Ronald E. Alley

Separation of temperature and emittance in remotely sensed radiance measurements

Abstract The remote determination of surface temperature and surface spectral emittance by use of airborne or satellite-borne thermal infrared instruments is not straightforward. The radiance measured is a function of surface temperature, the unknown surface spectral emittance, and absorption and emission in the intervening atmosphere. With a single measurement, the solution for temperature and spectral emittance is underdetermined. This article reviews two of the early approximate methods which have been fairly widely used to approach this problem.

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.

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.

A numerical simulation of soil temperature and moisture variations for a bare field

We simulated the diurnal variations of soil temperature and moisture content for a bare agricultural field in the San Joaquin Valley in California. The simulation pertained to the first 72 hours of drying, from saturation, of a sandy, clay loam soil. The results were compared with measurements of soil temperature and moisture content made at the field. Calculated and measured values of soil temperature trends agreed in general, but model results of moisture trends did not replicate observed diurnal effects evident at depths 4 centimeters or more below the surface.

Sensitivity of thermal inertia calculations to variations in environmental factors

Abstract The sensitivity of thermal inertia (TI) calculations to errors in the measurement or parameterization of a number of environmental factors is considered here. The factors include effects of radiative transfer in the atmosphere, surface albedo and emissivity, variations in surface turbulent heat flux density, cloud cover, vegetative cover, and topography. The error analysis is based upon data from the Heat Capacity Mapping Mission (HCMM) satellite for July 1978 at three separate test sites in the deserts of the western United States. Results show that typical errors in atmospheric radiative transfer, cloud cover, and vegetative cover can individually cause root-mean-square (RMS) errors of about 10% (with atmospheric effects sometimes as large as 30–40%) in HCMM-derived thermal inertia images of 20,000-200,000 pixels.

Preliminary Spectral and Geologic Analysis of Landsat-4 Thematic Mapper Data, Wind River Basin Area, Wyoming

A Landsat-4 Thematic Mapper (TM) image covering the Wind River Basin area, Wyoming, is under evaluation for stratigraphicand structural mapping, and for assessment of spectral and spatialcharacteristics using the six visible, near-infrared, and short-wavelengthinfrared bands.The image (path 36, row 30, ID # 40128-17232) was acquired on November 21, 1982. The data were obtained from the NASA Goddard Space Flight Center in radiometrically and geometrically correctedfullscene magnetic-tape format.To estimate equivalent Lambertian surface spectral reflectance, TMradiance data must first be calibrated to remove atmospheric andinstrumental effects. Reflectance measurements for homogeneousnatural and cultural targets in the scene were acquired during the period October 27-November 3, 1983, about one year after satellite dataacquisition. Scatterplots were prepared of image DN versus reflectancefor these sites. The scatterplots show the TM sensor system response islinear for the conditions of image acquisition and for the mix of terraincover types encountered.Low TM offset and gain settings result in encoded spectral data thatdo not occupy the full dynamic range (256 gray levels) of the TM.Calibration data acquired during the study were used to calculate new gainsand offsets that would improve scanner response for Earth scienceapplications.Analysis demonstrates that principal component images provideuseful structural and stratigraphic information. Principal componentscalculated from the correlation matix result from linear transformationsof ground reflectance. In images prepared from this transform, theseparation of spectral classes is largely independent of systematicatmospheric and instrumental factors.

AIS radiometry and the problem of contamination from mixed spectral orders

Abstract Airborne Imaging Spectrometer data from Mono Lake, CA, are studied in order to establish the spectral radiance of test areas under solar illumination. The objective is to provide a method of atmospheric correction for major absorbers from the spectrometer data themselves. Crucial to the analysis is radiometric calibration of the instrument. Good agreement is found between calculated and measured radiances for uniform surface targets (beaches), but simulations of atmospheric properties with LOWTRAN 6 lead to unreasonably low values of atmospheric precipitable water. Absorptions from carbon dioxide are not detected in the AIS data, but are strongly present in the LOWTRAN 6 model. The apparent low contrast of all atmospheric absorption bands leads to a study of contamination from overlapping spectral orders in the AIS data. The suspected contamination is shown unambiguously to be present beyond approximately 1500 nm and consists of an extra radiance term including atmospheric bands from the λ /2 wavelength interval. The magnitude of the contamination is a factor of 1.5–2 greater than the expected uncontaminated signal alone. The spectral band filling at 1400 nm, i.e., an apparent finite transmittance in a band expected to be saturated, cannot be accounted for by order mixing because of the 800-nm blocking filter used and must arise from some other cause. A rigorous removal of the unwanted spectral contamination does not seem possible for any data taken in the spectral region 1200–2500 nm. Rough estimates for observations in the interval 900–2100 nm might be pieced together if a suitable after-the-fact radiometric calibration of the instrument can be formulated.

Use Of Thermal-Inertia Properties For Material Identification

A knowledge of the thermal inertia of the Earths surface can be used in geologic mapping as a complement to surface reflectance data as provided by Landsat. Thermal inertia, a body property, cannot be determined directly but can be inferred from radiation temperature measurements made at various times in the diurnal heating cycle, combined with a model of the surface heating processes. We have developed such a model and applied it along with temperature measurements made in the field and from satellite to determine thermal properties of surface materials. An example from a test site in western Nevada is used to illustrate the utility of this technique.