The hydrosphere state (hydros) satellite mission: An earth system pathfinder for global mapping of soil moisture and land freeze/thaw

Dara Entekhabi, Eni G. Njoku, Paul Houser, Michael Spencer, Terence Doiron, Yunjin Kim, Joel Smith, Ralph Girard, Stephane Belair, Wade Crow, Thomas J. Jackson, Yann H. Kerr, John S. Kimball, Randy Koster, Kyle C. McDonald, Peggy E. O'Neill, Terry Pultz, Steve W. Running, Jiancheng Shi, Eric WoodJakob Van Zyl

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208 Scopus citations


The Hydrosphere State Mission (Hydros) is a pathfinder mission in the National Aeronautics and Space Administration (NASA) Earth System Science Pathfinder Program (ESSP). The objective of the mission is to provide exploratory global measurements of the earth's soil moisture at 10-km resolution with two- to three-days revisit and land-surface freeze/thaw conditions at 3-km resolution with one- to two-days revisit. The mission builds on the heritage of ground-based and airborne passive and active low-frequency microwave measurements that have demonstrated and validated the effectiveness of the measurements and associated algorithms for estimating the amount and phase (frozen or thawed) of surface soil moisture. The mission data will enable advances in weather and climate prediction and in mapping processes that link the water, energy, and carbon cycles. The Hydros instrument is a combined radar and radiometer system operating at 1.26 GHz (with VV, HH, and HV polarizations) and 1.41 GHz (with H, V, and U polarizations), respectively. The radar and the radiometer share the aperture of a 6-m antenna with a look-angle of 39° with respect to nadir. The lightweight deployable mesh antenna is rotated at 14.6 rpm to provide a constant look-angle scan across a swath width of 1000 km. The wide swath provides global coverage that meet the revisit requirements. The radiometer measurements allow retrieval of soil moisture in diverse (nonforested) landscapes with a resolution of 40 km. The radar measurements allow the retrieval of soil moisture at relatively high resolution (3 km). The mission includes combined radar/radiometer data products that will use the synergy of the two sensors to deliver enhanced-quality 10-km resolution soil moisture estimates. In this paper, the science requirements and their traceability to the instrument design are outlined. A review of the underlying measurement physics and key instrument performance parameters are also presented.

Original languageEnglish
Pages (from-to)2184-2195
Number of pages12
JournalIEEE Transactions on Geoscience and Remote Sensing
Issue number10
StatePublished - Oct 2004


Manuscript received November 3, 2003; revised April 6, 2004. This work was supported in part by the National Aeronautics and Space Administration and in part by the author home institutions. D. Entekhabi is with the Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: [email protected]). E. G. Njoku, M. Spencer, Y. Kim, J. Smith, K. C. McDonald, and J. van Zyl are with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA. P. Houser, T. Doiron, R. Koster, and P. E. O’Neill are with the Hydrologic Sciences Branch, National Aeronautics and Space Administration Goddard Space Flight Center (GSFC), Greenbelt, MD 20771 USA. R. Girard is with the Canadian Space Agency, Saint-Hubert, J3Y 8Y9 QC, Canada. S. Belair is with the Meteorological Service of Canada, Dorval, H98 IJ3 QC, Canada. W. Crow and T. J. Jackson are with the Hydrology and Remote Sensing Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705 USA. Y. H. Kerr is with the Centre d’Etudes Spatiales de la Biosphère, 31401 Toulouse Cedex 09, France. J. S. Kimball and S. W. Running are with the Division of Biological Sciences, University of Montana, Missoula, MT 59860 USA. T. Pultz is with the Canada Centre for Remote Sensing, Natural Resources Canada, Ottawa, ON K1A 0Y7, Canada. J . Shi is with the University of California, Santa Barbara, CA 93106 USA. E. Wood is with Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08854 USA. Digital Object Identifier 10.1109/TGRS.2004.834631

FundersFunder number
National Aeronautics and Space Administration


    • Land freeze/thaw
    • Microwave remote sensing
    • Satellites
    • Soil moisture


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