HyTES (Hyperspectral Thermal Emission Spectrometer)
HyTES is an airborne pushbroom imaging spectrometer developed at NASA/JPL (Jet Propulsion Laboratory) and funded by the NASA Instrument Incubator Program (IIP). The HyTES instrument will be used in support of the HyspIRI (Hyperspectral Infrared Imager) mission which was recommended by the NRC (National Research Council) in their Decadal Survey in 2007. 1)
The LWIR (Long Wave Infrared) is typically expressed as the wavelength range between 7 and 14 µm. The airborne instrument under development will operate from 7.5 µm to 12 µm. Spectral information from this wavelength range is extremely valuable for Earth Science research. The LWIR component of the HyspIRI mission will address science questions in five main science themes:
• Volcanos: What are the changes in the behavior of active volcanoes? Can we quantify the trace gases (CO2) released into the atmosphere by volcanoes and estimate its impact on Earth's climate? How can we help predict and mitigate volcanic hazards?
• Wildfires: What is the impact of global biomass burning on the terrestrial biosphere and atmosphere, and how is this impact changing over time? A LWIR sensor will allow us to measure temperature, emissivity, radiative flux, burn products, hot spots, etc.
• Water use and availability: As global freshwater supplies become increasingly limited, how can we better characterize trends in local and regional water use and moisture availability to help conserve this critical resource?
• Urbanization: How does urbanization affect the local, regional and global environment? Can we characterize this effect to help mitigate its impact on human health and welfare?
• Land surface composition and change: What is the composition and temperature of the exposed surface of the Earth? How do these factors change over time and affect land use and habitability?
Thermal spectroscopy acquires both the emission from the target source object as well as reflected and/or transmitted emission from surrounding and/or foreground objects.
Figure 1: Artist's rendition of the application spectrum to be covered by HyTES imagery (image credit: CalTech)
The following objectives pertain to the HyTES instrument: 2)
• Develop a calibrated, airborne imaging spectrometer operating between 7.5 and 12 µm with 256 spectral channels
• Develop an initial overall system design
- Optical design
- Thermal/mechanical design
- Data acquisition and recording system
• Evaluate key design elements using the existing JPL QWEST (Quantum Well Earth Science Testbed), in particular 7.5-12 µm QWIP FPA (Focal Plane Array)
• Design and fabricate: Overall system, optics, detectors, grating, mechanical, thermal, data recording and storage
• Assemble, test and calibrate in the laboratory
• Complete aircraft-integration
• Deploy from an airborne platform over test sites in the western USA
• Provide data to the HyspIRI Study Group.
QWEST, the precursor instrumentation of HyTES, is an end-to-end laboratory demonstration of both the thermal Dyson spectrometer as well as of the quantum well infrared focal plane technology. The QWEST testbed is providing enabling technology for the development of a fully operational airborne platform suitable for Earth science studies. It will have sufficient spatial and spectral resolution to allow scientists to acquire the necessary data to aid in the planning of future spaceborne missions. 3) 4) 5) 6)
The HyTES airborne instrument is a hyperspectral grating spectrometer in combination with a QWIP (Quantum Well Infrared Photodetector) and grating based spectrometer. The basic specifications of QWEST and HyTES are provided in Table 1 and an illustration is shown in Figure 2. Both systems will use large format detectors and have large spatial swath widths. The current optical design and grating works for the entire 7.5 - 12 µm spectral range but the existing QWIP FPA which is being used for preliminary testing in QWEST is only sensitive from 7.5-9.5 µm.
Table 1: Overview of instrument performance parameters
The system uses a mechanically cooled system consisting of a vacuum chamber, cryocoolers, thermal radiation shields, telescope/relay, and a spectrometer.
The HyTES instrument assembly is scheduled for completion in 2011.
Figure 2: Schematic view of the HyTES instrument assembly and its elements (image credit: NASA/JPL)
Figure 3: Mechanical layout of the HyTES instrument (image credit: NASA/JPL)
Figure 4: Illustration of the HyTES instrument scanhead (image credit: NASA/JPL)
Figure 5: Block diagram of the HyTES instrument assembly (image credit: NASA/JPL)
Optical design: HyTES is a pushbroom imaging spectrometer based on the Dyson optical configuration. The Dyson design allows for a very compact and optically fast system (f/1.6).
A single monolithic block is used in double pass where light from the slit enters at a narrow optical passageway and is transmitted through the rear power surface, diffracts off the grating and re-enters the block to totally internally reflect off the back surface which guides the spectrally dispersed radiation to focus at the QWIP location. This design minimizes the travel and form factor of the system. The actual block fabricated is shown in Figure 6. Broadband area coatings are used on all applicable light transmitting surfaces. The coatings allow 99.5% or better EOTIR (Earth Observing Thermal Infrared) light to transmit.
The block was fabricated from ZnSe, a robust material with a transparent wavelength region from 0.4 ~ 23 µm and an absorption coefficient between 10-3 and 10-4 cm-1. The ZnSe slab is produced by chemical vapor deposition.
Figure 6: HyTES conceptual layout of the Dyson spectrometer and the objective lens elements (image credit: NASA/JPL)
The telescope is an all-aluminum, diamond-turned, athermal. “snap-together” design featuring several accessories like: aperture baffle, 2 internal baffles, real slot baffle, and a precision made large shim.
Figure 7: The HyTES telescope (right), left is a cutaway view (image credit: NASA/JPL)
Figure 8: Telescope design provides a direct connection to the relay housing (image credit: NASA/JPL)
As shown in Figure 9, a single monolithic block is used in double pass where light from the slit enters at a narrow optical passageway and is transmitted through the rear power surface, diffracts off the grating and re-enters the block to totally internally reflect off the back surface which guides the spectrally dispersed radiation to focus at the QWIP location. This design minimizes the travel and form factor of the system.
Figure 9: Conceptual layout of the Dyson spectrometer and objective lens elements (image credit: NASA/JPL)
Figure 10: Photo og the HyTES spectrometer block (right) with the optical prescription (left), image credit: NASA/JPL
The slit was made using RIE (Reactive Ion Etching) of silicon nitride (Si3N4) films formed by LPCVD (Low Pressure Chemical Vapor Deposition).
QWIP array: The QWIP technology utilizes the photoexcitation of electrons between the ground state and the first excited state in the conduction band QW (Quantum Well). QWIPs have been successfully integrated into commercial handheld field units for more than a decade. This is the first integration of the QWIP with a spectrometer system for earth science studies requiring accurately calibrated data.
The detector pixel pitch of the FPA is 25 µm and the actual pixel area is 23 x 23 µm. Indium bumps were evaporated on top of the detectors for hybridization with a silicon ROIC (Readout Integrated Circuit). These QWIP FPAs were hybridized (via indium bump-bonding process) to a 640 x 512 pixel CMOS (Complementary Metal-Oxide Semiconductor) ROIC and biased at VB = –1.25 V. At temperatures below 72 K, the SNR (Signal-to-Noise Ratio) of the system is limited by array nonuniformity, readout multiplexer (i.e., ROIC) noise, and photocurrent (photon flux) noise. At temperatures above 72 K, the temporal noise due to the dark current becomes the limitation.
The project is currently running the system at 40 K to obtain a SNR advantage. The QWIP is known for its high spatial uniformity (<0.51%). This is a clear advantage over other detector technologies such as HgCdTe and InSb. A custom made LCC (Leadless Chip Carrier ) and titanium FPA clamp was designed to accommodate the close proximity of the FPA with the ZnSe block.
FPA (Focal Plane Array) fabrication:
• A cross section of the two-band QWIP (Quantum Well Infrared Photodetector) is used as the detector material is shown in Figure 11.
• Roughly half of the array will respond in the 7.5-10 µm range, and the other half will cover the 10-12 µm range.
Figure 11: HyTES pixel design - cross-sectional zoom at the transition point showing two bands with ¼ λ gratings on each (image credit: NASA/JPL)
Figure 12: QWIP and custom made clamp assembly to hold QWIP and LCC (image credit: NASA/JPL)
Diffraction grating: The diffraction grating design and fabrication is a key enabling technology for these spectrometers. JPL has developed electron-beam lithography techniques that allow fabrication of precisely blazed gratings on curved substrates having several millimeters of height variation. Gratings fabricated in this manner provide high efficiency combined with low scatter. The blazed grating for this LWIR Dyson spectrometer was fabricated in a thin layer of PMMA (Polymethyl Methacrylate) electron-beam resist coated on a diamond-turned concave ZnSe substrate. After exposure and development to the desired blaze angle, the resist was overcoated with gold for maximum infrared reflectance. A photograph of the grating and the simulated efficiency of the fabricated grating are shown in Figure 13, respectively. The design was optimized for maximum efficiency in the -1 order, and the other orders remain relatively weak across the band.
Figure 13: QWEST spectrometer grating (image credit: NASA/JPL)
Legend to Figure 13: On the left side is the photograph of the fabricated grating (annular E-beam focus zones are visible due to slight variation in scattering; unexposed rectangular areas near edge are due to the E-beam mount), the right side shows the simulated efficiency (calculated using PCGrate 6.1 software).
The HyTES predicted NEDT (Noise Equivalent Delta Temperature) is shown in Figure 14. A spectral calibration was performed using narrowband interference filters. This is an easy way to determine the position of the spectral bands and verify the full width at half maximum. For radiometric performance, a NIST (National Institute of Standards and Technology) traceable transfer calibration is performed on our electro-optic blackbody to verify its performance between the two end bracket temperatures of 4ºC and 40ºC. JPL has multiple NIST traceable blackbodies with a stability at 25ºC of ± 0.0007ºC and a thermistor standard probe with an accuracy of0.0015ºC over 0-60ºC and stability/yr of 0.005.
The data of Figure 14 shows that QWEST has very good linearity with many temperature measurements showing absolute errors below 0.1ºC. The NEDT for spectral channels at blackbody temperatures between 5ºC and 30ºC is measured as predicted. This implies that for a given temperature between this range QWEST has a mean NEDT of 124.7 mK.
Figure 14: Predicted NEDT for single pixel and 2 x 2 binned effective pixel. The performance meets expectation (image credit: NASA/JPL)
Table 2: HyTES instrument parameters (Ref. 6)
Figure 15: HyTES system electronics, data and control (image credit: NASA/JPL)
• Raw data from FPA with interleaved GPS and engineering data (L0)
• Geo-located radiance at sensor (L1)
• Future derived products:
- Surface radiance
- Surface emissivity
- Surface temperature.
• First flight of HyTES instrument in July 2012 (Ref. 6). HyTES successfully completed its first test flights on board a Twin Otter aircraft flown between Grand Junction, Colorado and Burbank, California in the week of July 16, 2012. Initial engineering data were acquired near Grand Junction. Based on an evaluation of those datasets, science datasets were acquired between Grand Junction and Burbank. They include acquisitions over different surface types and gas emissions (sulfur dioxide, methane, ammonia) to determine whether HyTES can be used for gas studies. 7)
Figure 16: Twin Otter 300 series with nadir view port (image credit: NASA/JPL)
Platform: Twin Otter (Ref. 6)
- Flight altitude range: 2000 - 4000 m
- Frame rate: variable 15 to 30 fps
- Swath width: 1.8 km - 3.6 km
- Pixel size: 2.9 - 5.8 m
Operator has console to monitor instrument status, change frame rate, start and stop acquisition. Data are stored on removable hard drives.
1) William R. Johnson, Simon J. Hook, Pantazis Mouroulis, Daniel W. Wilson, Sarath D. Gunapala, Vincent Realmuto, Andy Lamborn, Chris Paine, Jason M. Mumolo, Bjorn T. Eng, “HyTES: Thermal Imaging Spectrometer Development,” 2011 IEEE Aerospace Conference, Big Sky, MT, USA, March 5-12, 2011, URL: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/42293/1/11-0765.pdf
2) Simon J. Hook, Bjorn T. Eng, Sarath D. Gunapala, Cory J. Hill, William R. Johnson, Andrew U. Lamborn, Pantazis Mouroulis, Jason M. Mumolo, Christopher G. Paine, Vincent J. Realmuto, Daniel W. Wilson, “QWEST and HyTES: Two New Hyperspectral Thermal Infrared Imaging Spectrometers for Earth Science,” ESTF (Earth Science Technology Forum) 2010, Arlington, Va, USA, June 22-24, 2010, URL of presentation: http://esto.nasa.gov/conferences/estf2010/presentations/Eng_Hook_ESTF2010_B4P2.pdf, URL of paper: http://esto.nasa.gov/conferences/estf2010/papers/Eng_for_Hook_Simon_ESTF2010.pdf
3) William R. Johnson, Simon J. Hook, Pantazis Mouroulis, Daniel W. Wilson, Sarath D. Gunapala, Cory J. Hill, Jason M. Mumolo, Bjorn T. Eng, “Quantum well earth science testbed,” Infrared Physics & Technology, Volume 52, Issue 6, Nov. 2009, pp. 430-433
4) William R. Johnson, Simon J. Hook, Pantazis Mouroulis, Daniel W. Wilson, Sarath D. Gunapala, Cory J. Hill, Jason M. Mumolo, Vincent Realmuto, Bjorn T. Eng, “Towards HyTES: an airborne thermal imaging spectroscopy instrument,” Proceedings of SPIE, 'Imaging Spectrometry XIV,' Sylvia S. Shen; Paul E. Lewis, Editors, Vol. 7457, August 2009
5) Simon Hook, William R. Johnson, Bjorn T. Eng, Sarath D. Gunapala, Andrew U. Lamborn, Pantazis Z. Mouroulis, Christopher G. Paine, Alexander Soibel, Daniel W. Wilson, “The Hyperspectral Thermal Emission Spectrometer (HyTES): preliminary results,” 2011, URL: http://esto.nasa.gov/conferences/estf2011/papers/hook_hytes_estf2011.pdf
6) Simon Hook & The HyspIRI/HyTES/PHyTIR Team, “First flight of the Hyperspectral Thermal Emission Spectrometer (HyTES) airborne instrument,” 2012 HyspIRI Science Workshop, NASA Decadal Survey Mission, Washington, DC, USA, Oct. 16-18, 2012, URL: http://hyspiri.jpl.nasa.gov/downloads/2012_Workshop/day3/15_Hook-2012-10-17-HyTES.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.