Transcript Lecture 3: Sensors - San Jose State University

Lecture 6: Sensors And Scanner
Menglin Jin
Meteorology and Climate Science
San José State University
http://www.fas.org/irp/imint/docs/rst/Intro/Part2_5a.html
Terra Orbit and Scanning
• http://www.met.sjsu.edu/~jin/video/TerraOr
bit.mpg
We will learn
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Type of scanner
Type of detector
Spatial resolution
Spectral resolution
Swath width
Pixel
Field of view (FOV)
The Afternoon Constellation
“A-Train”
 The Afternoon constellation consists of 7 U.S. and international Earth Science
satellites that fly within approximately 30 minutes of each other to enable
coordinated science
 The joint measurements provide an unprecedented sensor system for Earth
observations
Sensor types (classification) in the
following two diagrams
• Radiometer—An instrument that quantitatively
measures the intensity of electromagnetic radiation in
some bands within the spectrum. Usually, a radiometer
is further identified by the portion of the spectrum it
covers; for example, visible, infrared, or microwave.
• Spectrometer—A device that is designed to detect,
measure, and analyze the spectral content of incident
electromagnetic radiation. Conventional imaging
spectrometers use gratings or prisms to disperse the
radiation for spectral discrimination.
• Sounder—An instrument that measures vertical
distributions of atmospheric parameters such as
temperature, pressure, and composition from
multispectral information.
Advanced Very High Resolution
Radiometer - AVHRR REVIEW
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Band 1 Visible Channel (0.63 µm)
Band 2 Visible Channel (0.86 µm)
Band 3A NIR channel 1.58 - 1.64 µm
Band 3B Shortwave IR Channel (3.74
µm)
• Band 4 IR Window Channel (10.8 µm)
• Band 5 IR Channel (12.0 µm)
Resolution at Nadir
Then
• Spectroradiometer
A radiometer that measures the intensity of radiation
in multiple wavelength bands (i.e., multispectral).
Many times the bands are of high-spectral resolution,
designed for remotely sensing specific
geophysical parameters
The term spectroradiometer is reserved for sensors
that collect the dispersed radiation in bands
rather than discrete wavelengths
NASA Earth System Science Remote Sensors
http://earthdata.nasa.gov/data/references/nasa-earth-system-science-remote-sensors
•Most air/space sensors are spectroradiometers.
again
• Imaging spectroradiometer
A radiometer that has a scanning capability to provide
a two-dimensional array of pixels from which an image
may be produced. Scanning can be performed mechanically
or electronically by using an array of detectors.
MODIS: The Moderate Resolution Imaging
Spectroradiometer
•we concentrate the discussion on optical-mechanical-electronic radiometers
and scanners, leaving the subjects of camera-film systems and active radar
for consideration elsewhere
Passive and Active Sensors
• Passive Sensor:
energy leading to radiation received comes from
an external source, e.g., the Sun or objects that
emit
• Active Sensor
energy generated from within the sensor system
is beamed outward, and the fraction returned is
measured; radar is an example
Imaging and non-imaging sensor
• Non-imaging:
measures the radiation received from all
points in the sensed target, integrates this,
and reports the result as an electrical
signal strength or some other quantitative
attribute, such as radiance
Imaging and non-imaging sensor
• Non-imaging:
measures the radiation received from all points
in the sensed target, integrates this, and reports
the result as an electrical signal strength or
some other quantitative attribute, such as
radiance
• Imaging
the electrons released are used to excite or
ionize a substance like silver (Ag) in film or to
drive an image producing device like a TV or
computer monitor or a cathode ray tube or
oscilloscope or a battery of electronic detectors
• Aperture:
In optics, an aperture is a hole
or an opening through which
light travels
The aperture determines how collimated
the admitted rays are, which is of great
importance for the appearance
at the image plane.
•Most remote sensing instruments (sensors) are
designed to measure photons
Principal: photoelectric effect
• There will be an emission of negative particles
(electrons) when a negatively charged plate of some
appropriate light-sensitive material is subjected to a
beam of photons. The electrons can then be made to
flow as a current from the plate, are collected, and then
counted as a signal
Principal: photoelectric effect
• There will be an emission of negative particles (electrons) when a
negatively charged plate of some appropriate light-sensitive material
is subjected to a beam of photons. The electrons can then be made
to flow as a current from the plate, are collected, and then counted
as a signal
• Albert Einstein’s experiment (see lecture 3, or next slide)
Principal: photoelectric effect
• There will be an emission of negative particles (electrons) when a
negatively charged plate of some appropriate light-sensitive material
is subjected to a beam of photons. The electrons can then be made
to flow as a current from the plate, are collected, and then counted
as a signal
• Albert Einstein’s experiment (see lecture 3, or next slide)
• Thus, changes in the electric current can be used to measure
changes in the photons (numbers; intensity) that strike the plate
(detector) during a given time interval.
• The kinetic energy of the released photoelectrons varies with
frequency (or wavelength) of the impinging radiation
• different materials undergo photoelectric effect release of electrons
over different wavelength intervals; each has a threshold wavelength
at which the phenomenon begins and a longer wavelength at which
it ceases.
photoelectric effect –measure
photon energy level
Review
• the discovery by Albert Einstein in 1905
•His experiments also revealed that regardless
of the radiation intensity, photoelectrons are
emitted only after a threshold frequency is exceeded
•for those higher than the threshold value (exceeding
the work function) the numbers of photoelectrons
released re proportional to the number
of incident photons
Handout “Detector types”
• from John Schott “Remote Sensing –The
Image Chain Approach”
Moving further down the classification tree, the optical setup for imaging sensors
will be either an image plane or an object plane set up depending on
where lens is before the photon rays are converged (focused), as shown in this illustration
a diagrammatic model of an electro-optical sensor that does not contain
the means to break the incoming radiation into spectral components
Scanning System
• If the scene is sensed point by point
(equivalent to small areas within the
scene) along successive lines over a finite
time, this mode of measurement makes up
a scanning system. Most non-camera
sensors operating from moving platforms
image the scene by scanning
2 Types of Scanner
• Cross-Track Scanner
the Whiskbroom Scanning
• Along-Track Scanner
the pushbroom scanning
Field of View (FOV)
• Sensors that instantaneously measure
radiation coming from the entire scene at
once are called framing systems. The eye,
a photo camera, and a TV vidicon belong
to this group. The size of the scene that is
framed is determined by the apertures and
optics in the system that define the field of
view, or FOV
Question: What is FOV of your eyes?
Cross-Track Scanner
the Whiskbroom Scanning
A general scheme of a typical
Cross-Track Scanner
Swath width
Class activity:
If w=1cm, calculate f
For Terra and GOES
Essential Components of
Cross-track Sensor
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1) a light gathering telescope that defines the scene dimensions at any
moment (not shown)
2) appropriate optics (e.g., lens) within the light path train
3) a mirror (on aircraft scanners this may completely rotate; on spacecraft
scanners this usually oscillates over small angles)
4) a device (spectroscope; spectral diffraction grating; band filters) to break
the incoming radiation into spectral intervals
5) a means to direct the light so dispersed onto an array or bank of
detectors
6) an electronic means to sample the photo-electric effect at each detector
and to then reset the detector to a base state to receive the next incoming
light packet, resulting in a signal stream that relates to changes in light
values coming from the ground targets as the sensor passes over the scene
7) a recording component that either reads the signal as an analog current
that changes over time or converts the signal (usually onboard) to a
succession of digital numbers, either being sent back to a ground station
Note: most are shared with Along Track systems
pixel
The cells are sensed one after another along the line.
In the sensor, each cell is associated with a pixel that is
tied to a microelectronic detector
Pixel is a short abbreviation for Picture Element
a pixel being a single point in a graphic image
Each pixel is characterized
by some single value of radiation
(e.g., reflectance) impinging on
a detector that is converted by
the photoelectric effect into electrons
MODerate-resolution Imaging
Spectroradiometer (MODIS)
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NASA, Terra & Aqua
– launched 1999, 2002
– 705 km polar orbits, descending (10:30
a.m.) & ascending (1:30 p.m.)
Sensor Characteristics
– 36 spectral bands (490 detectors)
ranging from 0.41 to 14.39 µm
– Two-sided paddle wheel 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
– onboard solar diffuser & solar diffuser
stability monitor
– 12 bit dynamic range (0-4095)
MODIS Onboard Calibrators
Solar
Diffuser
Spectral
Radiometric
Calibration
Assembly
Blackbody
Scan
Mirror
Space View
Port
Fold
Mirror
Nadir (+z)
MODIS Optical System
Visible
Focal
Plane
SWIR/MWI
R Focal
Plane
LWIR
Focal
Plane
NIR
Focal
Plane
Four MODIS Focal Planes
Shortwave IR/Midwave IR
Visible
Near-infrared
Longwave Infrared
MODIS Cross-Track Scan on
Terra
MODIS_Swath
http://eoimages.gsfc.nasa.gov/images/imagerecords/53000/53280/1.mov
Along-track Scanner
pushbroom scanning
the scanner does not have a mirror
looking off at varying angles.
Instead there is a line of small sensitive detectors
stacked side by side, each having some
tiny dimension on its plate surface;
these may number several thousand
2-D array
Multi-angle Imaging
SpectroRadiometer (MISR)
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NASA, EOS Terra
– Launched in 1999
– polar, descending orbit of 705 km,
10:30 a.m. crossing
Sensor Characteristics
– uses nine CCD-based pushbroom cameras viewing nadir and
fore & aft to 70.5°
– four spectral bands for each
camera (36 channels), at 446,
558, 672, & 866 nm
– resolutions of 275 m, 550 m, or
1.1 km
Advantages
– high spectral stability
– 9 viewing angles helps determine
aerosol by µ dependence (fixed t)
MISR Pushbroom Scanner
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• Family portrait
– 9 MISR cameras
– 1 AirMISR
camera
Orbital characteristics
– 400 km swath
– 9 day global coverage
– 7 min to observe each scene at all
9 look angles
MISR Provides New Angle on
Haze
• In this MISR view spanning from Lake Ontario to Georgia,
the increasingly oblique view angles reveal a pall of haze
over the Appalachian Mountains
spectral resolution
• The radiation - normally visible and/or Near and
Short Wave IR, and/or thermal emissive in
nature - must then be broken into spectral
intervals, i.e., into broad to narrow bands. The
width in wavelength units of a band or channel is
defined by the instrument's spectral resolution
• The spectral resolution achieved by a sensor
depends on the number of bands, their
bandwidths, and their locations within the EM
spectrum
Spectral filters
Absorption and Interference. Absorption filters pass
only a limited range of radiation wavelengths,
absorbing radiation outside this range.
Interference filters reflect radiation at wavelengths
lower and higher than the interval they transmit.
Each type may be either a broad or a narrow bandpass filters.
This is a graph distinguishing the two types.
Landsat Thematic Mapper
Bands
• Landsat collects monochrome images in each band by measuring
radiance & reflectance in each channel
– When viewed individually, these images appear as shades of gray
TRMM Satellite
Earth Science Mission Profile
1997-2003
eospso.gsfc.nasa.gov
Earth Science Mission Profile
2004-2010
eospso.gsfc.nasa.gov
MODIS Onboard Calibrators
Solar
Diffuser
Spectral
Radiometric
Calibration
Assembly
Blackbody
Scan
Mirror
Space View
Port
Fold
Mirror
Nadir (+z)
Calibration facility is part of sensor
Sample Calibration Curve Used to
Correlate Scanner Output with Radiant
Temperature Measured by a Radiometer
Color Composites
• The human eye is not
sensitive to ultraviolet or
infrared light
– To build a composite
image from remote
sensing data that makes
sense to our eyes, we
must use colors from the
visible portion of the EM
spectrum—red, green, and
blue
Chesapeake & Delaware Bays
R = 0.66 µm
G = 0.56 µm
B = 0.48 µm
May 28, 1999
Balti
more
Washi
ngton
“False Color” Composite Image
• To interpret radiance measurements in the infrared portion of the electromagnetic
spectrum, we assign colors to the bands of interest and then combine them into a
“false color” composite image
Terra
Launched December
18, 1999
MODIS
MOPITT
ASTER
MISR
CERES
Advanced Spaceborne Thermal
Emission & Reflection Radiometer
(ASTER)
• NASA & MITI, Terra
•
– 705 km polar orbit, descending
(10:30 a.m.)
Sensor Characteristics
– 14 spectral bands ranging from
0.56 to 11.3 µm
– 3 tiltable subsystems for
acquiring stereoscopic imagery
over a swath width of 60 km
– Spatial resolutions:
• 15 m (bands 1, 2, 3N, 3B)
• 30 m (bands 4 - 9)
• 90 m (bands 10 - 14)
– 4% reflectance calibration
accuracy (VNIR & SWIR)
– 2 K brightness temperature
accuracy (240-370 K)
VNIR (1,2,3N)
VNIR (3B)
TIR
SWIR
Class activity
Comparison of Landsat 7 and
ASTER
Wavelength Region
VNIR
SWIR
TIR
Terra/ASTER
Band No. Spectral Range
(µm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.52-0.60
0.63-0.69
0.76-0.86
1.60-1.70
2.145-2.185
2.185-2.225
2.235-2.285
2.295-2.365
2.360-2.430
8.125-8.475
8.475-8.825
8.925-9.275
10.25-10.95
10.95-11.65
Landsat 7/ETM+
Band No. Spectral Range
(µm)
1
0.45-0.52
2
0.52-0.60
3
0.63-0.69
4
0.76-0.90
5
1.55-1.75
7
2.08-2.35
6
10.4-12.5
Synergy Between Terra and Landsat 7 Data
(same day 705 km orbits ~ 30 minutes apart)
Landsat ETM+ input to Terra data
• Vegetation classification for MODIS & MISR biophysical
products
• Focus on global change hotspots detected by MODIS & MISR
• Linking Terra observations with 34+ year Landsat archive
• Radiometric rectification of MODIS data
MODIS
2330 km swath width
spatial resolution (250, 500, 1000 m)
MISR
spatial resolution (275, 550, 1100 m)
360 km
global coverage  2 days
global coverage  9 days
Landsat 7
spatial resolution (15, 30, 60 m)183 km
16 day orbital repeat
global coverage  seasonally
ASTER
spatial resolution (15, 30, 90 m)
45-60 day orbital repeat
global coverage  months to years
60 km swath
Terra input to Landsat ETM+ data
• Use of MODIS & MISR for improved atmospheric correction of ETM+
• Use of MODIS & MISR for temporal interpolation of ETM+ data
• Cross-calibration of ASTER, MISR, and MODIS
Aqua
Launched May 4, 2002
AMSR-E
MODIS
AMSU
AIRS
HSB
CERES
Advanced Microwave Scanning Radiometer
(AMSR-E)
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NASA, Aqua
– launched May 4, 2002
– 705 km polar orbits, ascending
(1:30 p.m.)
Sensor Characteristics
– 12 channel microwave radiometer
with 6 frequencies from 6.9 to 89.0
GHz with both vertical and
horizontal polarization
– Conical scan mirror with 55°
incident angle at Earth’s surface
– Spatial resolutions:
• 6 x 4 km (89.0 GHz)
• 75 x 43 km (6.9 GHz)
– External cold load reflector and a
warm load for calibration
• 1 K Tb accuracy
AMSR-E Conical Scan on Aqua
AMSR-E Composite Sea
Surface Temperature
Orange colors denote temperature necessary for hurricane formation
°C
35
28
-2
June 2002
MODIS “Science and Beauty” video
http://www.youtube.com/watch?v=1jqFxZI_2
XY