The cryospheric advanced sensor (CAS) is a developmental airborne (and, potentially, spaceborne) radar based instrumentation system for measuring and mapping the thickness of sea ice. A planned future version of the system would also provide data on the thickness of snow covering sea ice. Frequent measurements of the thickness of polar ocean sea ice and its snow cover on a synoptic scale are critical to understanding global climate change and ocean circulation.
The CAS system includes two relatively narrow-band chirped radar subsystems that operate about two different nominal middle frequencies in the very-high-frequency (VHF) range (e.g., 137 and 162 MHz). The radar targets the same surface area from two slightly different directions (see figure). The radar backscatter signals are processed to extract angular- and frequency-domain correlation functions (ACF/FCF) so that the system acts, in effect, as a combined spatial- and frequency-domain interferometric radar system.
The phase of the ACF/FCF varies with the thickness of the sea ice. To enable the utilization of the phase information to compute this thickness, the interactions between the radar waves and the seawater, ice, and snow cover are represented by a mathematical model: The snow, sea ice (including air bubbles and brine inclusions), and seawater are represented as layers, each characterized by an assumed thickness and a known or assumed complex-number index of refraction. Each interface (air/snow, snow/ice, and ice/water) is modeled as deviating from a plane by a surface roughness characterized by a Gaussian spectrum. The scattering of the radar waves from the interfaces is computed by use of small perturbation and Kirchhoff rough-surface submodels. The scattering from within the layers is computed by a Rayleigh volume scattering model. The ACF/FCF is computed from the scattered signals.
Assuming that the ACF/FCF obeys the model, the interferometric phase information can be inverted by use of a suitable computational inversion technique (e.g., a genetic algorithm or gradient descent or other least-squares technique) to obtain the thickness of the sea ice. In essence, the inversion amounts to seeking whichever value of sea-ice thickness used in the model yields the best match between (1) the ACF/FCF interferometric phase computed from the model and (2) the ACF/FCF measured interferometric phase.
This work was done by Ziad Hussein, Rolando Jordan, Kyle McDonald, Benjamin Holt, and John Huang of Caltech; Yasuo Kugo, Akira Ishimaru, and Sermsak Jaruwatanadilok of the University of Washington, Seattle; and Torry Akins and Prasad Gogineni of the University of Kansas, Lawrence, for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-42391
This Brief includes a Technical Support Package (TSP).
Interferometric System for Measuring Thickness of Sea Ice
(reference NPO-42391) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically focused on the "Interferometric System for Measuring Thickness of Sea Ice." It outlines the development and capabilities of a prototype fully polarimetric VHF radar system designed for technology demonstration and to investigate inversion models for estimating sea ice thickness. This system was tested during a field experiment conducted from a Twin Otter aircraft over the Arctic Ocean ice cover.
Key features of the radar system include its operational design tailored for aircraft deployment, with specific parameters such as a pulse repetition frequency (PRF) of 700 Hz, an average power radiated of 0.155 Watts, and an antenna gain of 9.0 dB. The system operates with a sampling rate of 480 Msps and has a cross-track and along-track resolution of 15 meters. The document also discusses the optimization techniques used in the radar system, highlighting the importance of convergence conditions in the estimation of ice thickness.
The CAS (Cryospheric Airborne System) project aims to develop reliable techniques for direct estimation of sea ice thickness by combining spatial and frequency domain interferometry. The project includes both field experiments and controlled laboratory experiments to verify algorithms and predictions related to ice thickness retrieval. The document emphasizes the challenges and considerations involved in implementing a cryospheric spaceborne mission, which would integrate the necessary frequencies to assess both sea ice thickness and snow cover characteristics.
In summary, the document serves as a comprehensive overview of the technological advancements in measuring sea ice thickness, detailing the design, operational parameters, and experimental validation of the radar system. It underscores the significance of this research in understanding cryospheric changes and contributes to broader scientific and technological applications in aerospace and environmental monitoring. The information is part of NASA's efforts to disseminate aerospace-related developments with potential wider applications, and it encourages further exploration of the technologies discussed.
