Lecture 6: Sensors And Scanner

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

Terra Orbit and Scanning

• http://www.met.sjsu.edu/~jin/video/TerraOrbit.mpg

We will learn

• Type of scanner• Type of detector• Spatial resolution• Spectral resolution• Swath width• Pixel• Field of view (FOV)

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

• 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

• 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

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.

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

•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 Sensorenergy 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

• 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

• 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

• 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

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 Scannerthe Whiskbroom Scanning

• Along-Track Scannerthe 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 Scannerthe Whiskbroom Scanning

A general scheme of a typical Cross-Track Scanner

Swath width

Class activity:

If w=1cm, calculate fFor Terra and GOES

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

http://eoimages.gsfc.nasa.gov/images/imagerecords/53000/53280/1.mov

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

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.

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

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

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

Class activity

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

AquaLaunched 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

MODIS “Science and Beauty” video

http://www.youtube.com/watch?v=1jqFxZI_2XY