William Ledeboer

California Institute of Technology, Pasadena, California, United States

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Publications (9)13.62 Total impact

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    ABSTRACT: Terrestrial Planet Finder Coronagraph, one of two potential architectures, is described. The telescope is designed to make a visible wavelength survey of the habitable zones of at least thirty stars in search of earth-like planets. The preliminary system requirements, optical parameters, mechanical and thermal design, operations scenario and predicted performance is presented. The 6-meter aperture telescope has a monolithic primary mirror, which along with the secondary tower, are being designed to meet the stringent optical tolerances of the planet-finding mission. Performance predictions include dynamic and thermal finite element analysis of the telescope optics and structure, which are used to make predictions of the optical performance of the system
    Preview · Article · Oct 2004 · Proceedings of SPIE - The International Society for Optical Engineering

  • No preview · Conference Paper · Apr 2004
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    ABSTRACT: On June 11, 2000, the first vicarious calibration experiment in support of the Multi-angle Imaging SpectroRadiometer (MISR) was conducted. The purpose of this experiment was to acquire in situ measurements of surface and atmospheric conditions over a bright, uniform area. These data were then used to compute top-of-atmosphere (TOA) radiances, which were correlated with the camera digital number output, to determine the in-flight radiometric response of the on-orbit sensor. The Lunar Lake Playa, Nevada, was the primary target instrumented by the Jet Propulsion Laboratory for this experiment. The airborne MISR simulator (AirMISR) on board a NASA ER-2 acquired simultaneous observations over Lunar Lake. The in situ estimations of top-of-atmosphere radiances and AirMISR measurements at a 20-km altitude were in good agreement with each other and differed by 9% from MISR measurements. The difference has been corrected by adjusting the gain coefficients used in MISR standard product generation. Data acquired simultaneously by other sensors, such as Landsat, the Terra Moderate-Resolution Imaging SpectroRadiometer (MODIS), and the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), were used to validate this correction. Because of this experiment, MISR radiances are 9% higher than the values based on the on-board calibration. Semiannual field campaigns are planned for the future in order to detect any systematic trends in sensor calibration.
    No preview · Article · Aug 2002 · IEEE Transactions on Geoscience and Remote Sensing
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    ABSTRACT: A vicarious reflectance-based calibration experiment for the Multiangle Imaging SpectroRadiometer (MISR) airborne simulator, AirMISR, is described as one precursor experiment of this type planned for postlaunch application to MISR itself. The experiment produces a set of multiangle near-top-of-atmosphere radiances that are compared with the multiangle AirMISR radiances, established using a laboratory calibration. The field and aircraft data were collected as part of an engineering test flight at Moffett Field, CA, on November 5, 1997. A concrete tarmac was used as the field target. Atmospheric optical depth data were collected adjacent to the target throughout the actual overflight period using a single Reagan solar radiometer. For logistical reasons, the surface hemispherical directional reflectance factor (HDRF) was determined 7 days later using the Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere III (PARABOLA III), along with the areally averaged spectral HDRF at normal incidence, obtained with an Analytical Spectral Devices (ASD) FieldSpec moderate resolution field spectrometer. AirMISR overflew the target under clear sky conditions though the aerosol turbidity was high (∼0.3 at 550 nm). Good to fair agreement has been obtained at all angles and wavelengths between the top-of-atmosphere (TOA) radiances calculated for the measured atmospheric and surface conditions and the radiances incident at AirMISR as determined from the laboratory calibration. Some systematic disagreements are present. The largest disagreements (∼15% in the blue) are found at the highest view angles and the smallest at nadir viewing (<5%). Possible explanations for the differences in radiances at large view angles are discussed.
    No preview · Article · Sep 2001 · Remote Sensing of Environment
  • W.A. Abdou · C.J. Bruegge · W.C. Ledeboer
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    ABSTRACT: Radiative closure experiments involving a comparison between surface-measured spectral ance anti the surface calculated according to a radiative transfer code at a desert site in Nevada under clear skies, yield the result that agreement between the two requires presence of an absorbing aerosol component with an imaginary refractive index equal to 0.03 and a mix by optical depth of small and large particles with log-normal size distributions. The mode radius of the small particle distribution is 0.03 and that of the large 0.5 The same aerosol model can be used for both day-to-day fits in one campaign year, and also between campaigns in different years. The high imaginary index required for the fits suggests presence of urban-type particles in the aerosol, but an alternative under study is to rely on absorbing iron oxide components in a dust fraction to account for some of the absorption. INTRODUCTION As part of the algorithm validation phase of preflight activities for the Imaging tro...
    No preview · Article · Feb 1999
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    ABSTRACT: An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assist in validation of the Earth Observing System (EOS) MISR experiment. Unlike the EOS MISR, which contains nine individual cameras pointed at discrete look angles, AirMISR utilizes a single camera in a pivoting gimbal mount. The AirMISR camera has been fabricated from MISR brassboard and engineering model components and, thus, has similar radiometric and spectral response as the MISR cameras. This paper provides a description of the AirMISR instrument and summarizes the results of engineering flights conducted during 1997
    Full-text · Article · Aug 1998 · IEEE Transactions on Geoscience and Remote Sensing
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    ABSTRACT: A plan for the ground-based validation of MISR aerosol retrieval is outlined. Activities occur in two phases: (1) pre-launch, work is focused on technique development and MISR algorithm validation using conventional ground-based methods and a MISR simulator (AirMISR) operating from the ER-2 aircraft to simulate MISR on-orbit observations. (2) Post-launch, the validation program relies on ground campaigns, underflights with the MISR simulator and the use of local measurements of aerosol loading and properties and irradiance measurements derived from the AERONET and ISIS networks
    Full-text · Conference Paper · Sep 1997
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    ABSTRACT: State of the are modeling tools for the Terrestrial Planet Finder (TPF) Coronagraph mission are being developed that will combine thermal, structural, and optical modeling in one package. This tool is discussed as well as specific modeling results highlighting the capabilities of integrated modeling for TPF.
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    ABSTRACT: Radiative closure experiments involving a comparison between surface-measured spectral irradi- ance anti the surface irradiance calculated according to a radiative transfer code at a desert site in Nevada under clear skies, yield the result that agreement between the two requires presence of an absorbing aerosol component with an imaginary refractive index equal to 0.03 and a 50:50 mix by optical depth of small and large particles with log-normal size distributions. The mode radius of the small particle distribution is 0.03 ~m and that of the large 0.5 Lm. The same aerosol model can be used for both day-to-day fits in one campaign year, and also between campaigns in differ- ent years. The high imaginary index required for the fits suggests presence of urban-type particles in the aerosol, but an alternative under study is to rely on absorbing iron oxide components in a dust fraction to account for some of the absorption.
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