Digital Imaging and Remote Sensing Laboratory Sensor Characteristics

Digital Imaging and Remote Sensing Laboratory

Sensor CharacteristicsSensor Characteristics

Sensor Characteristics 2Digital Imaging and Remote Sensing Laboratory

MODISMODIS

The MODerate resolution Imaging Spectrometer

instrument (MODIS) the first operational space-

based spectrometer. Its requirements for wide

spectral coverage (VIS to LWIR) wide field of view,

and a range of spectral resolutions resulted in a

conventional line scanner design with multiple lines

per rotation.


MODIS (cont’d)MODIS (cont’d)

Small linear arrays are located perpendicular to the

scan direction with individual filters for each band.

Multiple focal planes are used for the various

detector materials. 8, 16, or 32 lines will be

scanned per mirror sweep at 1000, 500, or 250 m

nominal GIFOV.


Sensors:Sensors:Bandpass Filter Spectrometers –Bandpass Filter Spectrometers –

Line Scan/WhiskbroomLine Scan/Whiskbroom

• MODIS: Moderate Resolution Imaging Spectroradiometer

Solar diffuser

Blackbody reference

Double-sidedscan mirror

Aperture cover

Spectroradiometric calibrator Main electronics

module

Space view & lunarcalibration port

Radiative cooler

Radiative cooler door & earth shield

Thermal blanket


MODISMODIS

•39 channels (36 bands 3 with 2 gains)•1500 km swath•repeat coverage of the globe every 2 days •cloud, sea, and land monitoring

http://modis.gsfc.nasa.gov/


MODIS MODIS (partial scene 3/6/00)(partial scene 3/6/00)


Types of multispectral imaging systemsTypes of multispectral imaging systems Spectral Line Scanners (cont’d)

The basic spectrometer designs are extensions of the whisk broom or line scanners and the push broom scanners


Airborne Imaging Spectrometer Airborne Imaging Spectrometer Spectral Line Scanners (cont’d)

One of the earliest experimental systems was

NASA’s Airborne Imaging Spectrometer (AIS) flown

in the mid 1980’s. It used the 2-d array design

originally with a 32 x 32 element detector and later

with a 64 x 64 element array (HgCdTe) operated

from 1.2 - 2.4 and 0.8 - 2.4 respectively.


Benefits of spectrometer data and the Benefits of spectrometer data and the limitation of AIS as an imagerlimitation of AIS as an imager

Spectral Line Scanners (con’t)


Comparison of AIS-1 and AIS-2 Comparison of AIS-1 and AIS-2 performance parametersperformance parameters

Spectral Line Scanners (cont’d)(cont’d)

IFOV, mrad 1.91 2.05

Ground IFOV, m at 6-km altitude 11.4 12.3

FOV, deg 3.7 7.3

Swath width, m at 6-km altitude 365 787

Spectral sampling interval, nm 9.3 10.6

Data rate, kbps 394 1670

Spectral sampling Short-wavelength mode, m 0.9-2.1 0.8-1.6 Long-wavelength mode, m 1.2-2.4 1.2-2.4


AVIRISAVIRIS Spectral Line Scanners (cont’d)

At that time, limitations in detector technology precluded

a large array and still limit 2-D array approaches.

NASA chooses a whisk broom array spectrometer for its

follow-on research activity. The airborne visible infrared

imaging spectrometer (AVIRIS) schematic design and

conceptual approach are shown in the following figures


Spectral Line Scanners

Linear array

Diffraction grating

Aperture

TelescopeOscillating scan mirror

Scan Track

Ground track


Spectral Line ScannersSpectral Line Scanners

• AVIRIS (airborne visible infrared imaging

spectrometer)

• MISI (Modular Imaging Spectrometer Instrument)

• CASI


Conceptual layout of the AVIRIS Conceptual layout of the AVIRIS optical systemoptical system



AVIRIS Performance characteristicsAVIRIS Performance characteristics Spectral Line Scanners (cont’d)(cont’d)

Spectral coverage 0.4-2.45Spectral sampling interval, nm 9.6-9.9Number of spectral bands 224IFOV, mrad 0.95Ground IFOV, m at 20-km altitude 20FOV, deg 30Swath width, km at 20-km altitude 10.5Number of cross-track pixels 614Data encoding, bits 10Data rate, Mbps 17Radiometric calibration accuracy, % Absolute 6

Spectral band-to-band 0.5Spectral calibration accuracy, nm 1-2

Parameter Performance


AVIRIS image cube of Moffet Field, CAAVIRIS image cube of Moffet Field, CASpectral Line Scanners (cont’d)(cont’d)

•224 channels•.4 m to 2.5 m•spectral bandwidth • ~10 nm

(Image courtesy of NASA JPL.)


AVIRIS signal-to-noise AVIRIS signal-to-noise


AVIRIS SceneAVIRIS SceneLake Ontario Lake Ontario Shoreline Shoreline RochesterRochesterEmbayment Embayment May 20, 1999May 20, 1999


MISI (Modular Imaging MISI (Modular Imaging Spectrometer Instrument)Spectrometer Instrument)



Modular Imaging Spectrometer Instrument (MISI)

Airborne line scanner70 VNIR channels5 thermal channelsNominal 2 milliradian FOV (20ft GSD at 10000ft)Sharpening bands in VIS and LWIR

spectrometers

thermal focal plane

scan mirror

On-board blackbody


thermal

MISI image of nuclear power plant discharge into Lake Ontario September 3, 1999

Three of MISI’s 70 VNIR channels


MISI Examples

Irodequoit Bay

Charlotte Pier

Ginna Power Plant

Sensor CharacteristicsDigital Imaging and Remote Sensing Laboratory

Push Broom Dispersion Systems

Pushbroom axis

Sp

ectr

al a

xis

Area arrays

Diffraction grating

Collimator

Slit

Optics

Ground Track

AIS (diffraction grating)HYDICE (prism)SEBASS (prism)Hyperion (EO-1)


HYDICE SensorHYDICE Sensor Push Broom Dispersion Systems (con’t)

The Hyperspectral Digital Imagery Collection

Experiment (HYDICE) uses a 2-d array push broom

approach with a prism monochromator. The optical

layout is on the following slide. The system is a

technology demonstration airborne test bed for

future satellite systems. The optics are designed to

fit in a mapping camera mount.



The system IFOV is 0.5 m rad and flies in a C141 at

2 to 14 km (nominal 6) with a GIFOV of 1 to 7

meters. The FOV is 8.94 degrees yielding coverage

of 0.3 to 2.2 km.



The prism design yields variable spectral

bandwidth as shown in Figure 2. The bandwidth in

the blue channels will be increased by averaging in

the spectral direction at the extreme end of the blue

to maintain a nominal bandwidth of approximately

10 nm.



Fig 2. Spectral bandwidth (FWHM) as a function of wavelength



The wide spectral range from 0.4 - 2.5 µm is

achieved with a single cooled InSb detector (65K)

array as shown in Figure 3. Special passivation and

anti reflection coating were developed to maintain

acceptable sensitivity and SNR over the entire

range.



Fig 3. Focal plane array architecture



The expected HYDICE SNR is shown in Figure 4 for

its spec point of a 5% reflector (N.B. this system

was designed for water sensors.)


SEBASS Sensor SEBASS Sensor HighlightsHighlights

Push Broom Dispersion Systems (con’t)

• Spatially Enhanced Broadband Array Spectrograph System

• Developed by the Aerospace Corporation

• Prototype Hyperspectral Infrared Sensor

• Material Identification using 3-5 and 8-14 µm signatures


SEBASS Sensor SEBASS Sensor GeometryGeometry

Push Broom Dispersion Systems (con’t)

• Pushbroom Scanner

• Disperses line image into its spectral components

• Detectors are 128x128 pixel “Blocked Impurity Band”

– manufactured by Rockwell International

– Built as part of NASA SIRTF effort

• Spatial Resolution of 0.5 and 3 meters – @1500 and 10000 feet respectively

• 1 milliradian per pixel IFOV (~7 degrees FOV)



Spectral purity issues:Spectral purity issues:spatial/temporal/sensor artifacts (smile)spatial/temporal/sensor artifacts (smile)

The SEBASS Sensor is a Pushbroom Scanner


Pushbroom axis

Sp

ectr

al a

xis

Area arrays

Diffraction grating

Collimator

Slit

Optics

Ground Track

Spectral purity issues:Spectral purity issues:spatial/temporal/sensor artifacts (smile)spatial/temporal/sensor artifacts (smile)

Push Broom Dispersion Systems


Spectral purity issues:spatial/temporal/sensor artifacts (smile)


Linear Wedge Filter Spectrometer

Atmospheric Corrector on EO-1

wedgefilter

2D array

wedge interference filter

side view of filter


Fourier transform instrumentsFourier transform instruments

At longer wavelengths, the spectral features become very narrow. This is particularly important in the 8-14 µm region where many gaseous absorption features are manifest. It can be difficult to achieve sufficient spectral resolution at these wavelengths. In the laboratory Fourier, transform spectrometers are often used for detailed characterization of the spectra at these wavelengths.



Fig 1. IFTS raw data cube



Figure 1 shows the concept behind an FTIR imaging spectrometer where a 2-d array is located at the image plane (interference plane). Each spatial 2-d sample represents a different time sample corresponding to a different location of the moving mirror in the interferometer and, therefore, a different interference pattern. For any pixel, the Fourier transform of the interference samples (interferogram) is the spectrum for that pixel. Thus, from the interferogram image cube, a conventional spectral image cube can be created by a 1-dimensional Fourier transform of each pixel.



Fig 2. A sketch of the optics of an Imaging Fourier

Transform Spectrometer



Figure 2 shows a conceptual diagram of an FTIR imaging instrument. The object plane would typically be the focal plane of the conventional collection optics. The 2-d array is located at the image plane.

The primary advantage of the imaging FT instrument is that spectral resolution is primarily a function of the number of samples taken. Therefore, high spectral resolution can be achieved without great cost in detector technology.



Note a major drawback of this approach is the

assumption of constant FOV during motion of the

mirror.


• Many variations in design of IFTS available

• Michelson

– Collects spectral information over time

– Spatial information collected like an image

• Sagnac

– Spectral information collected spatially (over one FPA dimension)

– Spatial info collected over other FPA dimension + pushbroom scanning



Michelson InterferometerMichelson Interferometer

• Frame camera– Must stare at one point

during the collection time

• Interferogram

collection method– Collect interference image

– Move mirror (change OPD)

– Change view angle

– Repeat

ObjectPlane

Image Plane

Fixed Mirror

MovingMirror

y

f

f’

y’


Michelson InterferometerMichelson Interferometer

• Input spectrum changes

with view angle and

pointing accuracy

• Collects one slice of

image cube at every time

interval


Sagnac InterferometerSagnac Interferometer

• Pushbroom Scanner

• Collect entire interferogram

over one axis of the FPA

• Each interferogram is

collected instantaneously

• Examples– FTHSI on MightySat II.1

– MTU sensor for water quality of GL

Mirrors

Spherical lensCylindrical lens

Beamsplitter

ApertureTelescope focus

detector


Spectral databases – mixed pixelsSpectral databases – mixed pixels

• Lab & Field Spectra – (diffuse hemispheric- BDRF

ASD)

• USGS

• EOS

• ASTER Spectral Library

• http://speclib.jpl.nasa.gov


brown silt loam

0

1

2

3

4

5

6

7

8

8 10 12 14

brown silt loam

0

0.5

1

1.5

2

2.5

3

3.5

8 9 10 11 12 13 14

conifer

deciduous

grass

from:ASTER Spectral Libraryhttp://speclib.jpl.nasa.gov


Grass

asphalt roofing

Brick

1.0

ASD FieldSpecASD FieldSpec


BRDFBRDF

While BRDF effects overall reflectance levels: to

first order spectral contrast in materials with similar

texture is not significantly impacted by normal

variations in viewing conditions. (In many cases,

this may not be a valid assumption: beach sand vs.

plowed field.)


BRDF (cont’d)BRDF (cont’d)


SensorLight trap

specular ray

sample

Incident flux

Integrating Sphere

Schematic concept for measuring total Schematic concept for measuring total and diffuse reflectance and diffuse reflectance

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Digital Imaging and Remote Sensing Laboratory Sensor Characteristics