A.R. Ehsani

Bio: A.R. Ehsani is an academic researcher from University of Arizona. The author has contributed to research in topics: Radiometer & Microwave radiometer. The author has an hindex of 1, co-authored 1 publications receiving 6 citations.

A Microprocessor Based Auto Sun-Tracking Multi-Channel Solar Radiometer System

Abstract: paper describes the design and fabricm'on of a microprocessor controlled auto sun-tracking lkhannel solar radiometer system which is used in measuring the solar spectral irradiance received at the Earth's suface. The instrument measurements are employed in determining the atmospheric optical depths at 10 discrete wavelengths through visible and near IR regions. me automated solar radiometer is capable of tracking the sun, and measuring and recording the output of 10 channels through the course of a day. Some pe formance and experimental results of the instrument are also presented in this paper.

Columnar aerosol single‐scattering albedo and phase function retrieved from sky radiance over the ocean: Measurements of Saharan dust

Abstract: [1] The single-scattering albedo and phase function of African mineral dust are retrieved at 14 wavelengths across the visible spectrum from ground-based measurements of the aerosol optical thickness and the sky radiance taken in the solar principal plane. The retrieval algorithm employs the radiative transfer equation to solve by iteration for these properties that best reproduce the observed sky radiance, and is therefore independent of particle shape. The estimated error in the retrieved single-scattering albedo is less than 0.02 due to the precision of the solar-reflectance-based calibration of the radiometer. The phase function retrieved at 860 nm is robust under simulations of expected experimental errors and may be used to characterize aerosol scattering at the directly measured scattering angles (i.e., Θ ≤ 155°). The phase function retrieved at 443 nm, however, is too sensitive to such errors to confidently describe the angular scattering at blue wavelengths. The single-scattering albedo displays a spectral shape expected of iron-bearing minerals but is much higher than climate models have assumed, indicating that wind-blown mineral dust cools Earth more than is generally believed. The method may be applied to any combination of airborne and ground-based measurements over the ocean and can complement more involved ground-based retrievals through its insensitivity to particle shape and ability to retrieve aerosol properties at relatively small aerosol optical depths.

Calibration and validation concept for the airborne PRISM experiment (APEX)

Abstract: The development of the Airborne PRISM Experiment (APEX) is supported by the European Space Agency (ESA) in view of an appropriate data simulator for future spaceborne hyperspectral instruments of the Agency. The terminology and conceptual design of the calibration and validation steps required for the APEX system are defined in this paper. The calibration concept for the APEX instrument is based on a standardized laboratory procedure in which spectral response, geometric response, as well as radiometric gain and offset values are determined. Additionally, in-flight calibration using sensor-internal means and vicarious calibration approaches will improve the reliability of the calibrated image data. All calibration-related parameters as well as the image data are kept and administered by the APEX Processing and Archiving Facility (PAF). A processing chain is defined which allows an efficient preparation of all the calibration parameters and fully reproducible processing of the acquired data from raw format to calibrated radiances. The validation concept for the imagery and its processing is based on sensor simulation, standard quality control procedures, and in-flight validation campaigns. The combination of all these efforts results in a consistent characterization of the APEX instrument performance and a reliable quality definition of the final image data products.

Ground spectroradiometric measurements in support of the validation of the calibration of digital airborne imaging spectrometer (dais 7915) data

Abstract: A standard reference test site is flown using the DAIS 7915 imaging spectrometer over Central Switzerland. Simultaneous to this overflight extensive ground measurements are carried out. A sun photometer, a radiosonde, and two spectroradiometers are used simultaneously during the overflight to support the in– flight calibration and validation of the DAIS 7915. The calibration of all instruments is discussed and the modelling process of the at–sensor radiances of the DAIS 7915 is described.

Experimental determination of adjacency effects over an eutrophic lake using a helicopter mounted spectroradiometer for the correction of imaging spectrometer data

Abstract: The amount of adjacency effect present in imaging spectrometer data over lakes is determined using a helicopter mounted spectroradiometer. The experiment – carried out over an eutrophic lake in Central Switzerland – included extensive ground data collection including water quality assessment and underwater spectroradiometric measurements. The experiment is flown over the same transect in 11 different altitudes above the lake. The radiometric measurements are converted to reflectance values using a Spectralon panel and a sun photometer. The contribution of the green vegetation on both ends of the transects to the water signal is measured and discussed.

Validation of hyperspectral imaging data from the barrax test site with brdf ground measurements in the reflective wavelength range

Abstract: During the DAISEX’99 campaign hyperspectral and multiangular images are taken with the airborne wide angle imaging spectrometer HyMap at the Barrax test site in Spain. For validation purposes, bidirectional ground measurements of dry bare soil, Alfalfa and barley are acquired with the Field Goniometer of the RSL, Zurich. The image data are atmospherically corrected using the ATCOR4 program developed at DLR. It removes the effects of angular dependent path radiance and atmospheric transmittance in airborne hyperspectral imagery. Using classification and a statistical approach, directional spectra are derived from single images. Thus the image spectra from different viewing angles can be compared with the corresponding goniometric ground spectra by calculating the anisotropy factor.