lecture 6: sensors and scanner
DESCRIPTION
Lecture 6: Sensors And Scanner. Menglin Jin Meteorology and Climate Science San Jos é 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/TerraOrbit.mpg. - PowerPoint PPT PresentationTRANSCRIPT
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Lecture 6: Sensors And Scanner
Menglin JinMeteorology and Climate Science
San José State University
http://www.fas.org/irp/imint/docs/rst/Intro/Part2_5a.html
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Terra Orbit and Scanning
• http://www.met.sjsu.edu/~jin/video/TerraOrbit.mpg
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We will learn
• Type of scanner• Type of detector• Spatial resolution• Spectral resolution• Swath width• Pixel• Field of view (FOV)
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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
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Sensor types (classification) in the following two diagrams
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• 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.
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Advanced Very High Resolution Radiometer - AVHRR
• 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
REVIEW
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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
NASA Earth System Science Remote Sensorshttp://earthdata.nasa.gov/data/references/nasa-earth-system-science-remote-sensors
The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths
•Most air/space sensors are spectroradiometers.
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again
• Imaging spectroradiometer
A radiometer that has a scanning capability to provide a two-dimensional array of pixels from which an imagemay be produced. Scanning can be performed mechanically or electronically by using an array of detectors.
MODIS: The Moderate Resolution Imaging Spectroradiometer
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•we concentrate the discussion on optical-mechanical-electronic radiometers and scanners, leaving the subjects of camera-film systems and active radar for consideration elsewhere
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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 Sensorenergy generated from within the sensor system is beamed outward, and the fraction returned is measured; radar is an example
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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
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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
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• 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.
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•Most remote sensing instruments (sensors) are designed to measure photons
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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
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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)
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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.
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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
Review
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Handout “Detector types”
• from John Schott “Remote Sensing –The Image Chain Approach”
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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.
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a diagrammatic model of an electro-optical sensor that does not contain the means to break the incoming radiation into spectral components
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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
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2 Types of Scanner
• Cross-Track Scannerthe Whiskbroom Scanning
• Along-Track Scannerthe pushbroom scanning
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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?
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Cross-Track Scannerthe Whiskbroom Scanning
A general scheme of a typical Cross-Track Scanner
Swath width
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Class activity:
If w=1cm, calculate fFor Terra and GOES
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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
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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
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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)
MODerate-resolution Imaging Spectroradiometer (MODIS)
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MODIS Onboard Calibrators
Fold Mirror
Space View Port
Blackbody
Spectral Radiometric Calibration Assembly
Nadir (+z)
Solar Diffuser
Scan Mirror
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MODIS Optical System
Visible Focal Plane
Tra
ck
Scan
SWIR/MWIR Focal Plane
NIRFocal Plane
LWIRFocal Plane
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Shortwave IR/Midwave IRVisible
Longwave InfraredNear-infrared
Four MODIS Focal Planes
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MODIS Cross-Track Scan on Terra
MODIS_Swath
http://eoimages.gsfc.nasa.gov/images/imagerecords/53000/53280/1.mov
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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
2-D array
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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 )
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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
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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
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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
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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.
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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
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TRMM Satellite
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Earth Science Mission ProfileEarth Science Mission Profile1997-20031997-2003
eospso.gsfc.nasa.gov
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Earth Science Mission ProfileEarth Science Mission Profile2004-20102004-2010
eospso.gsfc.nasa.gov
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MODIS Onboard Calibrators
Fold Mirror
Space View Port
Blackbody
Spectral Radiometric Calibration Assembly
Nadir (+z)
Solar Diffuser
Scan Mirror
Calibration facility is part of sensor
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Sample Calibration Curve Used to Correlate Scanner Output with Radiant
Temperature Measured by a Radiometer
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• 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
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Chesapeake & Delaware BaysR =0.66 µmG =0.56 µmB =0.48 µm Balti
more
Washington
May 28, 1999
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“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
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Terra
ASTER
Launched December 18, 1999
MODIS
CERESMISR
MOPITT
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• 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
Class activity
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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
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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
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AquaLaunched May 4, 2002
MODIS
CERESAIRS
AMSR-E
AMSU
HSB
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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
Advanced Microwave Scanning Radiometer (AMSR-E)
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AMSR-E Conical Scan on Aqua
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AMSR-E Composite Sea Surface Temperature
June 2002
-2
28
°C35
Orange colors denote temperature necessary for hurricane formation
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MODIS “Science and Beauty” video
http://www.youtube.com/watch?v=1jqFxZI_2XY