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United States Department of Agriculture

Agricultural Research Service

Title: Measuring Aerosol Optical Thickness with a Helicopter-Based Sunphotometer

Authors
item Walthall, Charles
item Halthore, Rangasayi - BROOKHAVEN NATIONAL LAB
item Loechel, Sara - UNIVERSITY OF MARYLAND
item Elman, Greg - SSAI, GREENBELT, MD
item Markham, Brian - NASA, GODDARD

Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 1999
Publication Date: N/A

Interpretive Summary: The use of airborne sunphotometers and radiometrically calibrated sensors with fixed wing remote sensing platforms has been shown in the past to be a reliable method of obtaining calibrated, atmospherically corrected surface reflectance. There are several sunphotometers available for use with fixed wing aircraft but none for helicopters. A sunphotometer for helicopters was designed and built in this study. The system uses a quad sensor to track the sun while the helicopter is moving or hovering. The electronics sample fast enough to collect useable data between passes of the main rotor blades. The sunphotometer was calibrated by making measurements on a mountaintop where the atmosphere is extremely clear and by intercomparison with well calibrated surface-based sunphotometers to convert measurements to atmospheric transmittance. The transmissivity of the atmosphere or aerosol optical thickness (AOT) is inferred in discrete channels throughout the visible and near infrared portions of the spectrum. Examination of aerosol optical thickness data collected with the system during the 1994 BOReal Ecosystem Atmosphere Study (BOREAS) in Northern Canada shows reasonable agreement with data collected using surface-based sunphotometers at fixed locations close to the airborne data collection locations. This device will result in better calibrated and atmospherically corrected remote sensing data derived using a helicopter as a measurement platform.

Technical Abstract: The use of airborne sunphotometers and radiometrically calibrated sensors with fixed wing remote sensing platforms has been shown in the past to be a reliable method of obtaining calibrated, atmospherically corrected surface reflectance. Here we describe an airborne sunphotometer for use with helicopters. Once positioned within 30 degrees of the direction of the sun, this system uses a quad sensor to track the sun. The electronics sample fast enough to collect useable data between passes of the main rotor blades. The sunphotometer was calibrated using Langley plot methods on a mountain site and by intercomparison with well calibrated surface sunphotometers to convert measured voltages to atmospheric transmittance. Aerosol optical thickness (AOT) is inferred in discrete channels throughout the visible and near infrared portions of the spectrum. Examination of aerosol optical thickness data collected with the system during the 1994 BOReal Ecosystem Atmosphere Study (BOREAS) shows reasonable agreement with data collected using surface-based sunphotometers at fixed locations close to the airborne data collection locations. This device will result in better calibrated and atmospherically corrected remote sensing data derived using a helicopter as a measurement platform.

Last Modified: 11/27/2014
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