromagnetic waves
*The visible portion of the spectrum is small, only from 0.4 µm to 0.7 µm. *Colors:
- blue 0.4 µm to 0.5 µm,
- green 0.5 µm to 0.6 µm
- red 0.6 µm to 0.7 µm.
*Ultraviolet (UV) energy adjoins the blue end of the visible portion, and
adjoining the red end, there are 3 different categories of infrared (IR) waves:
- near IR (0.7 µm – 1.3 µm),
- mid IR (1.3 µm – 3.0 µm)
- thermal IR (3.0 µm – 14.0 µm; sensed by us as heat
*Waves with wavelengths between 1 mm to 1 m are called microwaves.
*Most common sensors operate in the visible, IR or microwave portions.
*Energy of Quantum𝑄 =(ℎ 𝑥 𝑐)/𝝀
*The energy of a quantum of electromagnetic radiation is inversely proportional to its wavelength. Therefore, the longer the wavelength, the lower the energy of the wave –> radiation with long wavelengths is more difficult to sense–> This means that systems operating at long wavelength must
view large areas of the earth at any given time in order to reduce noise and to obtain a detectable energy signal.
spectral distribution of energy radiated from blackbodies
Stefan–Boltzmann Law:
The sun is the common source of electromagnetic radiation in passive remote sensing. All matter at temperature higher than 0 K (0 Kelvin) emits electromagnetic radiation with a different magnitude and spectral composition than the
sun. How much energy any object radiates is a function of the surface temperature of the object:
𝑀 = 𝜎 𝑇4
*Here M denotes total radiation exitance from the surface of a black body [W m-2], σ is the Stefan Boltzmann constant (5.76 x 10-8 W m-2 K-4) and T stands for the absolute Temperature [K].
*Total energy varies as T4, and therefore increases very rapidly. This law is expressed for an
energy source that behaves as a perfect blackbody. A blackbody is an ideal radiator that totally absorbs and remits all energy incident upon it
* Wien’s displacement law states that the spectral distribution of energy radiated from blackbodies peaks at a wavelength inversely proportional to its surface temperature:
𝜆𝑚 = 𝐴/𝑇
*Here, λm is the wavelength of
maximum spectral radiant exitance
(µm) and A is Wien’s displacement
length equal to 2898 µm K.
The surface temperature of the sun
is about 6000 K, leading to
λmax=0.5 µm, meaning that the
maximum is in the visible range.
Earth’s surface temperature is about
300 K. Hence, λmax equals 9.7 µm,
that is, the earth emits radiation with
a maximum in the thermal infrared
range.
radiant energy interactions
scattering
*In the context of land surface exploration –> unwanted, but unavoidable interactions.
*The state of the atmosphere affects the intensity and spectral composition of radiation available to any system.
*caused through the mechanisms of atmospheric scattering and absorption
*Scattering is the diffusion of radiation by particles and molecules in the atmosphere.
Rayleigh scatter
- radiation interacts with atmospheric molecules and other tiny particles < wavelength of the interacting
radiation.
- This type of scattering is wavelength dependent.
- It is inversely proportional to the 4th power of wavelength.
- As the wavelength decreases, the amount of scattering increases. (A blue sky is a manifestation of it. Sunlight interacts with the atmosphere, and the latter scatters the shorter (blue) wavelengths around four times more than the red (longer) ones. Therefore, at daytime we see a blue sky. At sunrises, long distance travel leads to almost complete scatter and we see mainly the long wavelengths (orange and red).
*Mie scatter occurs when the particles (pollen, dust, smoke) > wavelengths of radiation in contact with them. (white appearance of clouds)
- Non-selective scatter occurs when the diameter of the particles causing scatter (e.g., water
droplets, 5–100 µm) are much larger than the wavelength of the incident radiation. - not wavelength dependent
- primary cause of haze.
radiant energy interactions
absorption
Absorption:
- Results in an effective loss of energy to atmospheric constituents.
-The most efficient absorbers of solar radiation are H2O(g) (water vapor), CO2 and O3. They absorb electromagnetic energy at characteristic wavelengths.
= Remote sensing data acquisition is limited to spectral regions where radiation is (almost) not blocked, the so-called atmospheric windows
- One cannot select the sensor to be
used in any given remote sensing task arbitrarily, but one has to consider
1) spectral sensitivity of the sensor (relative efficiency of detection),
2) presence or absence of an
atmospheric window, and
3) source, magnitude and spectral composition of the energy available in this range.
radiant energy interactions
Interactions with surface features of the earth
*Reflectance
*Absorption
*Transmission
*There are three fundamental energy interactions: The incident energy can be
*reflected,
*absorbed
*and/or transmitted by the surface feature.
EI(λ)=ER(λ) + EA(λ) + ET(λ)
*proportions of the energy reflected, absorbed and transmitted vary for different surface features.
- They depend on material properties and conditions. This means that,
for a given feature, the proportion of reflected, absorbed and transmitted energy varies with wavelength.
–> Therefore, two features may be indistinguishable at one wavelength
but can be separated at another one.
- In the visible range, spectral variation in reflectance results in colors
(e.g., blue colors result from features reflecting more radiation in the blue part of the spectrum).
The geometry with which an object
reflects radiation
*Specular reflectors are flat surfaces that show mirror-like reflection. The angle of reflection = the angle of incidence.
*Diffuse (Lambertian) reflectors are
rough surfaces that reflect uniformly in all directions.
–>The reflection of earth features is usually between these extremes.
*Diffuse reflection contains
information on the “color” of the
reflecting surface, whereas specular
reflection does not.
*In remote sensing, we are usually interested in measuring the diffuse reflectance of features
Ground truth
- Remote sensing of the earth’s surface requires reference data (ground truth).
- collected in many forms, e.g., as aerial photographs when analyzing satellite images or field
campaigns. - Reference data might be used to:
- aid in the analysis and interpretation of remotely sensed data
- calibrate land cover or land use mapping procedures
- verify information extracted from remote sensing data.
- often expensive and time consuming to collect. They can be time-stable
(surface properties do not change or change only slowly in time, e.g., land cover) or time-critical
(dependent on the state of the surface, e.g., state of vegetation).
Components of an ideal RS system
1) Uniform energy source: irrespective of time and place.
2) Non-interfering atmosphere: an atmosphere that would not modify the energy from
the source in any manner.
3) Series of unique interactions at earth surface: unique to each and every earth surface
feature type and subtype of interest.
4) Super sensor: sensor highly sensitive to all wavelengths, yielding spatially detailed data
and brightness.
5) Real time processing and supply systems: processing can be performed
instantaneously.
6) Multiple data users: people with thorough system knowledge.
Explain the interaction of wavelength with atmosphere:
*The state of atmosphere affects the intensity and spectral compositon of radiation available to any system. the effects are caused through:
1) Scattering: is the diffusion of radiation by particles and molecules in the atmosphere.
- Rayleigh :particle < Wavelength e.g. blue sky
- Mie: particle > Wavelength e.g. cloud
- Non-selective particle»_space; Wavelength e.g. haze
2) Absorption: happens when radiation is absorbed by atmosphere constituents and results in loss of energy.
- The most efficient absorbers are H2O, Co2, O3.
-Fundamental interactions of incident energy: reflection, absorption, transmittance
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