A wide-band penetrating radar system for measuring the thickness of sea ice is under development. The need for this or a similar system arises as follows: Spatial and temporal variations in the thickness of sea ice are important indicators of heat fluxes between the ocean and atmosphere and, hence, are important indicators of climate change in polar regions. A remote-sensing system that could directly measure the thickness of sea ice over a wide thickness range from aboard an aircraft or satellite would be of great scientific value. Obtaining thickness measurements over a wide region at weekly or monthly time intervals would contribute significantly to understanding of changes in the spatial distribution and of the mass balance of sea ice.
A prototype of the system was designed on the basis of computational simulations directed toward understanding what signal frequencies are needed to satisfy partly competing requirements to detect both bottom and top ice surfaces, obtain adequate penetration despite high attenuation in the lossy sea-ice medium, and obtain adequate resolution, all over a wide thickness range. The prototype of the system is of the frequency-modulation, continuous-wave (FM-CW) type. At a given time, the prototype functions in either of two frequency-band/ operational-mode combinations that correspond to two thickness ranges: a lower-frequency (50 to 250 MHz) mode for measuring thickness greater than about 1 m, and a higher-frequency (300 to 1,300 MHz) mode for measuring thickness less than about 1 m. The bandwidth in the higher-frequency (lesser-thickness) mode is adequate for a thickness resolution of 15 cm; the bandwidth in the lower-frequency (greater-thickness) mode is adequate for a thickness resolution of 75 cm. Although a thickness resolution of no more than 25 cm is desired for scientific purposes, the 75-cm resolution was deemed acceptable for the purpose of demonstrating feasibility.
The prototype was constructed as a modified version of a 500-to-2,000-MHz FM-CW radar system developed previously for mapping near-surface internal layers of the Greenland ice sheet. The prototype included two sets of antennas: one for each frequency-band/mode. For Arctic and Antarctic field tests, the prototype was mounted on a sled that was towed across the ice. The Arctic field test was performed in the lower-frequency mode on ice ranging in thickness from 1 to 4 m. In the analysis of the results of the Arctic field test, a comparison of the radar-determined ice thicknesses with actual ice thicknesses yielded an overall mean difference of 14 cm and standard deviation of 30 cm. The Antarctic field test was performed in the higher-frequency mode; analysis of the results led to the conclusion that this mode is useful for measuring thicknesses between 0.5 and 1 m.
Several modifications have been conceived for implementation in further development toward an improved practical system:
- The system would function in a single frequency-band/mode (100 to 1,200 MHz) that would afford a resolution of about 15 cm.
- There would be a single antenna system that would be optimized for the entire 100-to-1,000-MHz frequency band.
- To enable ice-thickness surveys over larger areas, the system would be made capable of operating aboard a low-flying aircraft that could be either piloted or robotic.
- Data-processing techniques to deconvolve the system response have been developed on the basis of impulse-response measurements over a calm ocean. Implementation of these techniques in the system would enable correction for imperfections of the system and would thereby increase the effective sensitivity of the system.
This work was done by Prasad Gogineni and Pannir Kanagaratnam of the University of Kansas and Benjamin M. Holt of Caltech for NASA's Jet Propulsion Laboratory.
NPO-45565
This Brief includes a Technical Support Package (TSP).
Wide-Band Radar for Measuring Thickness of Sea Ice
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory detailing a Wide-Band Radar system designed for measuring the thickness of sea ice. This innovative radar technology is crucial for understanding climate change and its impacts on polar regions. The system operates in frequency-modulated continuous wave (FM-CW) mode, utilizing a YIG oscillator in a Phase Locked Loop (PLL) configuration to generate linear chirp signals across two frequency ranges: 50-250 MHz for measuring thicker ice (over 1 meter) and 300-1300 MHz for thinner ice (less than 1 meter).
The radar employs bowtie antennas with rounded edges, designed to minimize direct leakage signals that could interfere with the detection of weak returns from the sea-ice/water interface. The antennas are enclosed in a plexiglass cavity to prevent back-radiation and are mounted on a sled to facilitate mobility across the ice. The sled, measuring approximately 2 meters in length and 1 meter in width, is towed by a snow machine, allowing for data collection over transects ranging from 100 to 500 meters.
Data collection involves digitizing the received signals at a sampling rate of 50 MHz, which is then decimated to 5 MHz for processing. The system includes automatic gain control to ensure uniform signal power and employs various filtering techniques to enhance signal quality. The radar's design addresses challenges faced by earlier systems, such as non-linear wideband sources, by utilizing advanced digital synthesizers to achieve a highly linear frequency sweep, resulting in improved spectral response and reduced ringing.
The radar system aims for a vertical resolution of 75 cm for the low-frequency mode and 15 cm for the high-frequency mode, with a scientific measurement goal of 25 cm thickness resolution or better. This technology represents a significant advancement in radar capabilities, enabling more accurate assessments of sea ice thickness, which is vital for climate monitoring and understanding the dynamics of polar environments.
Overall, the document highlights the technical specifications, operational methodologies, and the scientific importance of the radar system, emphasizing its potential applications in both research and commercial sectors related to climate science and environmental monitoring.
