Northrop Grumman Space Technology (NGST) has developed and tested a Long-wave Hyperspectral Imaging Spectrometer (LWHIS) that operates in the 8 to 12.5 micron band. An overview of the system design has been described elsewhere. This paper describes the system characterization and radiometric calibration of this instrument using NGST’s Long-wave Hyperspectral Test Facility which uses a 1375K globar source assembly, a monochromator, a collimator and a fine pointing mirror to provide image quality and FPA alignment data. Image quality characterization results presented here include measurement of the instrument’s Modulation Transfer Function (MTF), spatial co-registration of spectral channels (spectral smile), cross-track spectral error (spatial smile), and spectral calibration. Radiometric calibration results for laboratory targets are also presented.
Northrop Grumman Space Technology (NGST), using internal funding, has designed, built and is testing a Long Wave Hyperspectral Imaging Spectrometer (LWHIS) that operates in the 8 to 12.5 micron band. This instrument was designed to be compatible with aircraft platforms so that flight data in this wavelength band can be used for phenomenological analysis. The instrument provides up to 256 contiguous spectral channels with 17 nm of dispersion per pixel (pixels are binned in normal operation to provide 128 spectral channels). The entrance aperture is 3.5 cm and feeds a F2/5 reflective triplet front end. The focal plane is a 256 x 256 array of 40 micron pixels which can be binned to form an 80 micron superpixel. With a fixed frame rate of 60 Hz, the instrument provides a ground sample distance of 1m at 1.1km altitude. This paper describes the physical characteristics of the design and presents the predicted performance based on NGST internal models. Design trades and test data will be presented. A more detailed look at the characterization and calibration of this instrument will be presented in a companion paper "Long Wave Hyperspectral Imaging Spectrometer -- System Characterization and Calibration."
The end-to-end calibration plan for the Hyperion EO-1 hyperspectral payload is presented. The ground calibration is traceable to a set of three high quantum efficiency p-n silicon photodiode trap detectors the responsivities of which are traceable absolutely to solid state silicon diode physical laws. An independent crosscheck of the radiance of the Calibration Panel Assembly used to flood the Hyperion instrument in field and aperture was made with a transfer radiometer developed at TRW. On-orbit measurements of the sun's irradiance as it illuminates a painted panel inside the instrument cover are compared to the radiance scale developed during pre-flight calibration. In addition, an on-orbit calibration lamp source is observed to trace the pre-flight calibration constants determined on the ground to the solar calibration determination.
The LANDSAT-7 ETM+ spectral bands centered at 479nm, 561 nm, 661 nm and 834 nm (bands 1, 2, 3, and 4) fall nicely across the Hyperion VNIR hyperspectral response region. They have bandwidths of 67nm, 78nm, 60 nm and 120 nm, respectively. The Hyperion spectral bandwidth of 10.2 nm results in 10 to 15 Hyperion spectral samples across each Landsat band in the VNIR. When the Hyperion spectral responses in the 10.2 nm bands are properly weighted to aggregate to a given Landsat band, the radiometric response of the Landsat band can be reproduced by Hyperion. Landsat bands 5 and 7 centered at 1650 and 2207 nm (with bandwidths of 190 and 250 nm respectively) fall in the Hyperion SWIR spectral response region. Hyperion spectral response for one area of a scene in Railroad Valley, NV on May 13, 2001 has been binned into Landsat bands and compared with Landsat values collected at the same time.
The Hyperion instrument mounted on the EO-1 spacecraft was launched November 21, 2000 into an orbit following LANDSAT-7 by 1 minute. Hyperion has a 7.5 km swath width, a 30 meter ground resolution and 10 nm spectral resolution extending from 400 nm to 2500 nm. The first portion of the mission was used to measure and characterize the on-orbit radiometric, spectral, image quality and geometric performance of the instrument. Lake Frome, a dry salt lake in South Australia was chosen as a calibration site for Hyperion. Surface spectral data were collected along a transect through the center of the lake prior to the Hyperion overpass. This paper discusses the incorporation of the Lake Frome ground measurements and analysis into the performance verification of the instrument.
KEYWORDS: Calibration, Space operations, Short wave infrared radiation, Sensors, Reflectivity, Image processing, Spectroscopy, Sun, Signal to noise ratio, Solids
The Moon provides an exo-atmospheric radiance source that can be used to determine trends in instrument radiometric responsivity with high precision. Lunar observations can also be used for absolute radiometric calibration; knowledge of the radiometric scale will steadily improve through independent study of lunar spectral photometry and with sharing of the Moon as a calibration target by increasing numbers of spacecraft, each with its own calibration history. EO-1 calibration includes periodic observation of the Moon by all three of its instruments. Observations are normally made with a phase angle of about 7 degrees (or about 12 hours from the time of Full Moon). Also, SeaWiFS has been making observations at such phase angles for several years, and observations of the Moon by instrument pairs, even if at different times, can be used to transfer absolute calibration. A challenge for EO-1 is pointing to include the entire full Moon in the narrow Hyperion scan. Three Hyperion observations in early 2001 covering an order-of-magnitude difference in lunar irradiance show good agreement for responsivity; The SWIR detector has undergone some changes in responsivity. Small discrepancies of calibration with wavelength could be smoothed using the Moon as a source. Off-axis scattered light response and cross-track response variations can be assessed using the lunar image.
The Hyperion Imaging Spectrometer is one of three principal instruments aboard the EO-1 spacecraft. Its mission as a technology demonstrator is to evaluate on-orbit issues for imaging spectroscopy and to assess the capabilities of a space-based imaging spectrometer for earth science and earth observation missions. The instrument provides earth imagery at 30 meter spatial resolution. 7.5 km swath width in 220 contiguous spectral bands at 10 nm spectral resolution. Spectral range is from 0.4 micrometers to 2.5 micrometers . The instrument includes internal and solar calibration sub- systems. This paper will review the design, construction and calibration of the Hyperion instrument. The on-orbit plans and operations will be presented along with updated calibration and characterization measurements.
This paper describes the calibration transfer path from primary standards representing fundamental physical quantities through the calibration radiance source used in Hyperion instrument level absolute calibration. The calibration transfer path and hardware design of the primary and secondary standards and their validation for end-to-end calibration of the sensor are presented. The primary standards reside at the TRW Radiometric Scale Facility and include two high quantum efficiency Silicon photodiode trap detectors; an electrically self-calibrated pyroelectric detector serves as a secondary standard for crosscheck. The end-to-end sensor calibration is accomplished with a Calibration Panel Assembly (CPA) source, which is illuminated by a NIST traceable FEL 1000 transfer standard lamp. An independent crosscheck of the Spectralon reflectance properties is made with a transfer radiometer. An error analysis of the transfer path is presented. The basic strategy of the Hyperion end-to-end calibration is to reduce the size of the sensor responsivity error tree and to provide control of systematic errors as much as possible through cross-calibration.
This paper presents the techniques and results of Hyperion laboratory characterization. Hyperion is a hyperspectral imager scheduled to fly on the Earth-Orbiter 1 (EO-1) spacecraft for the New Millennium project. The other payloads on the spacecraft are ALI (Advanced Land Imager) and AC (atmospheric corrector). The payloads were integrated into the spacecraft at Goddard Space Flight Center (GSFC). An End-to-End imaging test was conducted at GSFC which demonstrated integrity of Hyperion performance after environmental tests. The performance characterization procedures described here include: crosstrack MTF, spectral and spatial co-alignment, spectral wavelength calibration, signal to noise, polarization, spectral response function and scene generation. The characterization was carried out with the TRW Imaging Spectrometer Characterization Facility which is based on a 250 watt QTH lamp, a monochromator, a collimator and a fine pointing mirror. A selection of narrow slits and a knife edge are illuminated at the exit slit of the monochromator for sub-pixel performance characterization parameters such as MTF. Special attention is devoted to the spectral calibration technique using rare earth doped Spectralon panels. This was the technique used at the End-to-End test to verify spectral performance of Hyperion after GSFC environmental tests. It is a particular useful technique when the optical test setup does not allow for the use of a monochromator.
The TRW Imaging Spectrometer III airborne hyperspectral imager was competed in 1996. The spectrometer is a pushbroom sensor that gathers information in 384 contiguous spectral channels covering the 400nm to 2450nm wavelength range. TRWIS III was designed to fly on many different aircraft platforms and to meet critical performance requirements for image quality, co-registration of spectral samples, spectral calibration, noise and radiometric accuracy. Along with its first several seasons of operational demonstrations, the instrument has undergone laboratory performance validation, radiometric calibration, and system upgrades. This paper will describe the current TRWIS III system, the data calibration and correction system, and the instrument's applications to remote sensing.
The tremendous potential for hyperspectral imagery as a remote sensing tool has driven the development of TRW's TRWIS III hyperspectral imager. This instrument provides 384 contiguous spectral channels at 5 nm to 6.25 nm spectral resolution covering the 400 nm to 2450 nm wavelength range. The spectra of each pixel in the scene are gathered simultaneously at signal to noise ratios of several hundred to one for typical Earth scenes. Designed to fly on a wide range of aircraft and with variable frame rate, the ground resolution can be varied from approximately 50 cm to 11 m depending on the aircraft altitude and speed. Meeting critical performance requirements for image quality, co- registration of spectral samples, spectral calibration, noise, and radiometric accuracy are important to the success of the instrument. TRWIS III performance has been validated and the instrument has been radiometrically calibrated using TRW's Multispectral Test Bed. This paper discusses the characterization and calibration process and results of the measurements. An example of results from a flight at the end of 1996 is included.
The clouds and the Earth's radiant energy system (CERES) program continues the long term monitoring of the Earth's radiant energy budget begun by the Earth Radiation Budget Experiment (ERBE) scanning radiometer instruments. The CERES instrument contains three thermal detector based radiometers with broadband spectral responses. The relative spectral responses must be characterized at far infrared wavelengths out to 200 micrometers in support of absolute radiometric calibration. This will be accomplished with a Fourier transform spectrometer as a spectral source, relay optics and a vacuum chamber containing the sensors. This facility currently under development for the CERES program will measure end-to-end sensor spectral response relative to a spectrally flat well characterized reference detector also located in the vacuum chamber. Facility design and controls on the measurement process to assure spectral accuracy are discussed.
The clouds and the earth's radiant energy system (CERES) experiment will provide consistent data bases of radiation and cloud fields. The CERES instrument consists of a scanning thermistor bolometer package with built-in flight calibration systems. Two bolometer packages will be launched on the earth observing system (EOS) platforms to measure the Earth/atmosphere-reflected solar shortwave and Earth/atmosphere-emitted long wave radiances with measurement accuracy goals approaching 1.0% and 0.5%, respectively. In each package, there are three different bolometers. All bolometers will be calibrated in a unique TRW vacuum facility equipped with blackbodies, a cryogenically cooled active-cavity radiometer, shortwave sources, and other specialized calibration devices. The blackbodies are tied to the International Temperature Scale of 1990 (ITS'90). Using math models, the calibration measurements will define the instrument filtered gains and offsets. This paper outlines the CERES instrument design and radiometric calibrations.
Stability of the solar constant makes the Sun an attractive on-orbit calibration source for radiometers operating at visible and near IR wavelengths. Direct viewing of the Sun provides a radiance or irradiance that is significantly above the dynamic range of most earth observing system (EOS) radiometers, thereby requiring attenuated viewing of the Sun. To provide radiometric repeatability, the attenuator used must be stable over time at all in-band wavelengths, uniformly flood the radiometer aperture and field of view, and be invariant over the range of solar viewing angles. The Earth Radiation Budget Experiment (ERBE) radiometers flown in the mid-1980s carried a mirror attenuator mosaic (MAM) to attenuate the solar energy. This device, consisting of specularly reflective, closely packed concave hemispheres with a black mask covering the area between the spheres, was successfully used to calibrate the ERBE shortwave (0.3 to 3.5 micrometers ) and total (0.3 to > 50 micrometers ) radiometer channels. For CERES, the calibration accuracy requirements have been tightened (+/- 1% shortwave, +/- 0.5% total channel, end-of-life, 1 (sigma) ). While the stability and uniformity demonstrated by the ERBE MAM are sufficient for CERES, the variation with solar incidence angle is not. Improvements to the ERBE design have been made for CERES and sample MAMs have been fabricated and tested. The results of this study as well as the features and details of the MAM design are addressed.
Enhancements which have been made in the Radiometric Calibration Facility (RCF) of the CERES payload are described. These include narrow field blackbody and wide field of view blackbody sources, an active cavity radiometer, and a shortwave reference source. They permit the RCF to calibrate the CERES instruments to better than +/- 0.5 percent absolute radiometric accuracy in the 3.5 to above 50 micron wavelength region and to +/- 1.0 percent in the 0.3-3.5 micron region.
The Cloud and the Earth's Radiant Energy System (CERES) program continues the long term monitoring of the Earth's energy budget begun by the Earth Radiation Budget Experiment (ERBE) scanning radiometer instruments. The radiometic ground calibration sources employed for ERBE were designed to cover the very large (all Earth) field of view of the non-scanning radiometers. The ERBE radiometer ground and flight calibration proved to be more accurate than the requirement. The ground calibration sources to be used for CERES will be optimally designed to calibrate the much more narrow field of view of the scanning radiometer to improve on the absolute calibration performance. In addition, the shortwave calibration will be made in narrow bands to eliminate uncertainty in the spectral shape of the shortwave calibration source. Each shortwave band will be absolutely calibrated by transfer to a blackbody using a cryogenic active cavity radiometer fitted with the same telescope optics as the CERES radiometers.
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