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Sensor Characteristics

Sensor Characteristics. MODIS.

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Sensor Characteristics

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  1. Sensor Characteristics

  2. MODIS 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. Sensor Characteristics

  3. 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. Sensor Characteristics

  4. Sensors:Bandpass Filter Spectrometers –Line Scan/Whiskbroom • MODIS: Moderate Resolution Imaging Spectroradiometer Spectroradiometric calibrator Solar diffuser Main electronics module Blackbody reference Space view & lunar calibration port Double-sided scan mirror Radiative cooler Radiative cooler door & earth shield Aperture cover Thermal blanket Sensor Characteristics

  5. MODIS • 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/ Sensor Characteristics

  6. MODIS (partial scene 3/6/00) Sensor Characteristics

  7. The basic spectrometer designs are extensions of the whisk broom or line scanners and the push broom scanners Types of multispectral imaging systems Spectral Line Scanners (cont’d) Sensor Characteristics

  8. 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. Sensor Characteristics

  9. Benefits of spectrometer data and the limitation of AIS as an imagerSpectral Line Scanners (con’t) Sensor Characteristics

  10. Comparison of AIS-1 and AIS-2 performance parametersSpectral Line Scanners (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 Sensor Characteristics

  11. AVIRISSpectral 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 Sensor Characteristics

  12. Spectral Line Scanners Linear array Oscillating scan mirror Telescope Aperture Diffraction grating Scan Track Ground track Sensor Characteristics

  13. Spectral Line Scanners • AVIRIS (airborne visible infrared imaging spectrometer) • MISI (Modular Imaging Spectrometer Instrument) • CASI Sensor Characteristics

  14. Conceptual layout of the AVIRIS optical systemSpectral Line Scanners (cont’d) Sensor Characteristics

  15. AVIRIS Performance characteristicsSpectral Line Scanners (cont’d) Parameter Performance Spectral coverage 0.4-2.45 Spectral sampling interval, nm 9.6-9.9 Number of spectral bands 224 IFOV, mrad 0.95 Ground IFOV, m at 20-km altitude 20 FOV, deg 30 Swath width, km at 20-km altitude 10.5 Number of cross-track pixels 614 Data encoding, bits 10 Data rate, Mbps 17 Radiometric calibration accuracy, % Absolute 6 Spectral band-to-band 0.5 Spectral calibration accuracy, nm 1-2 Sensor Characteristics

  16. AVIRIS image cube of Moffet Field, CASpectral Line Scanners (cont’d) • 224 channels • .4 m to 2.5 m • spectral bandwidth • ~10 nm (Image courtesy of NASA JPL.) Sensor Characteristics

  17. AVIRIS signal-to-noise Sensor Characteristics

  18. AVIRIS SceneLake Ontario Shoreline RochesterEmbayment May 20, 1999 Sensor Characteristics

  19. MISI (Modular Imaging Spectrometer Instrument)Spectral Line Scanners (cont’d) Sensor Characteristics

  20. Modular Imaging Spectrometer Instrument (MISI) Airborne line scanner 70 VNIR channels 5 thermal channels Nominal 2 milliradian FOV (20ft GSD at 10000ft) Sharpening bands in VIS and LWIR spectrometers thermal focal plane On-board blackbody scan mirror Sensor Characteristics

  21. MISI image of nuclear power plant discharge into Lake Ontario September 3, 1999 Three of MISI’s 70 VNIR channels thermal Sensor Characteristics

  22. Ginna Power Plant Charlotte Pier MISI Examples Irodequoit Bay Sensor Characteristics

  23. Push Broom Dispersion Systems AIS (diffraction grating) HYDICE (prism) SEBASS (prism) Hyperion (EO-1) Diffraction grating Pushbroom axis Spectral axis Collimator Slit Area arrays Optics GroundTrack

  24. HYDICE SensorPush 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. Sensor Characteristics

  25. HYDICE SensorPush Broom Dispersion Systems (con’t) 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. Sensor Characteristics

  26. HYDICE SensorPush Broom Dispersion Systems (con’t) 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. Sensor Characteristics

  27. HYDICE SensorPush Broom Dispersion Systems (con’t) Fig 2. Spectral bandwidth (FWHM) as a function of wavelength Sensor Characteristics

  28. HYDICE SensorPush Broom Dispersion Systems (con’t) 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. Sensor Characteristics

  29. HYDICE SensorPush Broom Dispersion Systems (con’t) Fig 3. Focal plane array architecture Sensor Characteristics

  30. HYDICE SensorPush Broom Dispersion Systems (con’t) 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.) Sensor Characteristics

  31. SEBASS Sensor HighlightsPush 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 Sensor Characteristics

  32. SEBASS Sensor GeometryPush 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) Sensor Characteristics

  33. Sensor Characteristics

  34. Spectral purity issues:spatial/temporal/sensor artifacts (smile) The SEBASS Sensor is a Pushbroom Scanner Sensor Characteristics

  35. Spectral purity issues:spatial/temporal/sensor artifacts (smile) Push Broom Dispersion Systems Diffraction grating Pushbroom axis Spectral axis Collimator Slit Area arrays Optics GroundTrack Sensor Characteristics

  36. Spectral purity issues:spatial/temporal/sensor artifacts (smile) Sensor Characteristics

  37. Linear Wedge Filter SpectrometerAtmospheric Corrector on EO-1 2D array wedge interference filter wedge filter side view of filter Sensor Characteristics

  38. Fourier 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. Sensor Characteristics

  39. Fourier transform instruments Fig 1. IFTS raw data cube Sensor Characteristics

  40. Fourier transform instruments 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. Sensor Characteristics

  41. Fourier transform instruments Fig 2. A sketch of the optics of an Imaging Fourier Transform Spectrometer Sensor Characteristics

  42. Fourier transform instruments 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. Sensor Characteristics

  43. Fourier transform instruments Note a major drawback of this approach is the assumption of constant FOV during motion of the mirror. Sensor Characteristics

  44. Fourier transform instruments • 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 Sensor Characteristics

  45. Fixed Mirror y f Moving Mirror Object Plane f’ Image Plane y’ Michelson 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 Sensor Characteristics

  46. Michelson Interferometer • Input spectrum changes with view angle and pointing accuracy • Collects one slice of image cube at every time interval Sensor Characteristics

  47. Aperture Mirrors Telescope focus Spherical lens Cylindrical lens Beam splitter detector Sagnac 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 Sensor Characteristics

  48. Spectral databases – mixed pixels • Lab & Field Spectra – (diffuse hemispheric- BDRF ASD) • USGS • EOS • ASTER Spectral Library • http://speclib.jpl.nasa.gov Sensor Characteristics

  49. from: ASTER Spectral Library http://speclib.jpl.nasa.gov Sensor Characteristics

  50. ASD FieldSpec 1.0 Grass Brick asphalt roofing Sensor Characteristics

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