Transcript Slide 1

2009-10 CEGEG046 / GEOG3051
Principles & Practice of Remote Sensing (PPRS)
7: scanning redux, photography, lidar
Dr. Mathias (Mat) Disney
UCL Geography
Office: 113, Pearson Building
Tel: 7670 0592
Email: [email protected]
www.geog.ucl.ac.uk/~mdisney
Recap
• Last week
– storage/transmission
– pre-processing stages (raw data to products)
– sensor scanning mechanisms
• This week
– scanning mechanisms redux
– photography
– time-resolved signals (e.g. LiDAR)
2
Scanning mechanisms: examples
• Discrete detectors and scanning mirrors
– Landsat MSS, TM, ETM+, NOAA GOES, AVHRR, ATSR
• Multispectral linear arrays
– SPOT (1-3) HRV, HRVIR & SPOT-VGT, IKONOS, ASTER & MISR (both
on board NASA Terra)
• Imaging spectrometers using linear and area arrays
– AVIRIS, CASI, MODIS (on NASA Terra and Aqua)
From: http://ceos.cnes.fr:8100/cdrom/ceos1/irsd/pages/datacq4.htm & Jensen (2000)
3
Scanning mechanisms: examples
•MODIS scan mirror
http://modis.gsfc.nasa.gov/about/scanmirror.php
•Continuously rotating and double-sided
•SEVIRI (Spinning Enhanced Vis and IR Imager)
on board MSG
•Whole satellite rotates
•Vertical scan plus rotation = image
4
Scanning mechanisms: continued
• Image frame created by scanning detector footprint
•n pixels per line, pixel size r * r
nr
•Along track speed v ms-1 so footprint
travels distance r in r/v secs
line
r
pixel
• One line of data must be acquired in <=
r/v secs
•Typical v?
•Orbital period T ~ 100 mins, Earth radius
~ 6.4x103m
Frame
•v = 2*6.4x103 / 100*60 = 6.7x103ms-1
Across track
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Scanning mechanisms: single detector
• Even if we obtain 1 line in r/v secs say.....
• Significant along-track displacement from start to end of x-track scan
line
X-track scan
(whiskbroom)
Start
rv
Platform has moved r in
rv secs
6
Scanning mechanisms: single detector
• Zig-zag mechanism
– active scan lasts r/2v secs
– n pixels per line, so “dwell time” (seconds per
pixel) is r/2nv secs/pixel
– ok for low res e.g. AVHRR, as large r
– But problems for mod - high res.
– E.g. Landsat MSS, r = 70m, v = 7x103ms-1
n=3000 so dwell time = 70/2*3000*7x103 =
1.7secs (OK for SNR)
– BUT with single detector, required length of scan
cycle r/v is 10msecs (70/7x103)
– = 100 scan cycles per second
– TOO FAST!
Active scan
r/2
r
flyback
Speed, v
7
Scanning mechanisms: e.g. MSS
• MSS has 4x6 array of receptors - 4 bands, 6 receptors per band
• 6 lines scanned simultaneously
– ‘footprint’ of single receptor follows a zig-zag track
– ~30 cycles per second
T=0
WEST
T = 53ms
Active scan
474m
EAST
T = 73.4ms
185km (swath width)
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Scanning mechanisms: boustrophedon
Active
• Alternative right  left, left  right
– 2 n line pixels scanned in r/v secs
–  r/2nv secs/pixel
– For TM for e.g. r = 30m v = 20/3 x 103ms-1
n = 6000
–  dwell time 0.38 sec (not long enough
for good SNR)
– scan cycle ~4.5 msecs (~220 per second)
– Way too fast i.e. single detector operation
inadequate for TM
– use 6 detectors per band (vis), and 16
lines at a time in vis, 4 at a time in thermal
– 100 detectors total
From: http://rst.gsfc.nasa.gov/Intro/Part2_20.html
r
Active
Active
Speed, v
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Photography
• Largely obsolete due to electromechanical sensors
• Still used for
– some mapping and monitoring applications
• partic. aerial surveys and photogrammetry
– BUT requirement to get film back and process it
– Pan-chromatic (B&W) and colour (vis and some IR) but limited spectrally
– Radial image distortion away from focal point
• Relatively easy to correct if camera geometry known
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Photography
– E.g. Wild RC10 aerial camera + tracker software as used by NERC
Airborne Research and Survey Facility
– www.nerc.ac.uk/arsf
– Software allows pilot to gauge coverage and overlap
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Photography
•
•
•
•
AP of Barton Bendish, Norfolk
Acquired 1997 by NERC
aircraft
Scan of original
Note flight info and fiducial
marks @ corners
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Photography: parameters
• Photographic camera uses whole-frame image capture
– near instantaneous snapshot of projected field-of view on ground
– i.e. IFOV == whole FOV
– Imaged region (A) focused by lens/mirror system onto focal plane
(C)
– Spectral sensitivity from 0.3 to 0.9m i.e. Uv/vis/NIR
From: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter2/chapter2_7_e.html
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Photography: parameters
•Large and small apertures in camera system
•aperture compared to diameter of lens
FROM: http://cdoswell.com/tips2.htm
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Photography: parameters
•Focal length of photographic system
•pros and cons
•Amount of light v. depth of field
FROM: http://cdoswell.com/tips2.htm
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Optical mechanisms: e.g. MSS
• MSS optical system uses reflecting (Cassegrain) telescope
– Lens with hole in centre (concave)
– Convex focusing mirror
Detector
plane
Principal plane
23cm
9cm diam mirror
f = 82cm
Equivalent to a lens of
focal length 82cm
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Photography: parameters
• Normally adjust 4 parameters
– focus - by altering position of focusing lens relative to focal plane
– F-stop (f-number), defined as f/d i.e. Focal length / effective diameter of lens
opening
– Shutter speed
• e.g. 1/2000, 1/1000, 1/500 .... 1/2, 1/1, 2/1, 4/1 seconds
• Faster shutter = less motion blur, but less light
– Film “speed” - exposure level over which film responds (ISO/ASA number)
• Faster film responds to lower light BUT poorer spatial resolution
• ISO 25-100 (slow), 200-1000 (faster)
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Photography: parameters
• General film exposure equation
– E = exposure in Joules (J) mm-2, s = intrinsic scene brightness,
in J mm-2s-1, d = diameter of lens opening in mm, t = time in
seconds, f = lens focal length, mm
– So E is measure of recorded energy
• E increases with d2 , s and t
• E decreases with f2
– Note that any lens system diffraction limited i.e. can’t resolve
objects smaller than s/D
• s = distance of object from object-side focal point; D =
demagnification (Altitude/focal length i.e. D = 1/magnification =
1/s/f = f/s)
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Photography
• Historical archives of photography
– many military applications now declassified
– e.g. Surveillance (U2, Cuba, Bay of Pigs.....)
– Vietnam, N. Korea etc. etc.
?
From: Dr. S. Lewis, PhD thesis, 2003 UCL.
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Time-resolved signals: LIDAR
• Light Detection And Ranging
–
–
–
–
optical wavelength analogue of RADAR
active remote sensing
used for laser altimetry (height measurement) but also other information
Why use optical???
• Velocity of light ~ 3x108 ms-1
– one light year = 9.46 × 1015 m (10 trillion m)
– used for cosmological distances BUT also useful for smaller distances
– Light travels ~ 30cm in 1 nanosecond (10-9s)
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Time-resolved signals: LIDAR
Laser footprint

LIDAR – light detection and ranging - optical equivalent of RADAR
 First/last (discrete) return LIDAR
 Full waveform LIDAR more information BUT harder to
generate & interpret
 See Baltsavias paper for lidar equations
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Waveform LIDAR
• If we can resolve more than just
first/last return
– record shape of returning waveform?
– Waveform LIDAR
– Contains information about e.g.
Vegetation canopy structure
– Requires v. accurate timing information
– Again, typically green or red 
From:http://denali.gsfc.nasa.gov/research/laser/slicer/slicer.html
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Time-resolved signals: LIDAR
•
So for LIDAR
–
–
•
range of target from sensor (and source) is time of round trip for a pulse of light
return pulse very weak (function of surface reflectance) & (usually) spread out
LIDAR
–
–
–
–
laser light from source (coherent - narrow range of wavelengths) - typically 670-700nm
Spreads out as it is a wave (e.g. 10 to 100m spots on surface)
Roughness variation within spot (IFOV) mean energy returns sooner from some bits than
others
Needs short, powerful laser pulses
•
safety?
From: http://www.nasa.gov/offices/oce/appel/knowledge/publications/VCL.html
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Lidar signal: single birch tree
 More examples at:
 http://www2.geog.ucl.ac.uk/~mdisney/3Dmovies/
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Lidar signal: single birch tree, materials
 More examples at:
 http://www2.geog.ucl.ac.uk/~mdisney/3Dmovies/
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E.g. First/last return LIDAR data
•
Structural information from LIDAR
•
Possibly in situ laser scanning
•
Information?
–
Canopy height
–
Canopy gap fraction and vertical
profile of foliage
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E.g. Waveform LIDAR data
•
Canopy height AND density information
– intensity of return related to density
– from http://ltpwww.gsfc.nasa.gov/eib/projects/airborne_lidar/slicer.html
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LIDAR missions?
• SLICER
– Scanning Lidar Imager of Canopies by Echo
Recovery
– http://denali.gsfc.nasa.gov/research/laser/slicer/s
licer.html
• MOLA
– Mars Orbital LIDAR altimeter on Mars Global
Surveyor
– V. Accurate info on Martian topography
– Clues to geological formation
• GLAS
– Geoscience Laser Altimeter System on IceSAT
• Altimetry uses only first and last return
signal
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ICESat (aka: Laser Altimetry Mision) The Ice, Cloud, and Elevation Satellite
•
Launched Jan 12, 2003
– Jan 15, 2003 Earth pointing
•
Measures
–
–
–
–
–
–
•
•
ice sheet elevations
changes in elevation through time
height profiles of clouds and aerosols
land elevations
vegetation cover
approximate sea-ice thickness.
Geoscience Laser Altimeter System (GLAS)
- sole instrument
Combinination surface lidar with dual
wavelength cloud and aerosol lidar
Images and info from http://icesat.gsfc.nasa.gov/
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Time-resolved signals: LIDAR
• VCL didn’t get launched
– NASA budget cuts
– http://earthobservatory.nasa.gov/Library/VCL/VCL.html
– http://www.geog.umd.edu/vcl/vcltext.html
• ASCOPE – Proposed ESA Explorer mission (didn’t get selected)
– http://www.esa.int/esaCP/SEMHQH9ATME_index_0.html
• DESDyni – Deformation, Ecosystem Structure and Dynamics of Ice
– L-band interferometric SAR
– Canopy lidar
• But being applied in airborne projects
– rapid way to generate information on standing biomass
– Wood volume per hectare
• Used in carbon studies
• useful for forestry, inventory etc. etc.
From: http://earthobservatory.nasa.gov/Library/VCL/VCL_2.html
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E.g. Waveform LIDAR modelling
•
Use Monte
Carlo Ray
Tracing to
model LIDAR
signal of GLAS
ICEsat
•
Images
courtesy of U.
Heyder
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Simulating spaceborne LIDAR: ASCOPE
Based on field measurements in UK, Sweden, Finland
See Disney et al (2009) IEEE TGRSS, ASCOPE paper
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Ground-based laser scanning?
• Tripod-mounted LIDAR
– developed for surveying
– BUT has uses for collecting information on forest density and structure
– Typically records point cloud from several known locations then use
software to reconstruct scene in 3D
From: http://www.geospatialonline.com/geospatialsolutions/article/articleDetail.jsp?id=65014&pageID=4
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The next generation! ECHIDNA
• Scanning (multi-beam) ground-based LIDAR
– Developed by Jupp et al. at CSIRO (Aus.) specifically for vegetation
From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005.
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ECHIDNA
From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005.
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ECHIDNA
•Generalise hemispherical information
•But much more than for photography (discriminate canopy compnents)
From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005.
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ECHIDNA
From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005.
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Simulating ground-based (canopy) LIDAR
• Hemispherical
full waveform terrestrial laser scanner.
Abisko, Sweden
– Generates volumetric canopy data.
Echidna: Ground-based full-waveform scanning
White Fir, Sierra Nevada (A. Strahler)
Jupp et al. (2009) Estimating forest LAI profiles and structural parameters using a ground-based laser called
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Echidna, Tree Physiology 29(2) 171-181
Steve Hancock EPSRC, NCEOI
LIDAR sounding (up/down)
• For studying atmospheric aerosols, clouds etc.
–
–
–
–
–
Use backscatter properties of atmosphere
e.g. LITE (1994 Shuttle mission)
Upward looking? e.g. ELF
Coherence of laser gives narrow beam
better azimuthal sampling than thermal, RADAR
From:http://alg.umbc.edu/elf/elf.html
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Ground-based: GPR
• Ground penetrating RADAR
– gives v. accurate information on sub-surface density and structure
– Use for surveying hidden pipes for e.g.
– Archaeology
• Hidden graves
• dinosaur tracks!
– Geophysics
• ice and snow density & movement
– Hidden objects?
• Landmines...
From:www.geomodel.com &&
http://www.du.edu/~lconyer/picketwire_canyonlands_dinosaur_.htm
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Summary
• Sensor scanning mechanisms
– Limitations (dwell-time/SNR, scan rate)
– Striping of detector lines and arrays
– CCD
• Photography
– Becoming less widely-used but still some applications
• Time-resolved: LiDAR
– For altimetry AND imaging (veg. structure) – higher vertical resolution than
RADAR
• Ground-based
– Upward-looking for atmospheric studies
– GPR for sub-surface surveying: archaeology, geophysical dynamics
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REVISION
MISCELLANEOUS EXAMPLES,
TOPICS
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Revision: orbits and swaths
• Example: polar orbiter period, if h = 705x103m
– T = 2[(6.38x106 +705x103)3 / (6.67x10-11*5.983x1024)]1/2
– T = 5930.6s = 98.8mins
• Example: show separation of successive ground tracks
~3000km
–
–
–
–
–
Earth angular rotation = 2/24*60*60 = 7.27x10-5 rads s-1
So in 98.8 mins, point on surface moves 98.8*60*7.27x10-5 = .431 rads
Remember l =r* for arc of circle radius r &  in radians
So l = (Earth radius + sat. altitude)* 
= (6.38x106 +705x103)* 0.431 = 3054km
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Revision: Planck’s Law
•Planck was able to explain energy spectrum of blackbody
•Based on quantum theory rather than classical mechanics
E   
2c 2 h
5
1
e
hc
kT
1
•dE()/d gives constant of Wien’s Law
•E() over all  results in Stefan-Boltzmann relation
•Blackbody energy function of , and T
http://www.tmeg.com/esp/e_orbit/orbit.htm
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Revision: Planck’s Law
•Explains/predicts shape of blackbody curve
•Use to predict how much energy lies between given 
•Crucial for remote sensing
http://hyperphysics.phy-astr.gsu.edu/hbase/bbrc.html#c1
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Atmospheric “windows”
Atmospheric
windows
•As a result of strong  dependence of absorption
•Some  totally unsuitable for remote sensing as most
radiation absorbed
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Revision: the atmosphere
•SCATTERING: caused by presence of particles (soot, salt, etc.) and/or
large gas molecules present in the atmosphere
•Rayleigh, Mie, Non-selective
•ABSORPTION: gaseous components (CO2, CO, CH4, H2O etc)
•Very strong function of wavelength
•Atmospheric windows
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Revision: the surface: BRDF
•Reflectance of most real surfaces is a function of not only , but viewing and
illumination angles
•Described by the Bi-Directional Reflectance Distribution Function (BRDF)
•BRDF of area A defined as: ratio of incremental radiance, dLe, leaving
surface through an infinitesimal solid angle in direction (v, v), to
incremental irradiance, dEi, from illumination direction ’(i, i) i.e.
BRDF(Ω, Ω' ) 

dLe (Ω, Ω' )
sr 1
dEi (Ω' )

• is viewing vector (v, v) are view zenith and azimuth angles; ’ is illum.
vector (i, i) are illum. zenith and azimuth angles
•So in sun-sensor example,  is position of sensor and ’ is position of sun
After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.
48
Revision: the surface: BRDF
•Note that BRDF defined over infinitesimally small solid angles , ’ and
 interval, so cannot measure directly
•In practice measure over some finite angle and  and assume valid
viewer
exitant solid
angle 
incident solid
angle 
incident
diffuse
radiation
direct irradiance
(Ei) vector 
v
i
2-v
surface tangent
vector
i
surface area A
Configuration of viewing and illumination vectors in the viewing
hemisphere, with respect to an element of surface area, A.
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From: http://www.geog.ucl.ac.uk/~mdisney/phd.bak/final_version/final_pdf/chapter2a.pdf
Revision: examples
• Planck function
– Gravitational force Fg = GMEms/RsE2
• where G is universal gravitational constant (6.67x10-11 Nm2kg2); ME is Earth
mass (5.983x1024kg); ms is satellite mass (?) and RsE is distance from
Earth centre to satellite i.e. 6.38x106 + h where h is satellite altitude
– Centripetal (not centrifugal!) force Fc = msvs2/RsE
• where vs is linear speed of satellite (=sRsE where  is the satellite angular
velocity, rad s-1)
– for stable (constant radius) orbit Fc = Fg
–  GMEms/RsE2 = msvs2/RsE = ms s2RsE2 /RsE
– so s2 = GME /RsE3
From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
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Revision problems: Planck’s Law
•Fractional energy from 0 to  F0? Integrate Planck function
•Note Eb(,T), emissive power of bbody at , is function of product
T only, so....
Radiant energy from 0 to 
E0  , T 
Eb  , T 
F0  , T  
  d  , T 
4
5
T

T
0
T
Total radiant energy
for  =0 to  = 
51
Revision: Planck’s Law example
•Q: what fraction of the total power radiated by a black body
at 5770 K fall, in the UV (0    0.38µm)?
•Need table of integral values of F0
•So, T = 0.38m * 5770K = 2193mK
T (mK x103)
•Or 2.193x103 mK i.e. between 2 and 3
2
3
4
5
6
8
10
12
14
16
18
20
•Interpolate between F0 (2x103) and F0 (3x103)




F00.38  , T   F00.38 2 x103
2.193 2

 0.193
F00.38 3x103  F00.38 2 x103
3 2


F00.38  , T   0.067
 0.193
0.273  0.067
F0(T)
(dimensionless)
.067
.273
.481
.634
.738
.856
.914
.945
.963
.974
.981
.986
•Finally, F00.38 = 0.193*(0.273-0.067)+0.067 = 0.11
•i.e. ~11% of total solar energy lies in UV between 0 and 0.38m
52
Orbits: examples
• Orbital period for a given instrument and height?
– Gravitational force Fg = GMEms/RsE2
• where G is universal gravitational constant (6.67x10-11 Nm2kg2); ME is Earth
mass (5.983x1024kg); ms is satellite mass (?) and RsE is distance from
Earth centre to satellite i.e. 6.38x106 + h where h is satellite altitude
– Centripetal (not centrifugal!) force Fc = msvs2/RsE
• where vs is linear speed of satellite (=sRsE where  is the satellite angular
velocity, rad s-1)
– for stable (constant radius) orbit Fc = Fg
–  GMEms/RsE2 = msvs2/RsE = ms s2RsE2 /RsE
– so s2 = GME /RsE3
From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
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Orbits: examples
• Orbital period T of satellite (in s) = 2/
– (remember 2 = one full rotation, 360°, in radians)
– and RsE = RE + h where RE = 6.38x106 m
– So now T = 2[(RE+h)3/GME]1/2
• Example: geostationary altitude? T = ??
– Rearranging: h = [(GME /42)T2 ]1/3 - RE
– So h = [(6.67x10-11*5.983x1024 /42)(24*60*60)2 ]1/3 - 6.38x106
– h = 42.2x106 - 6.38x106 = 35.8km
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