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Passive Microwave Remote Sensing of Soil MoistureMicrowave remote sensing provides a unique capability for direct observation of soil moisture. Remote measurements from space afford the possibility of obtaining frequent, global sampling of soil moisture over a large fraction of the Earth's land surface. Microwave measurements have the benefit of being largely unaffected by cloud cover and variable surface solar illumination, but accurate soil moisture estimates are limited to regions that have either bare soil or low to moderate amounts of vegetation cover. A particular advantage of passive microwave sensors is that in the absence of significant vegetation cover soil moisture is the dominant effect on the received signal. The spatial resolutions of passive Microwave soil moisture sensors currently considered for space operation are in the range 10-20 km. The most useful frequency range for soil moisture sensing is 1-5 GHz. System design considerations include optimum choice of frequencies, polarizations, and scanning configurations, based on trade-offs between requirements for high vegetation penetration capability, freedom from electromagnetic interference, manageable antenna size and complexity, and the requirement that a sufficient number of information channels be available to correct for perturbing geophysical effects. This paper outlines the basic principles of the passive microwave technique for soil moisture sensing, and reviews briefly the status of current retrieval methods. Particularly promising are methods for optimally assimilating passive microwave data into hydrologic models. Further studies are needed to investigate the effects on microwave observations of within-footprint spatial heterogeneity of vegetation cover and subsurface soil characteristics, and to assess the limitations imposed by heterogeneity on the retrievability of large-scale soil moisture information from remote observations.
Document ID
20000004216
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
Njoku, Eni G.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA United States)
Entekhabi, Dara
(Massachusetts Inst. of Tech. Cambridge, MA United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 1996
Publication Information
Publication: Journal of Hydrology
Publisher: Elsevier
Volume: 184
ISSN: 0022-1694
Subject Category
Earth Resources And Remote Sensing
Funding Number(s)
CONTRACT_GRANT: NAGW-2942
Distribution Limits
Public
Copyright
Other

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