Si Chee Tsay

Airborne Scanning Spectrometer for Remote Sensing of Cloud, Aerosol, Water Vapor, and Surface Properties

An airborne scanning spectrometer was developed for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.2mm. The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. The spectrometer scans a swath width of 37 km, perpendicular to the aircraft flight track, with a 2.5-mrad instantaneous field of view. Images are thereby produced with a spatial resolution of 50 m at nadir from a nominal aircraft altitude of 20 km. Nineteen of the spectral bands correspond closely to comparable bands on the Moderate Resolution Imaging Spectroradiometer ( MODIS ) , a facility in- strument being developed for the Earth Observing System to be launched in the late 1990s. This paper describes the optical, mechanical, electrical, and data acquisition system design of the MODIS Airborne Simulator and presents some early results obtained from measurements acquired aboard the National Aeronautics and Space Administration ER-2 aircraft that illustrate the performance and quality of the data produced by this instrument.

MODTRAN4: multiple scattering and bidirectional reflectance distribution function (BRDF) upgrades to MODTRAN

Radiance multiply scattered from clouds and thick aerosols is a significant component in the short wave IR through the visible region of the electro-optical (EO) spectrum. In MODTRAN, until very recently, multiple scattering predictions could not vary with the azimuth of the line-of-sight (LOS), although the single scattering component of the radiance did take the azimuthal variation into account. MODTRAN has now been upgraded to incorporate the dependence of multiple scattering (MS) on the azimuth of the LOS. This was accomplished by upgrading the interface between MODTRAN and DISORT, which is used as an MS subroutine in MODTRAN. Results from the upgraded MODTRAN are compared against measurements of radiance in a cloudy sky in the 1.5 - 2.5 micrometer region. Furthermore, taking advantage of DISORT, the upgraded version of MODTRAN can accommodate parameterized BRDFs (Bi-Directional Reflectance Distribution Functions) for surfaces. Some results, which demonstrate the new MODTRAN capabilities, are presented. Additionally, MS results from MODTRAN are compared to results obtained from a Monte-Carlo model.

Cloud Detection over the Arctic Region Using Airborne Imaging Spectrometer Data during the Daytime

Detection of clouds over arctic regions from current satellite radiometric measurements in the visible and IR atmospheric window regions, such as those of Advanced Very High Resolution Radiometer and Landsat, is often difficult due to the high albedos of snow- and ice-covered surfaces in the visible and the nearly isothermal temperature profiles in the lower atmosphere. In this paper the authors show that the water vapor absorption channel at 1.38 mm is effective in detecting high clouds over snow- and ice-covered surfaces in the Arctic. Low-level clouds can be detected from surface snow and sea ice using a narrow channel centered at 1.5 mm with a width of approximately 10 nm because of the dark background that results from strong absorption by snow and sea ice. Imaging data with contiguous spectral coverage between 0.4 and 2.5 mm acquired with the Airborne Visible/Infrared Imaging Spectrometer during the Arctic Radiation Measurements in Column Atmosphere-Surface System in Alaska in 1995 are analyzed. The authors have observed that as wavelength increases from 1.38 mm the atmospheric water vapor absorption becomes weaker and weaker and the low-level clouds and surface tundra are increasingly seen. It is always possible to locate a narrow channel in the spectral range of 1.38‐1.50 mm with appropriate water vapor absorption strength to separate water and ice clouds from surface snow, sea ice, and tundra. The simple cloud-masking technique described here is directly applicable to cloud detection during the daytime from hyperspectral imaging data over arctic regions, which will be acquired with future satellite sensors.

MODIS Airborne Simulator radiometric calibration

Over the past few years, the MODIS airborne simulator (MAS) has been providing imagery for EOS scientific algorithm development. Primarily flown aboard NASAs ER-2 aircraft, the MAS provides high spatial resolution (50 m at nadir) in 50 spectral channels from 0.55 to 14.2 micrometer, overlapping many MODIS and ASTER channels. This paper focuses on calibration of the short-wave (0.55 - 2.38 micrometer) channels, both radiometric and spectral, and calibration of the integrating sources. Also discussed is the dependence of the short-wave calibration on instrument temperature, showing significant reduction in the thermal sensitivity after recent instrument enhancements and upgrades. The procedures for intercomparison of MAS and AVIRIS (airborne visible/infrared imaging spectrometer) data are also discussed. Some limited comparisons for flights over Alaska (June 1995) are presented, although this analysis is in its initial stages and quantitative results are preliminary.

Status and calibration of the MODIS airborne simulator for earth remote sensing applications

The MODIS airborne simulator (MAS), a scanning spectrometer built by Daedalus Enterprises for NASA Goddard Space Flight Center and Ames Research Center, is used for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.3 micrometers . The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. Nineteen of the channels on MAS have corresponding spectral channels on the moderate resolution imaging spectroradiometer (MODIS), an instrument being developed for the Earth Observing system (EOS) to be launched in the late 1990s. Flown aboard NASAs ER-2 aircraft, the MAS has a 2.5 mrad instantaneous field of view and scans perpendicular to the aircraft flight track with an angle of +/- 43 degree(s) about nadir. From a nominal ER-2 altitude of 20 km, images have a spatial resolution of 50 m at nadir and a 37 km swath width. We report on the status of the instrument, discuss recent design changes, and provide comparisons with MODIS. We also summarize MAS calibration work, especially efforts to calibrate those channels with strong water vapor absorption.

Observations using the airborne Linear Etalon Imaging Spectral Array (LEISA): 1- to 25-micron hyperspectral imager for remote sensing applications

In this presentation we describe flight results for an airborne IR hyperspectral imager used as a test bed for LEISA, a compact spaceborne wedged filter spectrometer. The moderate spectral resolution Linear Etalon Imaging Spectral Array (LEISA) is a low-mass, low-power, low-cost infrared spectral imager for spacecraft applications. LEISA uses a state-of-the- art wedged infrared filter (a linear variable etalon, LVE) in conjunction with a detector array to obtain hyperspectral image cubes. The LEISA concept has been described previously in Reuter et al., 1997, SPIE Vol. 2957, pp 154 - 161, EUROPTO Conference on: Advanced and Next-Generation Satellites II., 23 - 26 September, 1996, Taormina, Italy. A LEISA type instrument, the Atmospheric Corrector (LAC), will fly on NASAs EO-1 spacecraft to be launched in Dec. 1999. The airborne version of LEISA covers the spectral region from 1.0 to 2.5 microns at a constant resolving power ((lambda) /(Delta) (lambda) ) of approximately 250 (i.e. 4 nm 1.0 microns and 10 nm 2.5 microns). The single pixel spatial resolution is 2 milliradians. This corresponds to 2 meters 1 km altitude and 20 meters 10 km. The instrument has been operated throughout this altitude range. The instrument has a swath width of approximately 29 degrees. A 256 X 256 element NICMOS (Near Infrared Camera Multi-Object Spectrometer) HgCdTe detector array is used as the focal plane. The focal plane is enclosed in a small cryogenic dewar at liquid Nitrogen temperature. Results will be presented for three series of airplane flights: Lubbock Texas (USA) June - September 1997, Lubbock Texas (USA) July - September 1998, Bethlehem Orange Free State (South Africa) March 1999. Issues to be discussed include pre-, and post-flight calibration, image registration and spectral image reconstruction. The relationship of these measurements to future spaceborne hyperspectral imagers will also be discussed.

Radiative transfer in the cloudy atmosphere: modeling radiative transport

Pertinent issues concerning cloud-radiation interactions that are relevant to studies of climate are discussed in terms of cloud optical properties. These optical properties are classified either inherent or apparent; the former are functions of cloud microphysics, the latter come about from the illumination of the cloud by radiation. The connection between the two sets of optical properties is discussed under the format of radiative transfer. The state of our lack of understanding of this connection is illustrated using examples derived from recent observational studies. Further evidence is presented that questions the validity of one dimensional radiative transfer theory as applied to the earths atmosphere.