Gregg Vane

Imaging Spectrometry for Earth Remote Sensing

Imaging spectrometry, a new technique for the remote sensing of the earth, is now technically feasible from aircraft and spacecraft. The initial results show that remote, direct identification of surface materials on a picture-element basis can be accomplished by proper sampling of absorption features in the reflectance spectrum. The airborne and spaceborne sensors are capable of acquiring images simultaneously in 100 to 200 contiguous spectral bands. The ability to acquire laboratory-like spectra remotely is a major advance in remote sensing capability. Concomitant advances in computer technology for the reduction and storage of such potentially massive data sets are at hand, and new analytic techniques are being developed to extract the full information content of the data. The emphasis on the deterministic approach to multispectral data analysis as opposed to the statistical approaches used in the past should stimulate the development of new digital image-processing methodologies.

The airborne visible/infrared imaging spectrometer (AVIRIS)

Abstract The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is a facility consisting of a flight system, a ground data system, a calibration facility, and a full-time operations team. The facility was developed by the Jet Propulsion Laboratory (JPL) under funding from the National Aeronautics and Space Administration (NASA). NASA also provides funding for operations and maintenance. The flight system is a whisk-broom imager that acquires data in 224 narrow, contiguous spectral bands covering the solar reflected portion of the electromagnetic spectrum. It is flown aboard the NASA high altitude ER-2 research aircraft. The ground data system is a facility dedicated to the processing and distribution of data acquired by AVIRIS. It operates year round at JPL. The calibration facility consists of a calibration laboratory at JPL and a suite of field instruments and procedures for performing inflight calibration of AVIRIS. A small team of engineers, technicians and scientists supports a yearly operations schedule that includes 6 months of flight operations, 6 months of routine ground maintenance of the flight system, and year-round data processing and distribution. Details of the AVIRIS system, its performance history, and future plans are described.

Terrestrial imaging spectroscopy

Abstract A review of progress made in the new field of imaging spectroscopy is presented based on the nine papers making up the special issue of this journal. Background material on the motivation for the new approach to earth remote sensing is discussed. The history, design, and performance of the pioneering sensor for terrestrial high resolution remote sensing, the Airborne Imaging Spectrometer (AIS), are presented. Concluding this paper is a discussion of plans for the future of imaging spectroscopy of the earth.

Terrestrial imaging spectrometry: current status, future trends

A review of recent progress in the field of imaging spectrometry is presented based on the 14 articles comprising the special issue of this journal. The results presented were achieved through research done with data from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), the first imaging spectrometer to cover the full solar reflected portion of the spectrum. The majority of the early work in imaging spectrometry prior to AVIRIS focused largely on geological applications and specifically surface mineral identification. In the past 5 years, the range of applications has expanded into the scientific disciplines of ecology, hydrology, oceanography, and atmospheric science. Significant progress has also been made in sensor design and calibration, and information extraction. NASA plans to place high spectral resolution sensors in earth orbit within the next few years; two have been flown already on recent planetary missions and have proven to be of great value to the study of planetary surfaces and atmospheres. The work presented in this issue will lead directly to more effective utilization of imaging spectrometry in the study of the earth. We present a discussion of future trends in imaging spectrometry at the conclusion of this article.

Airborne imaging spectrometer: A new tool for remote sensing

Thc first of a new class of remote sensing instruments is described. The Airborne Imaging Spectrometer represents the first use of two-dimensional integrated infrared area arrays in a scientific application. The instrument images 32 cross-track pixels simultaneously, each in 128 spectral bands la the 1.2- to 2.4-μ region. The IFOV of the instrument is 1.9 mrad per pixel and the spectral sampling interval is 9.6 nm. Plans include upgrading the detector from the current 32 × 32 element HgCdTe CCD array to a 64 × 64 element array in 1985. Science and engineering data are currently being actively gathered with the instrument.

Airborne Imaging Spectrometer-2: Radiometric Spectral Characteristics And Comparison Of Ways To Compensate For The Atmosphere

A field experiment and its results involving AIS-2 data for Rogers Lake, CA are described. The radiometry and spectral calibration of the instrument are critically examined in light of laboratory and field measurements. Three methods of compensating for the atmosphere in the search for ground reflectance are compared. We find, preliminarily, that the laboratory-determined responsivities are 30 to 50% less than expected for conditions of the flight for both short-and long-wavelength observations. The spectral sampling interval is 20 to 30 nm. The combined system-atmosphere-surface signal-to-noise ratio, as indexed by the mean response divided by the standard deviation for selected areas, lies between 40 and 110, depending upon how scene averages are taken, and is 30% less for flight conditions than for the laboratory. Atmospheric and surface variations may contribute to this difference. It is not possible to isolate instrument performance from the present data. As for methods of data reduction, the so-called scene average or log-residual method fails to recover any feature present in the surface reflectance, probably because of the extreme homogeneity of the scene. The empirical line method returns predicted surface reflectances that are systematically high but within a few percent of actual observed values using either calibrated or uncalibrated data. LOWTRAN-6, acting as an approximate theoretical model of the atmosphere for these exercises, predicts reflectance values 30 to 50% below the measured ones, based on the lower than expected radiances under solar illumination given by the instrument. This emphasizes the importance of accurate radiometric calibration in the study of surface or atmospheric properties.

First Results From The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)

After engineering flights aboard the NASA U-2 research aircraft in the winter of 1986-87 and spring of 1987, extensive data collection across the United States was begun with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) in the summer of 1987 in support of a NASA data evaluation and technology assessment program. This paper presents some of the first results obtained from AVIRIS. Examples of spectral imagery acquired over Mountain View and Mono Lake, California, and the Cuprite Mining District in western Nevada are presented. Sensor performance and data quality are discussed, and in the final section of the paper, plans for the future are described.

Spectral and radiometric calibration of the Airborne Visible/Infrared Imaging Spectrometer

The laboratory spectral and radiometric calibration of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) used in the radiometric calibration of all AVIRIS science data collected in 1987 is described. The instrumentation and procedures used in the calibration are discussed and the calibration accuracy achieved in the laboratory as determined by measurement and calculation is compared with the calibration requirements. Instrument performance factors affecting radiometry are described. The paper concludes with a discussion of future plans.

IN-FLIGHT RADIOMETRIC CALIBRATION OF THE AIRBORNE VISIBLE/INFRARED IMAGING SPECTROMETER (AVIRIS)

A reflectance-based method was used to provide an analysis of the in-flight radiometric performance of AVIRIS. Field spectral reflectance measurements of the surface and extinction measurements of the atmosphere using solar radiation were used as input to atmospheric radiative transfer calculations. Five separate codes were used in the analysis. Four include multiple scattering, and the computed radiances from these for flight conditions were in good agreement. Code-generated radiances were compared with AVIRIS-predicted radiances based on two laboratory calibrations (pre- and post-season of flight) for a uniform highly reflecting natural dry lake target. For one spectrometer (C), the pre- and post-season calibration factors were found to give identical results, and to be in agreement with the atmospheric models that include multiple scattering. This positive result validates the field and laboratory calibration technique. Results for the other spectrometers (A, B and D) were widely at variance with the models no matter which calibration factors were used. Potential causes of these discrepancies are discussed.

Comparison of laboratory calibrations of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) at the beginning and end of the first flight season

Spectral and radiometric calibrations of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) were performed in the laboratory in June and November, 1987, at the beginning and end of the first flight season. This paper describes those calibrations and the changes in instrument characteristics that occurred during the flight season as a result of factors such as detachment of the optical fibers to two of the four AVIRIS spectrometers, degradation in the optical alignment of the spectrometers due to thermally-induced and mechanical warpage, and breakage of a thermal blocking filter in one of the spectrometers. These factors caused loss of signal in three spectrometers, loss of spectral resolution in two spectrometers, and added uncertainty in the radiometry of AVIRIS. Results from in-flight assessment of the laboratory calibrations are presented. The paper concludes with a discussion of improvements made to the instrument since the end of the first flight season and plans for the future. Improvements include (1) a new thermal control system for stabilizing spectrometer temperatures, (2) kinematic mounting of the spectrometers to the instrument rack, and (3) new epoxy for attaching the optical fibers inside their mounting tubes.