Tim Durham, Harris Corporation
H.P. Marshall, Boise State University
Leung Tsang, University of Washington
Paul Racette, NASA Goddard Space Flight Center
Quenton Bonds, NASA Goddard Space Flight Center
Felix Miranda, NASA Glenn Research Center
Ken Vanhille, Nuvotronics Corporation
Measurements of surface snow distribution are needed to improve our knowledge of the global hydrologic cycle. Measurement of the spatial extent of snow cover, and how much water is stored frozen in the natural reservoir of the seasonal snowpack, is critical for hydrological modeling. However, spatial and temporal snow information is sparse. Potential improvements in the estimates of snow water equivalent (SWE) and its spatial/temporal variability in mountain regions have significant implications; the spatial distribution of SWE is currently one of the greatest uncertainties in streamflow prediction. In areas such as the western U.S. and South America, snow can account for 50-80 percent of the annual runoff. One billion people worldwide depend on seasonal snow for their water supply.
Knowledge of SWE distribution is critical for improved planning of reservoir operations as well as conservation measures by water managers; however, our current ability to estimate the spatial and temporal distribution of SWE is limited. Surface snow cover also plays an important role in the climate. Snow is nearly a perfect reflector of energy from the sun, typically absorbing less than 10 percent, while snow-free areas absorb more than 50 percent. Snow distribution information is therefore an important component of the global energy balance and vital for climate modeling.åÊ
The NASA Snow and Cold Land Processes (SCLP) Earth science mission concept, as outlined in the National Academy of Sciences Earth Science Decadal Survey , calls for measuring SWE to determine and track the freshwater stored in the seasonal snowpack at sub-km resolution and global extent. Specifically, the SCLP concept calls for a combination of synthetic aperture radar and radiometry over a broad band of frequencies between the X- and Ka-bands.The challenge and potential for remote sensing of seasonal snow is illustrated in Figure 1. This figure shows the comparison between forward modeling results from the dense media radiative transfer (DMRT) model, compared with airborne backscatter observations from the second Cold Lands Processes Experiment (CLPX) . The snow properties used as input to the forward DMRT model results were obtained from manual snow-pit and depth measurements during the concurrent ground-based field campaign. The comparisons show that the DMRT simulations are in good agreement with both co-polarization and cross-polarization data. The backscatter observations and model results both increase with snow depth and SWE, confirming that backscatter is sensitive to snow depth and SWE; however, inverting backscatter for depth and SWE requires exceptional calibration of the radar instrument, as well as a-priori estimates of grain size distribution, to achieve the few centimeter SWE accuracies called for by SCLP. There exists a basic need within the snow science community for remote sensing instrumentation on which to base techniques for measuring the geo-spatial variability of SWE while advancing core technologies that enable a technically feasible space mission.
Technology for Snow Measurements
To address the measurement needs of the snow science community, Harris Corp. teamed with investigators from NASA Goddard Space Flight Center (GSFC), NASA Glenn Research Center, University of Washington (UWA), Boise State University (BSU), and Nuvotronics Corp. The team partnered under an Earth Science Technology Office Instrument Incubator Program (IIP) 2010 award to develop broadband antenna technology that enables active and passive remote sensing using a common antenna aperture spanning the frequencies of interest for SCLP. The outcome of this IIP is an airborne, wideband instrument for snow measurement (WISM) comprised of a dual-frequency (X- and Ku-bands) synthetic aperture radar and a dual-frequency (K- and Ka-bands) radiometer. All measurement bands share a common antenna aperture that utilizes the novel current sheet array (CSA) antenna feed combined with a focusing offset parabolic dish.
The WISM CSA antenna feed consists of 56 individual PolyStrata layers assembled to provide the full antenna feed. It is composed of 36 dual polarized 5:1 bandwidth radiating elements and a fully integrated passive beam former. There are more than 500 part-to-part interconnects in the feed that operate from DC to greater than 40 GHz and four baluns operating over the same frequency range. There are 82 X-to-Ka-band splitters and eight Ku-to-Ka-band splitters. The total length of the transmission lines for the full antenna feed exceeds 12 meters of rectangular coaxial cable. The WISM CSA antenna feed is possibly the most integrated fixed-beam, wideband array built to date operating at microwave/millimeter-wave frequencies. A picture of the antenna and sample pattern at Ka band are shown in Figure 2.
The single antenna feed enables the co-boresighting of the dual-frequency radar and dual-frequency radiometer at the four widely spaced frequencies. The dual-frequency (9.6 GHz and 17.2 GHz) synthetic aperture radar is fully polarimetric. The radiometer operates at 18.7 GHz and 36.5 GHz with single-linear polarization. Concurrent operation of the radar and radiometer is achieved by interleaving the signals in a single cycle at the chosen pulse repetition frequency. The modulation scheme allows simultaneous ground sensing by both instruments at both bands, thereby optimizing the benefit of the co-boresighted beams. A key advantage to the broad-band CSA technology is its applicability to space. A single 8-40 GHz reflector feed offers significant size and weight savings over separate feed horns while providing co-alignment of the antenna beams. One of the main difficulties during the first CLPX was the spatial-temporal coincidence between radar and radiometer observations; during the second CLPX, only the radar was used. WISM will overcome this limitation by producing coincident radiometer and radar observations at sub-km resolution for the first time.
Applicability to Snow Science
The investigation team is planning a series of air- and ground-based experiments to demonstrate the utility of WISM. The objective is to advance the state-of-the-art in remote sensing of surface snow and retrieval algorithms using the wideband capabilities of the antenna and multi-band utility of the instrument. BSU has integrated a previously developed array antenna with its custom FMCW radar systems for ultra-wideband radar measurements from the ground. These measurements have demonstrated the capabilities of the wideband array antenna approach for measuring snow properties. The ultra-wideband allows fine vertical resolution near-ground measurements such that reflections from the snow surface, layers, and ground can be independently observed. Travel-time in dry snow depends on both density and depth, and can be used to estimate SWE to within 5 percent . FMCW radar measurements also track stratigraphy between snow pits, providing accurate layer thickness estimates as shown in Figure 3 . These estimates can be useful for constraining backscatter inversion algorithms, which are typically narrowband and integrate all backscatter from the snowpack for SWE and depth.
BSU will make weekly observations from a fixed tower during the 2013-2014 snow season using FMCW radars, a rugged, portable 1-26.5 GHz vector network analyzer, and the new WISM antenna. Future work is planned for an airborne campaign for WISM in Colorado at one of the CLPX study sites in 2014-15. Coincident with planned WISM airborne measurements, BSU and NASA GSFC will perform an intensive, ground-based measurement campaign with both standard manual measurements and FMCW radar observations covering 6-18 GHz, similar to the campaigns performed during CLPX-I, -II, and -III (2003-2010) and ESA CoReH2O campaigns in Austria, Canada, Colorado and Alaska (2013).
The combination of radar and radiometric measurements spanning the 8-40 GHz spectrum shows promise for quantifying the geospatial distribution of surface snow. WISM is a broadband, airborne instrument that utilizes a novel ultra-wideband antenna feed array. We anticipate the instrument developed will serve as a valuable new scientific research tool to improve capability for making SWE measurements from space. The novel CSA ultra-wideband antenna technology has been developed for future Earth science missions requiring multiband or wideband microwave/mm-wave instruments. åÊåÊåÊ Earth Science and Applications from Space: Urgent Needs and Opportunities to Serve the Nation Committee on Earth Science and Applications from Space: A Community Assessment and Strategy for the Future, National Academy of Sciences National Research Council, 2005, available for free download at http://www.nap.edu/catalog/11281.html.  åÊåÊåÊ Xu, X., L. Tsang, and S. Yueh (2012), ÛÏElectromagnetic Models of Co/Cross-polarization of Bicontinuous/DMRT in Radar Remote Sensing of Terrestrial Snow at X- and Ku-band for CoReH2O and SCLP Applications,Û Selected Topics in Applied Earth Observations and Remote Sensing, IEEE Journal of, vol.5, no.3, pp. 1024-1032.  åÊMarshall, H.-P., G. Koh, and R. Forster. Estimating alpine snowpack properties using FMCW radar. ANNALS OF GLACIOLOGY, VOL 40,157ÛÒ162. 2005.  åÊåÊåÊ Marshall, H.-P., M. Schneebeli, and G. Koh. Snow stratigraphy measurements with high-frequency FMCW radar: Comparison with snow micro-penetrometer. COLD REGIONS SCIENCE AND TECHNOLOGY, 47(1-2):108ÛÒ117, 2007.
Dr. Tim Durham is an antenna systems engineer at Harris Corp. Government Communications Systems Division in Melbourne, Florida. He works on design and development of antennas and arrays. He is the principal investigator for the WISM Instrument Incubator Program.
Dr. H.P. Marshall is an associate professor at Boise State University, where he designs andåÊdeploys microwave radar to study radiative transfer and wave propagation in snow. He hasåÊled ground-based radar calibration and validation campaigns since 2003 for NASA and ESA snow remote sensing projects.
Dr. Leung Tsang is a professor of Electrical Engineering at University of Washington, Seattle.åÊ His expertise is in microwave remote sensing, electromagnetics and waves in random media and rough surfaces.åÊHe authored four books on these subjects.åÊHeåÊis a recipient ofåÊthe 2012 Pecora Award and the 2013 IEEE Electromagnetics Award.
Dr. Paul Racette is a member of the senior technical staff at the NASA Goddard Space Flight Center, where he leads technology development for microwave remote sensing of Earth’s environment. He’s also editor-in-chief of Earthzine.
Dr. Quenton Bonds is an engineer in the Microwave Instrument Technology Branch at the NASA Goddard Space Flight Center, where he designs and develops microwave sensors.
Dr. FÌ©lix A. Miranda is the chief of the Antenna and Optical Systems Branch in the Communications, Instrumentation and Controls Division at the NASA Glenn Research Center in Cleveland, Ohio. His primary interests are antenna technology and microwave-integrated circuits and devices for space and ground-based communications.
Dr. Ken Vanhille is a senior member of the technical staff at Nuvotronics. He has more than 10 years of experience in antennas and microwave electronics spanning frequencies from less than 1 GHz to greater than 300 GHz. He has been instrumental in the development and commercialization of the PolyStrata technology.
The material is based upon work supported by NASA under grant award NNX11AF27G
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of NASA.