Digital Imaging and Remote Sensing Laboratory Sensor Characteristics

Digital Imaging and Remote Sensing Laboratory 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.

Digital Imaging and Remote Sensing Laboratory 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.

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

Digital Imaging and Remote Sensing Laboratory 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

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 6 MODIS (partial scene 3/6/00)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 7 Types 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

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 8 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 and respectively.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 9 Benefits of spectrometer data and the limitation of AIS as an imager Benefits of spectrometer data and the limitation of AIS as an imager Spectral Line Scanners (con’t)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 10 Comparison of AIS-1 and AIS-2 performance parameters (cont’d) Comparison of AIS-1 and AIS-2 performance parameters Spectral Line Scanners (cont’d) IFOV, mrad Ground IFOV, m at 6-km altitude FOV, deg Swath width, m at 6-km altitude Spectral sampling interval, nm Data rate, kbps Spectral sampling Short-wavelength mode,  m Long-wavelength mode,  m

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 11 AVIRIS AVIRIS 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

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 12 Spectral Line Scanners Linear array Diffraction grating Aperture Telescope Oscillating scan mirror Scan Track Ground track

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 13 Spectral Line Scanners AVIRIS ( airborne visible infrared imaging spectrometer) MISI (Modular Imaging Spectrometer Instrument) CASI

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 14 Conceptual layout of the AVIRIS optical system (cont’d) Conceptual layout of the AVIRIS optical system Spectral Line Scanners (cont’d)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 15 AVIRIS Performance characteristics (cont’d) AVIRIS Performance characteristics Spectral Line Scanners (cont’d) Spectral coverage Spectral sampling interval, nm Number of spectral bands 224 IFOV, mrad0.95 Ground IFOV, m at 20-km altitude20 FOV, deg30 Swath width, km at 20-km altitude10.5 Number of cross-track pixels614 Data encoding, bits10 Data rate, Mbps17 Radiometric calibration accuracy, % Absolute6 Spectral band-to-band0.5 Spectral calibration accuracy, nm1-2 Parameter Performance

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 16 AVIRIS image cube of Moffet Field, CA (cont’d) AVIRIS image cube of Moffet Field, CA Spectral Line Scanners (cont’d) 224 channels.4  m to 2.5  m spectral bandwidth ~10 nm (Image courtesy of NASA JPL.)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 17 AVIRIS signal-to-noise

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 18 AVIRIS Scene Lake Ontario Shoreline Rochester Embayment May 20, 1999

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 19 MISI (Modular Imaging Spectrometer Instrument) (cont’d) MISI (Modular Imaging Spectrometer Instrument) Spectral Line Scanners (cont’d)

Digital Imaging and Remote Sensing Laboratory 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 scan mirror On-board blackbody

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 21 thermal MISI image of nuclear power plant discharge into Lake Ontario September 3, 1999 Three of MISI’s 70 VNIR channels

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 22 MISI Examples Irodequoit Bay Charlotte Pier Ginna Power Plant

Digital Imaging and Remote Sensing Laboratory Push Broom Dispersion Systems Pushbroom axis Spectral axis Area arrays Diffraction grating Collimator Slit Optics Ground Track AIS (diffraction grating) HYDICE (prism) SEBASS (prism) Hyperion (EO-1)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 24 HYDICE Sensor HYDICE 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 25 HYDICE Sensor HYDICE Sensor Push 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 26 HYDICE Sensor HYDICE Sensor Push 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 27 HYDICE Sensor HYDICE Sensor Push Broom Dispersion Systems (con’t) Fig 2. Spectral bandwidth (FWHM) as a function of wavelength

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 28 HYDICE Sensor HYDICE Sensor Push Broom Dispersion Systems (con’t) The wide spectral range from 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 29 HYDICE Sensor HYDICE Sensor Push Broom Dispersion Systems (con’t) Fig 3. Focal plane array architecture

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 30 HYDICE Sensor HYDICE Sensor Push 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.)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 31 SEBASS Sensor Highlights SEBASS Sensor Highlights 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

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 32 SEBASS Sensor Geometry SEBASS Sensor Geometry 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 and feet respectively 1 milliradian per pixel IFOV (~7 degrees FOV)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 33

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 34 Spectral purity issues: spatial/temporal/sensor artifacts (smile) The SEBASS Sensor is a Pushbroom Scanner

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 35 Pushbroom axis Spectral axis Area arrays Diffraction grating Collimator Slit Optics Ground Track Spectral purity issues: spatial/temporal/sensor artifacts (smile) Push Broom Dispersion Systems

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 36 Spectral purity issues: spatial/temporal/sensor artifacts (smile)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 37 Linear Wedge Filter Spectrometer Atmospheric Corrector on EO-1 wedge filter 2D array wedge interference filter side view of filter

Digital Imaging and Remote Sensing Laboratory 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 39 Fourier transform instruments Fig 1. IFTS raw data cube

Digital Imaging and Remote Sensing Laboratory 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 41 Fourier transform instruments Fig 2. A sketch of the optics of an Imaging Fourier Transform Spectrometer

Digital Imaging and Remote Sensing Laboratory 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.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 43 Fourier transform instruments Note a major drawback of this approach is the assumption of constant FOV during motion of the mirror.

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 44 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 Fourier transform instruments

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 45 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 Object Plane Image Plane Fixed Mirror Moving Mirror y f f’ y’

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 46 Michelson Interferometer Input spectrum changes with view angle and pointing accuracy Collects one slice of image cube at every time interval

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 47 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 Mirrors Spherical lens Cylindrical lens Beam splitter Aperture Telescope focus detector

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 48 Spectral databases – mixed pixels Lab & Field Spectra – (diffuse hemispheric- BDRF ASD) USGS EOS ASTER Spectral Library

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 49 from: ASTER Spectral Library

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 50 Grass asphalt roofing Brick 1.0 ASD FieldSpec

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 51 BRDF 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.)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 52 BRDF (cont’d)

Digital Imaging and Remote Sensing Laboratory Sensor Characteristics 53 Sensor Light trap specular ray sample Incident flux Integrating Sphere Schematic concept for measuring total and diffuse reflectance