lecture 5: sensors and scanner
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Lecture 5: Sensors And Scanner
Professor Menglin Jin
San Jose State University
The Afternoon ConstellationThe Afternoon Constellation“A-Train“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
•Most remote sensing instruments (sensors) are designed to measure photons
•we concentrate the discussion on optical-mechanical-electronic radiometers and scanners, leaving the subjects of camera-film systems and active radar for consideration elsewhere
Non-Photographic Sensor Systems
• 1800 Discovery of the IR spectral region by Sir William Herschel. • 1879 Use of the bolometer by Langley to make temperature measurements of
electrical objects. • 1889 Hertz demonstrated reflection of radio waves from solid objects. • 1916 Aircraft tracked in flight by Hoffman using thermopiles to detect heat
effects. • 1930 Both British and Germans work on systems to locate airplanes from their
thermal patterns at night. • 1940 Development of incoherent radar systems by the British and United States
to detect and track aircraft and ships during W.W.II. • 1950's Extensive studies of IR systems at University of Michigan and elsewhere.
1951 First concepts of a moving coherent radar system. • 1953 Flight of an X-band coherent radar. • 1954 Formulation of synthetic aperture concept (SAR) in radar. • 1950's Research development of SLAR and SAR systems by Motorola, Philco,
Goodyear, Raytheon, and others. • 1956 Kozyrev originated Frauenhofer Line Discrimination concept. • 1960's Development of various detectors which allowed building of imaging and
non-imaging radiometers, scanners, spectrometers and polarimeters. • 1968 Description of UV nitrogen gas laser system to simulate luminescence.
Passive and Active Sensors
• Passive Sensor:
energy leading to radiation received comes from an external source, e.g., the Sun
• 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
since the radiation is related to specific points in the target, the end result is an image [picture] or a raster display [for example: the parallel horizontal lines on a TV screen])
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
• Imagingthe 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
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
• 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”
two broadest classes of sensors
• Passive sensorenergy leading to radiation received comes
from an external source, e.g., the Sun
• Active Sensor energy generated from within the sensor
system is beamed outward, and the fraction returned is measured
Example: radar
• Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum
• spectrometer When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to an array of detectors
•spectroradiometer The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths
•Most air/space sensors are spectroradiometers.
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.
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
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
Cross-Track Scannerthe Whiskbroom Scanning
A general scheme of a typical Cross-Track Scanner
Essential Components of Cross-track Sensor
• 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
• 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)
MODerate-resolution Imaging Spectroradiometer (MODIS)
MODIS Onboard Calibrators
Fold Mirror
Space View Port
Blackbody
Spectral Radiometric Calibration Assembly
Nadir (+z)
Solar Diffuser
Scan Mirror
MODIS Optical System
Visible Focal Plane
Tra
ck
Scan
SWIR/MWIR Focal Plane
NIRFocal Plane
LWIRFocal Plane
Shortwave IR/Midwave IRVisible
Longwave InfraredNear-infrared
Four MODIS Focal Planes
MODIS Cross-Track Scan on Terra
MODIS_Swath
MISR_Swath
Along-track Scannerpushbroom 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
Along-track, or Pushbroom, Multispectral System Operation
Multi-angle Imaging SpectroRadiometer (MISR)
• NASA, EOS Terra– Launched in 1999– polar, descending orbit of 705 km,
10:30 a.m. crossing• Sensor Characteristics
– uses nine CCD-based push-broom 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 )
MISR Pushbroom Scanner• Orbital characteristics
– 400 km swath– 9 day global coverage– 7 min to observe each scene at
all 9 look angles
• Family portrait– 9 MISR cameras– 1 AirMISR
camera
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.
Enhanced Thematic Mapper Plus (ETM+)
• NASA & USGS, Landsat 7– launched April 15, 1999– 705 km polar orbit, descending
(10:00 a.m.)• Sensor Characteristics
– 7 spectral bands ranging from 0.48 to 11.5 µm
– 1 panchromatic band (0.5-0.9 µm)
– cross-track scan mirror with 185 km swath width
– Spatial resolutions:• 15 m (panchromatic)• 30 m (spectral)
– Calibration:• 5% reflectance accuracy• 1% thermal IR accuracy• onboard lamps,
blackbody, and shutter• solar diffuser
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 ProfileEarth Science Mission Profile1997-20031997-2003
eospso.gsfc.nasa.gov
Earth Science Mission ProfileEarth Science Mission Profile2004-20102004-2010
eospso.gsfc.nasa.gov
Satellites in Geosynchronous Orbits are used as Relay Satellites for LEO
SpacecraftImaging
System (e.g., Landsat)
Communication relay system
Communication relay
system (e.g., TDRSS)
GEO
LEOGround station
Sample Calibration Curve Used to Correlate Scanner Output with Radiant
Temperature Measured by a Radiometer
• 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
Color Composites
Chesapeake & Delaware BaysR =0.66 µmG =0.56 µmB =0.48 µm Balti
more
Washington
May 28, 1999
“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
ASTER
Launched December 18, 1999
MODIS
CERESMISR
MOPITT
• 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)
Advanced Spaceborne Thermal Emission & Reflection Radiometer
(ASTER)
SWIR
VNIR (1,2,3N)
VNIR (3B) TIR
Wavelength RegionBand No. Spectral Range
(µm)Band No. Spectral Range
(µm)VNIR 1 0.45-0.52
1 0.52-0.60 2 0.52-0.602 0.63-0.69 3 0.63-0.693 0.76-0.86 4 0.76-0.90
SWIR 4 1.60-1.70 5 1.55-1.755 2.145-2.185 7 2.08-2.356 2.185-2.2257 2.235-2.2858 2.295-2.3659 2.360-2.430
TIR 10 8.125-8.475 6 10.4-12.511 8.475-8.82512 8.925-9.27513 10.25-10.9514 10.95-11.65
Terra/ASTER Landsat 7/ETM+
Comparison of Landsat 7 and ASTER
Synergy Between Terra and Landsat 7 DataSynergy Between Terra and Landsat 7 Data(same day 705 km orbits ~ 30 minutes apart)(same day 705 km orbits ~ 30 minutes apart)
spatial resolution (275, 550, 1100 m)
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
183 km
2330 km swath widthspatial resolution (250, 500, 1000 m) global coverage⇒2 days
360 km global coverage⇒9 days
spatial resolution (15, 30, 60 m)Landsat 7 16 day orbital repeatglobal coverage⇒seasonally
spatial resolution (15, 30, 90 m)ASTER 45-60 day orbital repeatglobal coverage⇒months to years
60 km swath
MODIS
MISR
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
MODIS
CERESAIRS
AMSR-E
AMSU
HSB
• 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
Advanced Microwave Scanning Radiometer (AMSR-E)
AMSR-E Conical Scan on Aqua
AMSR-E Composite Sea Surface Temperature
June 2002
-2
28
°C35
Orange colors denote temperature necessary for hurricane formation
Satellite online visualization (class Activity)
• Satellite rainfall observations are very useful to reveal the rain intensity and spatial distribution over the globe. Tropical rainfall measurement mission (TRMM) is one NASA program to monitor rainfall from the space bake to 1998. Use the Monthly TRMM and Other Data Sources Rainfall Estimate (3B43 V6) (http://disc2.nascom.nasa.gov/Giovanni/tovas/TRMM_V6.3B43.shtml), to answer the following questions:
– Plot spatial distribution of rainfall at CA area (25-40°N, 110-125°W) using data from May 1998 to May 2009. Where do you see the highest rainfall in this area? How much there?
– Plot the time series of accumulated rainfall for the same CA area above during the same time. Which month does CA have the highest rainfall and which month CA have the lowest rainfall? How much are the highest and lowest rainfall respectively?
– Plot the rainfall over the globe spatial distribution (180°W-180°E, 50°N-50°S) for July 2008 and December 2008, respectively. Describe at least three major differences of the rainfall pattern of these two months.
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