A document discusses a hyperspectral imaging instrument package designed to be carried aboard a helicopter. It was developed to map the depths of Greenland’s supraglacial lakes. The instrument is capable of telescoping to twice its original length, allowing it to be retracted with the door closed during takeoff and landing, and manually extended in mid-flight. While extended, the instrument platform provides the attached hyperspectral imager a nadir-centered and unobstructed view of the ground.
Before flight, the instrument mount is retracted and securely strapped down to existing anchor points on the floor of the helicopter. When the helicopter reaches the destination lake, the door is opened and the instrument mount is manually extended. Power to the instrument package is turned on, and the data acquisition computer is commanded via a serial cable from an onboard user-operated laptop to begin data collection. After data collection is complete, the instrument package is powered down and the mount retracted, allowing the door to be closed in preparation for landing.
The present design for the instrument mount consists of a three-segment telescoping cantilever to allow for a sufficient extended length to see around the landing struts and provide a nadir-centered and unobstructed field of view for the hyperspectral imager. This instrument works on the premise that water preferentially absorbs light with longer wavelengths on the red side of the visible spectrum. This property can be exploited in order to remotely determine the depths of bodies of pure freshwater. An imager flying over such a lake receives light scattered from the surface, the bulk of the water column, and from the lake bottom. The strength of absorption of longer-wavelength light depends on the depth of the water column. Through calibration with in situ measurements of the water depths, a depth-determining algorithm may be developed to determine lake depth from these spectral properties of the reflected sunlight.
This work was done by Alberto E. Behar and Moogega Cooper of Caltech; John Adler of NOAA; and Tobias Jacobson of the University of Southern California for NASA’s Jet Propulsion Laboratory. NPO-48141
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Airborne Hyperspectral Imaging System
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Overview
The document outlines a research initiative conducted by NASA's Jet Propulsion Laboratory (JPL) focused on the study of supraglacial lakes in Greenland, particularly during the warmer months when parts of the ice cap melt. These temporary lakes can drain rapidly into cracks or moulins in the ice, and it is hypothesized that if the water reaches the glacier-bedrock interface in liquid form, it may facilitate the glacier's movement by lubricating its base. Understanding the volume of water in these lakes is crucial for accurately modeling their impact on the mass balance and dynamics of the Greenland ice sheet.
To assess the volume of these lakes, researchers utilize satellite imagery, specifically from the MODIS (Moderate Resolution Imaging Spectroradiometer), to determine the surface area of the lakes. However, estimating lake volumes requires depth measurements, which are obtained through a specialized instrument designed for remote sensing. This instrument, carried by a helicopter, includes a hyperspectral imager, a GPS/inertial measurement unit (GPS/IMU), and a data-logging computer. The hyperspectral imager captures light reflected from the lake, and by analyzing the wavelength-dependent absorptivity of water, researchers can derive depth maps for the lakes.
The document details the operational aspects of the research, including the flight plans and methodologies used to collect data over selected supraglacial lakes. A series of overlapping passes were performed to ensure complete coverage, and the data collected will be processed to create geo-referenced datasets. A depth-calculating algorithm, based on an in-situ calibrated absorptivity model, will be employed to extrapolate depth maps for each observed lake.
The expedition was deemed successful, with the instrument mount functioning as intended and no vibration issues reported, instilling confidence in the platform's performance for future surveys. The research is part of a broader effort to understand the dynamics of Greenland's ice sheet and the implications of supraglacial lakes on glacial movement and climate change.
Overall, this document highlights the innovative use of airborne hyperspectral imaging technology in environmental research, emphasizing its significance in understanding the complex interactions between climate, ice dynamics, and water systems in polar regions.
