J. Pearlman

Hyperion, a space-based imaging spectrometer

The Hyperion Imaging Spectrometer was the first imaging spectrometer to routinely acquire science-grade data from Earth orbit. Instrument performance was validated and carefully monitored through a combination of calibration approaches: solar, lunar, earth (vicarious) and atmospheric observations complemented by onboard calibration lamps and extensive prelaunch calibration. Innovative techniques for spectral calibration of space-based sensors were also tested and validated. Instrument performance met or exceeded predictions including continued operation well beyond the planned one-year program.

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.

Estimation and validation of land surface broadband albedos and leaf area index from EO-1 ALI data

The Advanced Land Imager (ALI) is a multispectral sensor onboard the National Aeronautics and Space Administration Earth Observing 1 (EO-1) satellite. It has similar spatial resolution to Landsat-7 Enhanced Thematic Mapper Plus (ETM+), with three additional spectral bands. We developed new algorithms for estimating both land surface broadband albedo and leaf area index (LAI) from ALI data. A recently developed atmospheric correction algorithm for ETM+ imagery was extended to retrieve surface spectral reflectance from ALI top-of-atmosphere observations. A feature common to these algorithms is the use of new multispectral information from ALI. The additional blue band of ALI is very useful in our atmospheric correction algorithm, and two additional ALI near-infrared bands are valuable for estimating both broadband albedo and LAI. Ground measurements at Beltsville, MD, and Coleambally, Australia, were used to validate the products generated by these algorithms.

Millimeter-wave imaging using preamplified diode detector

A 2-pixel imaging array was developed to demonstrate millimeter-wave imaging. Each pixel consists of a Q-band Vivaldi antenna and a preamplified diode detector, using InGaAs pseudomorphic HEMT MMIC low-noise amplifiers (LNAs) and a beam-lead Schottky-diode detector. The approach does not require local oscillator (LO) power, is compatible with MMIC technology, and can reduce the complexity and manufacturing cost of millimeter-wave imaging arrays. The preamplified diode detector exhibited 17-V/ mu W responsivity at 44 GHz and -75-dBm tangential sensitivity at 1-MHz video bandwidth. The array demonstrated millimeter-wave imaging of three vehicles in a parking lot. >

Improving the analysis of hyperion red-edge index from an agricultural area

The benefits of EO-1 data, and especially Hyperion hyperspectral data, are being studied at sites in the Coleambally Irrigation Area of Australia where a seasonal time series has been developed. Hyperion can provide effective measures of agricultural performance through the use of spectral indices if systematic and random noise is managed and such noise management methods have been established for Coleambally. Among the sources of noise specific to Hyperion is the spectral “smile” which affects the location of the red-edge -- an important index in agricultural assessment. We show how this phenomenon, which arises from the pushbroom technology of Hyperion, affects the data and discuss how its effects can be overcome to provide stable and accurate measures of the red-edge and related indices. HyMap airborne data are used to evaluate the results of the methods studied. This paper also shows how future pushbroom instruments should consider the wavelength sampling step in their design if it is intended to remove the “smile” effects by a systematic software processing.

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.