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 1
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.
Remote Sensing – Space Science Teachers Summit 2008
Richard 2
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
Remote Sensing – Space Science Teachers Summit 2008
Richard 3
Everyday example: Digital camera
Remote Sensing – Space Science Teachers Summit 2008
Richard 4
Functional Classes of Sensors
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Richard 5
Element of optical sensors characteristics
Sensor
Spectral Characteristics
Spectral bandwidth (λ )
Resolution (∆ λ )
Out of band rejection
Polarization sensitivity
Scattered light
Remote Sensing – Space Science Teachers Summit 2008
Radiometric Characteristics
Detection accuracy
Signal to noise
Dynamic range
Quantization level
Flat fielding
Linearity of sensitivity
Noise equivalent power
Geometric Characteristics
Field of view
Instan. Field of view
Spectral band registration
Alignments
MTF’s
Optical distortion
Richard 6
Resolving Power
Na spectral lines
Na D-lines
Instrument & Detector
Remote Sensing – Space Science Teachers Summit 2008
D1=589.6 nm
D2=589.0 nm
Richard 7
Schematic Wave of Radiation
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 ?
E = η? φ ?
Ε=
λ=
ηχ
λ
χ
φ
c = 299, 792, 458 µ / σεχ
η = 6.626069 ? 10 −34 ϑ ?σεχ
Visible Spectrum
0.4
0.5
0.6
0.7
Wavelength (µm)
Remote Sensing – Space Science Teachers Summit 2008
Richard 8
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?
Remote Sensing – Space Science Teachers Summit 2008
Richard 9
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.
Remote Sensing – Space Science Teachers Summit 2008
Richard 10
Operational Classes of Sensors
Remote Sensing – Space Science Teachers Summit 2008
Richard 11
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)
Remote Sensing – Space Science Teachers Summit 2008
Richard 12
Scanning Types
Remote Sensing – Space Science Teachers Summit 2008
Richard 13
Multi or Hyper-spectral?
• Multidimensional data “cube”
– Spatial information
– Spectral information
• Full spectrum
– Hyperspectral
• Partial spectrum
– Multispectral
Remote Sensing – Space Science Teachers Summit 2008
Richard 14
EM derived information
Remote Sensing – Space Science Teachers Summit 2008
Richard 15
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.
Remote Sensing – Space Science Teachers Summit 2008
Richard 16
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.
Remote Sensing – Space Science Teachers Summit 2008
Richard 17
Earth’s Albedo
•Albedo is defined as the reflectance using the incident light source from the Sun
Remote Sensing – Space Science Teachers Summit 2008
Richard 18
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
Remote Sensing – Space Science Teachers Summit 2008
Richard 19
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
Remote Sensing – Space Science Teachers Summit 2008
Richard 20
Blackbody Radiation Curves
Remote Sensing – Space Science Teachers Summit 2008
Richard 21
The Sun’s spectrum
UV
Vis
IR
Radiometric definitions
Irradiance : Radiant power incident per unit area upon a surface (W/m
Spectral Irradiance : Irradiance per unit wavelength interval (W/m2/nm)
Remote Sensing – Space Science Teachers Summit 2008
Richard 22
The Sun’s spectrum
with Planck distributions at different temperatures
UV
Vis
Remote Sensing – Space Science Teachers Summit 2008
IR
M. Planck
Richard 23
Black body radiation
• Planck distributions
Hot objects emit A LOT more
radiation than cool objects
QuickTimeﾪ and a
YUV420 codec decompressor
are needed to see this picture.
I (W/m2) = σ
x T4
The hotter the object, the
shorter the peak wavelength
T x λ max =
constant
Remote Sensing – Space Science Teachers Summit 2008
Richard 24
Spectral Characteristics of Energy Sources
and Sensing Systems
Remote Sensing – Space Science Teachers Summit 2008
Richard 25
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 formulaΡαδιαντ?ενεργψￊοφￊαν ￊοβϕεχτ
Emissivity =
Ραδιαντￊενεργψￊοφￊαￊβλαχκￊβοδψ
ωιτηￊτηε ￊσαµ ε ￊτεµ περατυρε ￊασￊτηε ￊοβϕεχτ
Remote Sensing – Space Science Teachers Summit 2008
Richard 26
Atmospheric Absorption in the Wavelength
Range from 1 to 15 µm
Remote Sensing – Space Science Teachers Summit 2008
Richard 27
Atmospheric Observation Modes
Remote Sensing – Space Science Teachers Summit
Richard ￊￊ
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)
Remote Sensing – Space Science Teachers Summit 2008
Richard 29
Optical thickness
• The optical thickness of the atmosphere (τ t)
is the integrated value σext with altitude
τ t (l ) = ?s
ext
dz
0
Total attenuation in a vertical path from the top of the
atmosphere down to the surface
Ι
−τ τ ( λ )
T = =ε
Ιο
Remote Sensing – Space Science Teachers Summit 2008
Richard 30
< 2% RE
Altitude (km)
Atmospheric absorption
of solar radiation
~99% penetrates
to the troposphere
stratosphere
troposphere
Altitude “contour” for attenuation by
a factor of 1/e
I(km) = 37% x Io
Remote Sensing – Space Science Teachers Summit 2008
Richard 31
Global Ozone Monitoring
•
The Total Ozone Mapping Spectrometer (TOMS) samples backscatter
UV at six wavelengths and provides a contiguous mapping of total
column ozone.
Remote Sensing – Space Science Teachers Summit 2008
Richard 32
Composition of atmospheric
transmission
Remote Sensing – Space Science Teachers Summit 2008
Richard 33
Atmospheric Scattering
• Factors influencing atmospheric
transmittance
– Atmospheric molecules (size << λ)
• CO2, O3, N2, etc.
– Aerosols (size >λ)
• Water drops (fog & haze), smog, dust, etc.
Remote Sensing – Space Science Teachers Summit 2008
Richard 34
Scattering
• Rayleigh scattering
– Scattering by atmospheric molecules with
size << λ
– Scattering coefficient σs
1
σs ? 4
l
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 wavelength
is called Mie scattering (think of a movie projector with dust)
Remote Sensing – Space Science Teachers Summit 2008
Richard 35
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)
Remote Sensing – Space Science Teachers Summit 2008
Richard 36
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)
Remote Sensing – Space Science Teachers Summit 2008
Richard 37
Understanding the Earth’s Energy Budget
Solar radiation is the Earth’s only incoming energy source. The balance between the Earth’s incoming
and outgoing energy controls daily weather as well as longterm weather patterns (i.e. climate). Since we
are 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 2008
Richard 38
Radiation Balance
Remote Sensing – Space Science Teachers Summit 2008
Richard 39
Radiation Balance
Remote Sensing – Space Science Teachers Summit 2008
Richard 40
Radiation Balance
Remote Sensing – Space Science Teachers Summit 2008
Richard 41
Earth’s Energy Balance
Remote Sensing – Space Science Teachers Summit 2008
Richard 42
So, just how “bright” is the Sun?
If T = 5780 K @ Sun’s
surface
Then the Sun’s
emission from the photosphere is
I Sun = σ ?ξￊΤ
4
ISun ~ 63,000,000 W/m2
(6.3 kW / cm2)
What does this mean for Earth?
Remote Sensing – Space Science Teachers Summit
Richard
Surface areaￊ=?4π Ρ12ΑΥ
2
Surface areaￊ=?4πρΣυν
63 MW/m2
here
rSun = 696, 000?κµ
How much
here?
R1AU = 149, 600, 000?κµ
I@ Earth ? 1360?Ω / µ
2
Historically know as “Earth’s Solar
Constant”
Remote Sensing – Space Science Teachers Summit 2008
Richard 44
“It is ridiculous to try to measure variations in a constant”
- Dove & Maury (ca. 1890)
famous oceanographers
Remote Sensing – Space Science Teachers Summit 2008
Richard 45
SORCE
Solar Radiation and Climate Experiment
http://lasp.colorado.edu/sorce/
A Mission of Solar Irradiance
for Climate Research
Launched January 25, 2003
Daily measurements of
• Total Solar Irradiance (TSI)
• Solar Spectral Irradiance (SSI)
0.1 nm-27nm & 115 - 2400 nm
Remote Sensing – Space Science Teachers Summit
Richard
Total Irradiance Monitor (TIM)
Four
Radiometers
TIM Instrument
Detector
Head
Board
Heat Sink
Vacuum Door
Shutter
Precision Aperture
Remote Sensing – Space Science Teachers Summit 2008
Light Baffles
Radiometer
(Cone)
Vacuum Shell
Richard 47
1360 W/m2
Remote Sensing – Space Science Teachers Summit 2008
QuickTimeﾪ and a
YUV420 codec decompressor
are needed to see this picture.
Richard 48
30 year TSI record from space
The “constant” variable
Remote Sensing – Space Science Teachers Summit 2008
Richard 49
Solar Cycle
0.1% = 1.4 W/m2
∆ T of ~1.5 °C on Sun
Remote Sensing – Space Science Teachers Summit 2008
Richard 50
Clouds and the Earth’s Radiant
Energy System (CERES)
• 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
Remote Sensing – Space Science Teachers Summit 2008
CERES Swath Movie
Richard 51
CERES Results
• Longwave (thermal) radiation
• Longwave
(thermal) & simultaneous Shortwave (reflecte
Remote Sensing – Space Science Teachers Summit 2008
Richard 52
“If the Sun had no magnetic field…
it would be as boring as most astronomers
seem to believe it is”
- R. Leighton
Astrophysicist, CalTech
Remote Sensing – Space Science Teachers Summit 2008
Richard 53
The Sun’s magnetism is ultimately
responsible
for all manifestations of solar activity
Sunspots
CME’s
Flares
Remote Sensing – Space Science Teachers Summit 2008
Erupting prominences
Coronal loops
Richard 54
The Sun’s spectrum
UV
Vis
Remote Sensing – Space Science Teachers Summit 2008
IR
Richard 55
Magnetic Fields and Sunspots
P. Zeeman
G. E. Hale
λ
G.E. Hale, June 1908
Remote Sensing – Space Science Teachers Summit 2008
Richard 56
The formation of sunspots
Animation
Hale provided the first proof that sunspots
are the seats of strong magnetic fields
QuickTimeﾪ and a
YUV420 codec decompressor
are needed to see this picture.
TRACE image
Remote Sensing – Space Science Teachers Summit 2008
Richard 57
The Sun’s Magnetic Cycle
Hale’s polarity Law (1919)
Well-organized large scale magnetic field
Changes polarity approximately every 11 years
(22 year magnetic cycle)
N
S
S
N
t=0
Remote Sensing – Space Science Teachers Summit 2008
t = 3 yrs
t = 9 yrs
t = 11 yrs
Richard 58
“Seeing” the Sun’s magnetic fields
QuickTimeﾪ and a
YUV420 codec decompressor
are needed to see this picture.
SOHO MDI Magnetograms
Remote Sensing – Space Science Teachers Summit 2008
Richard 59