An interferometric synthetic aperture radar (SAR) onboard processor concept and algorithm has been developed for the Ka-band radar interferometer (KaRIn) instrument on the Surface and Ocean Topography (SWOT) mission. This is a mission-critical subsystem that will perform interferometric SAR processing and multi-look averaging over the oceans to decrease the data rate by three orders of magnitude, and therefore enable the downlink of the radar data to the ground.
The onboard processor performs demodulation, range compression, coregistration, and re-sampling, and forms nine azimuth squinted beams. For each of them, an interferogram is generated, including common-band spectral filtering to improve correlation, followed by averaging to the final 1×1-km ground resolution pixel. The onboard processor has been prototyped on a custom FPGAbased cPCI board, which will be part of the radar’s digital subsystem.
The level of complexity of this technology, dictated by the implementation of interferometric SAR processing at high resolution, the extremely tight level of accuracy required, and its implementation on FPGAs are unprecedented at the time of this reporting for an onboard processor for flight applications.
This work was done by Daniel Esteban-Fernandez, Ernesto Rodriguez, Eva Peral, Duane I. Clark, and Xiaoqing Wu of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47789
This Brief includes a Technical Support Package (TSP).
Onboard Interferometric SAR Processor for the Ka-Band Radar Interferometer (KaRIn)
(reference NPO-47789) is currently available for download from the TSP library.
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Overview
The document outlines the Technical Support Package for the Onboard Interferometric SAR Processor for the Ka-Band Radar Interferometer (KaRIn) as part of the Surface and Ocean Water Topography (SWOT) mission, developed by NASA's Jet Propulsion Laboratory (JPL). The SWOT mission aims to enhance our understanding of oceanography and hydrology by providing detailed measurements of water bodies and ocean circulation.
The primary goals of the SWOT mission include characterizing ocean mesoscale and submesoscale circulation with spatial resolutions of 10 km or greater, and creating a global inventory of terrestrial water bodies, such as lakes, reservoirs, wetlands, and rivers. The mission seeks to measure changes in freshwater storage and river discharge at various time scales, including sub-monthly, seasonal, and annual intervals.
The KaRIn system employs a Doppler-sharpened, multi-squint Synthetic Aperture Radar (SAR) processing technique to achieve high-resolution data while minimizing the output data rate. This innovative approach allows for the generation of interferograms for each beam, facilitating accurate height measurements and co-registered all-weather imagery. The system operates with a 200 MHz signal bandwidth, achieving a slant range resolution of 0.75 m and a maximum pulse repetition frequency of 5 kHz per channel.
The SWOT mission is designed to operate from a 970 km altitude orbit with a 78-degree inclination, following a 22-day repeat cycle. It features two 60 km cross-track swaths, with a single-look ground resolution ranging from 10 to 70 m in range and approximately 250 m in azimuth. The on-board processor is crucial for reducing the radar's output data rate, which can reach up to 290 Mbps or approximately 166 GB per orbit during the longest ocean passes.
The document emphasizes the collaboration between NASA and the French space agency CNES in developing the SWOT mission, building on the heritage of previous missions such as the Wide Swath Ocean Altimeter. The advancements in technology and processing capabilities showcased in this document highlight the potential for significant contributions to our understanding of global water resources and ocean dynamics.
Overall, the SWOT mission represents a significant step forward in Earth observation technology, with the potential to provide critical data for environmental monitoring and management.
