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Remote Sensing – Space Science Teachers Summit 2008 Richard 1
Remote Sensing“How we know what we know”
A Brief Tour
Dr. Erik Richard
Dr. Jerald Harder
LASP
Remote Sensing – Space Science Teachers Summit 2008 Richard 2
Remote Sensing
• The measurement of physical variables (usually light or sound) from outside of a medium to infer properties (other physical variables) of the medium.
• Electro-magnetic radiation which is reflected or emitted from (or absorbed by) an object is the usual source of remote sensing data. However any media such as gravity or magnetic fields can be utilized in remote sensing.
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Measurement Fundamentals
• Key Instrument Components
– Sensing device, or sensor– Transducer
• Translates a sensed quantity (i.e. photons, acoustic waves, etc.) into a measurable quantity (e.g. voltage, current, displacement etc.)
– Readout device
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Element of optical sensors characteristics
Sensor
Spectral bandwidth (λ ) Resolution (∆ λ )
Out of band rejectionPolarization sensitivity
Scattered light
Detection accuracySignal to noiseDynamic range
Quantization levelFlat fielding
Linearity of sensitivityNoise equivalent power
Field of viewInstan. Field of view
Spectral band registrationAlignments
MTF’sOptical distortion
Spectral Characteristics Radiometric Characteristics Geometric Characteristics
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Resolving Power
Na spectral lines
Na D-linesD1=589.6 nmD2=589.0 nmInstrument & Detector
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Schematic Wave of Radiation
Visible Spectrum
Wavelength (µm)0.4 0.5 0.6 0.7
Electromagnetic (EM) energy at a particular wavelength l (in vacuum) has an associated frequency f and photon energy E.Thus, the EM spectrum may be expressed equally well in terms of any of these three quantities:
c = φρεθυενχψ? ωαϖελενγτη ? λ =χφ
E = η ? φ ? Ε =ηχλ
c = 299,792,458 µ / σεχ
η = 6.626069 ? 10−34 ϑ?σεχ
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The electromagnetic spectrum
• Remote sensing uses the radiant energy that is reflected and emitted from Earth at various “wavelengths” of the electromagnetic spectrum
• Our eyes are only sensitive to the “visible light” portion of the EM spectrum
• Why do we use nonvisible wavelengths?
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Passive or Active?
• Passive sensor– energy leading to radiation received comes from
an external source • e.g., direct Sun, reflected Sun, thermal emission etc.
• Active sensor– Energy generated from within the sensor system,
beamed outward, and the fraction returned is measured.
• e.g. laser LIDAR, microwaves, RADAR, SONAR, etc.
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Scanning or Non-scanning?
• Scanning mode– Motion across the scene over a time interval
(think of your video recorder)
• Non-scanning– Holding the sensor fixed on the scene or
target of interest as it is sensed in a brief moment (think of your digital camera)
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Multi or Hyper-spectral?
• Multidimensional data “cube”– Spatial information– Spectral information
• Full spectrum– Hyperspectral
• Partial spectrum– Multispectral
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Spectral Reflectance
• Spectral reflectance is assumed to be different with respect to the type of land cover. This is the principle that in many cases allows the identification of land covers with remote sensing by observing the spectral reflectance (or spectral radiance) from a distance far removed from the surface.
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Spectral Reflectance
• Shown below are three curves of spectral reflectance for typical land covers; vegetation, soil and water. As seen in the figure, vegetation has a very high reflectance in the near infrared region, though there are three low minima due to absorption. Soil has rather higher values for almost all spectral regions. Water has almost no reflectance in the infrared region.
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Earth’s Albedo
•Albedo is defined as the reflectance using the incident light source from the Sun
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MODIS
• MODIS: MODerate-resolution Imaging Spectroradiometer
• NASA Terra & Aqua satellites– Launched 1999, 2002– 705 km polar orbits, descending (10:30 am) & ascending (1:30 pm)
• Sensor Characteristics– 36 spectral bands ranging from 0.41 to 14.385 µm– Cross-track scan mirror with 2330 km swath width– Spatial resolutions
• 250 m (bands 1-2)• 500 m (bands 3-7)• 1000 m (bands 8-36)
– 2% reflectance calibration accuracy
movie
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Black Body Radiation
• An object radiates unique spectral radiant flux depending on the temperature and emissivity of the object. This radiation is called thermal radiation because it mainly depends on temperature. Thermal radiation can be expressed in terms of black body theory.
• Black body radiation is defined as thermal radiation of a black body, and can be given by Planck's law as a function of temperature T and wavelength
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The Sun’s spectrum
UV Vis IR
Radiometric definitions Irradiance : Radiant power incident per unit area upon a surface (W/m2)Spectral Irradiance : Irradiance per unit wavelength interval (W/m2/nm)
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M. Planck
The Sun’s spectrum with Planck distributions at different temperatures
UV Vis IR
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Black body radiation
• Planck distributions
QuickTimeᆰ and aYUV420 codec decompressor
are needed to see this picture.
Hot objects emit A LOT moreradiation than cool objects
The hotter the object, theshorter the peak wavelength
I (W/m2) = σ x T4
T x λ max = constant
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Spectral Characteristics of Energy Sourcesand Sensing Systems
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Emissivity
• In remote sensing, a correction for emissivity should be made because normal observed objects are not black bodies. Emissivity can be defined by the following formula-
Emissivity=Ραδιαντ?ενεργψᅧοφᅧανᅧοβϕεχτ
Ραδιαντᅧενεργψᅧοφᅧαᅧβλαχκᅧβοδψωιτηᅧτηεᅧσαµ εᅧτεµ περατυρεᅧασᅧτηεᅧοβϕεχτ
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Atmospheric Absorption in the WavelengthRange from 1 to 15 µm
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Transmittance of the Atmosphere
• Transmission of solar radiation through the atmosphere is affected by– Absorption– Scattering
• The reduction of radiation intensity is called extinction (expressed as extinction coefficient, σext)
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Optical thickness
• The optical thickness of the atmosphere (τ t) is the integrated value σext with altitude
τ t (l ) = s extdz0
?Total attenuation in a vertical path from the top of the atmosphere down to the surface
T =ΙΙο
= ε−ττ (λ )
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Alt
itude (
km)
Altitude “contour” for attenuation bya factor of 1/e
~99% penetrates to the troposphere
I(km) = 37% x Io
troposphere
stratosphere
Atmospheric absorption of solar radiation
< 2% RE
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Global Ozone Monitoring
• The Total Ozone Mapping Spectrometer (TOMS) samples backscatter UV at six wavelengths and provides a contiguous mapping of total column ozone.
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Composition of atmospheric transmission
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Atmospheric Scattering
• Factors influencing atmospheric transmittance
– Atmospheric molecules (size << λ)• CO2, O3, N2, etc.
– Aerosols (size >λ)• Water drops (fog & haze), smog, dust, etc.
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Scattering
• Rayleigh scattering– Scattering by atmospheric molecules with
size << λ– Scattering coefficient σs
σ s ? 1
l 4
The strong wavelength dependence of the scattering (~λ-4) means that blue light is scattered much more than red light.
Scattering by aerosols with larger size than the wavelengthis called Mie scattering (think of a movie projector with dust)
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Radiometry
• Radiant energy– Energy carried by EM radiation (J)
• Radiant flux– Radiant energy transmitted per unit time (W)
• Radiant intensity– Radiant flux from a point source per unit solid
angle in a radial direction (W sr-1)
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Radiometry con’t
• Irradiance– Radiant flux incident upon a surface per unit area
(Wm-2)
• Radiant emittance– Radiant flux radiated from a surface per unit area
(Wm-2)
• Radiance– Radiant intensity per unit projected area in a
radial direction (Wm-2sr-1)
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Understanding the Earth’s Energy Budget
Solar radiation is the Earth’s only incoming energy source. The balance between the Earth’s incomingand outgoing energy controls daily weather as well as longterm weather patterns (i.e. climate). Since weare dealing only with electromagnetic radiation as a heat transfer mechanism, we can start by applying the basic laws of radiation physics to begin to understand the Earth-Sun system and the Earth’s energy budget
Remote Sensing – Space Science Teachers Summit Richard
If T = 5780 K @ Sun’s surface Then the Sun’s emission from the photosphere is
ISun = σ ?ξᅧΤ 4
So, just how “bright” is the Sun?
ISun ~ 63,000,000 W/m2
What does this mean for Earth?
(6.3 kW / cm2)
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63 MW/m2 here
How much here?
I@Earth ? 1360?Ω / µ 2
Historically know as “Earth’s Solar Constant”
Surfaceareaᅧ=?4π Ρ1ΑΥ2
Surfaceareaᅧ=?4πρΣυν2
R1AU = 149,600,000?κµ
rSun = 696,000?κµ
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“It is ridiculous to try to measure variations in a constant”
- Dove & Maury (ca. 1890)
famous oceanographers
Remote Sensing – Space Science Teachers Summit Richard
SORCESolar Radiation and Climate Experiment
A Mission of Solar Irradiancefor Climate Research
Daily measurements of• Total Solar Irradiance (TSI)• Solar Spectral Irradiance (SSI) 0.1 nm-27nm & 115 - 2400 nm
Launched January 25, 2003
http://lasp.colorado.edu/sorce/
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Radiometer (Cone)Precision Aperture
Total Irradiance Monitor (TIM)
TIM InstrumentFour Radiometers
Vacuum Shell
Detector Head Board
Light Baffles
Shutter
Vacuum Door Heat Sink
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QuickTimeᆰ and aYUV420 codec decompressor
are needed to see this picture.
1360 W/m2
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30 year TSI record from space
The “constant” variable
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Solar Cycle0.1% = 1.4 W/m2
∆ T of ~1.5 °C on Sun
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• NASA, TRMM, Terra & Aqua– launches 1997, 1999, 2002– 350 km orbit (35° inclination), 705
km polar orbits, descending (10:30 a.m.) & ascending (1:30 p.m.)
• Sensor Characteristics– 3 spectral bands
» Shortwave (0.3-5.0 µm)» Window (8-12 µm)» Total (0.3->200 µm)
– Spatial resolution:» 20 km
– ±78° cross-track scan and 360° azimuth biaxial scan
– 0.5% calibration accuracy– onboard blackbodies & solar
diffuser
Clouds and the Earth’s RadiantEnergy System (CERES)
CERES Swath Movie
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CERES Results
• Longwave (thermal) radiation
• Longwave (thermal) & simultaneous Shortwave (reflected) radiation
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“If the Sun had no magnetic field…it would be as boring as most astronomersseem to believe it is”
- R. Leighton Astrophysicist, CalTech
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CME’s
Coronal loops
Erupting prominences
Sunspots
The Sun’s magnetism is ultimately responsible
for all manifestations of solar activity
Flares
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Magnetic Fields and Sunspots
P. Zeeman
G.E. Hale, June 1908
G. E. Hale
λ
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The formation of sunspots
TRACE image
Animation
QuickTimeᆰ and aYUV420 codec decompressor
are needed to see this picture.
Hale provided the first proof that sunspotsare the seats of strong magnetic fields
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t = 0 t = 3 yrs t = 9 yrs t = 11 yrs
N
S
S
N
Hale’s polarity Law (1919)
Well-organized large scale magnetic field
Changes polarity approximately every 11 years(22 year magnetic cycle)
The Sun’s Magnetic Cycle
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