Peter J. Jarecke

Overview of the Hyperion Imaging Spectrometer for the NASA EO-1 mission

The New Millennium Program (NMP) is an initiative to demonstrate advanced technologies and designs that show promise for dramatically reducing the cost and improving the quality of instruments and spacecraft for future space missions. The EO-1 platform hosts the Advanced Land Imager (ALI), the Hyperion Imaging Spectrometer and the LEISA Atmospheric Corrector (LAC) payloads. It was launched on November 24, 2000 and is now in an orbit one minute behind Landsat 7. Hyperion has a 7.5 km swath width, a 30 meter ground resolution and 10 nm spectral resolution. The initial mission for Hyperion is to measure and characterize on-orbit performance as thoroughly as possible and to compare with ground acceptance test data. This will be followed by activities of the EO-1 Science validation team to assess the utility of space-based hyperspectral data. This paper gives an overview of the technical innovation and on-orbit characterization scope for the EO-1 Hyperion Instrument and planned operations.

EO-1/Hyperion Hyperspectral Imager Design, Development, Characterization, and Calibration

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.

Performance characterization of the Hyperion Imaging Spectrometer instrument

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.

Radiometric calibration transfer chain from primary standards to the end-to-end Hyperion sensor

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.

EO-1 Hyperion hyperspectral aggregation and comparison with EO-1 Advanced Land Imager and Landsat 7 ETM+

The EO-1 spacecraft, launched November 21, 2000 into a Sun-synchronous orbit behind Landsat 7, hosts advanced technology demonstration instruments, whose capabilities are currently being assessed by the user community for future missions. A significant part of the program is to perform data comparisons between the Hyperion Imaging Spectrometer and Advanced Land Imager (ALI) payloads on the EO-1 spacecraft with the Landsat 7 ETM+ sensor. To perform the absolute radiometric comparison, an aggregation method was developed and used to combine the proper portions of the Hyperion 10 nm bands to emulate the broader multispectral response of the ALI and ETM+. The aggregation capability allows radiometric comparisons between different instruments with different spectral response functions. This paper reviews the aggregation methodology, and presents absolute comparison results. A demonstration of the replication of portions of ALI and ETM+ scenes through a synthesis of Hyperion hyperspectral data is also provided. The capability of synthesizing broad band data from a hyperspectral sensor will enable users to test the selection of broad bands for future missions by using various combinations of Hyperion bands.

Initial lunar calibration observations by the EO-1 Hyperion imaging spectrometer

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.

Aggregation of Hyperion hyperspectral spectral bands into Landsat-&ETM+ spectral bands

The Landsat 7 ETM+ spectral bands centered at 479 nm, 561 nm, 661 nm and 834 mn (Bands 1, 2, 3, and 4) fall nicely across the Hyperion VNIR hyperspectral response region. They have bandwidths of 67 nm, 78 nm, 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. This is done for Bands 2, 3 and 4 on the scene 7 of Lake Frome, Australia collected simultaneously by Hyperion and Landsat on January 21, 2001. The initial comparison of the radiances from Hyperion synthesized into the ETM+ bands with the Landsat-7 ETM+ radiances showed differences of 15-23%, with ETM+ being higher. Prior to launch a laboratory standard comparison showed differences of 10-13% in the same direction.

Radiometric calibration validation of the Hyperion instrument using ground truth at a site in Lake Frome, Australia

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.

Updated results from performance characterization and calibration of the TRWIS III hyperspectral imager

The tremendous potential for hyperspectral imagery as a remote sensing tool has driven the development of TRWs 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 TRWs 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.

On-orbit solar radiometric calibration of the Hyperion instrument

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 suns 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.