Overview Background Andria L Bilich 1, Andrew G Slater 2, Kristine M Larson 3 1 National Geodetic Survey, NOAA/NOS, Boulder CO; corresponding author:

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Presentation transcript:

Overview Background Andria L Bilich 1, Andrew G Slater 2, Kristine M Larson 3 1 National Geodetic Survey, NOAA/NOS, Boulder CO; corresponding author: 2 NSIDC/CIRES, Univ of Colorado, Boulder CO; 3 Aerospace Engineering, Univ of Colorado, Boulder CO Snow Depth with GPS: Case Study from Minnesota C33E Snow Depth Comparison Winter Data Sources GPS-IR Theory Although originally designed to enable accurate positioning and time transfer, the Global Positioning System (GPS) has also proved useful for remote sensing applications. In this study, GPS signals are used to measure snow depth via GPS interferometric reflectometry (GPS-IR). In GPS-IR, a GPS antenna receives the desired direct signal as well as an indirect signal which reflects off of the ground / snow surface. In this study, we use GPS- IR to create a new snow depth product for the state of Minnesota over the winter of This is one of the first studies to compute GPS snow depth over a large regional-scale network. We chose Minnesota for this study because the state Department of Transportation runs a large network of continuously operating reference stations (CORS) with many desired characteristics: freely available data (anonymous FTP) good GPS station distribution with reasonable proximity to COOP weather stations many GPS stations located adjacent to farm fields with few sky obstructions Trimble receiver models with L2C SNR data (best data source for GPS- IR) Free GPS stations already installed, operating for another purpose Spatial coverage Multiple satellites & arcs at each station Each arc samples ~ 50 sq meters Temporal coverage Satellites rise/set throughout day Each arc ~ 2 hr average Latency – minutes to hours (after all data for satellite pass have been recorded) GPS-IR Compared to SNODAS and COOP Start with raw GPS SNR data Isolate reflected component Lomb periodogram of reflected component Dominant frequency is proportional to antenna- snow distance Do this for each available GPS satellite arc The composite SNR signal recorded by the GPS receiver is post-processed to yield the distance between the antenna and the reflecting surface, that is, distance to the snow surface. Technique Advantages Validation Characteristics of a Good GPS-IR Station  Antenna height of 1.5 meters or more above snow-free ground surface [taller if lots of snow expected]  Flat or slightly titled ground surface [snow height retrieval error negligible for slope angles < 5  ]  Bare ground lacking obstructions for 50 meters or more distance from antenna References / Acknowledgements Larson, K.M., E. Gutmann, V. Zavorotny, J. Braun, M. Williams, and F. Nievinski, Can We Measure Snow Depth with GPS Receivers?, Geophys. Res. Lett., 36, L17502, doi: /2009GL039430, 2009 Gutmann, E., K. M. Larson, M. Williams, F.G. Nievinski, and V. Zavorotny, Snow measurement by GPS interferometric reflectometry: an evaluation at Niwot Ridge, Colorado, Hydrologic Processes, in press Larson, K.M. and F.G. Nievinski, GPS Snow Sensing: Results from the EarthScope Plate Boundary Observatory, GPS Solutions, under review. Interpretation GPS caveats: Careful site selection is important  Many CORS are in urban settings  Seasonal vegetation effects can be significant Not all satellites at a rural CORS may be useful (trees, roads, tall hillsides, etc. can create obstructions) Future Work  Assess data from other states, both CORS and Plate Boundary Observatory networks  Automate into a semi- operational product  Begin applications (e.g. improve NWS snow data assimilation)  Determine GPS-IR utility (e.g. via increased streamflow forecast skill) Advantages to the GPS- IR snow depth technique: GPS inversion only requires 1-time calibration based on land cover Provides snow information for model validation, calibration or assimilation Provides an estimate of spatial variability; particularly useful for assimilation Automated, objective method with direct repeatability Potential nationwide network of opportunity Data available in near real-time Interference is the key to GPS-IR. Direct + reflected creates oscillations whose frequency is proportionate to reflector distance. Reflected signal Direct signal 1 in. 2 in. Reflected component of SNR in spring and summer, PRN12 at ASHL. GPS-IR Method 50 cm snow The reflections group at CU is supported by NSF EAR , NSF AGS ; AES is supported by NOAA grant NA11OAR We thank MN-DOT, especially Blaine McKeever, for providing the raw GPS data files. Please see our website at for more information. Data collected at Niwot Ridge LTER in Colorado, at a 3,500-m altitude alpine tundra site. Gray dots are independent estimates of daily tracks for GPS satellite PRN 05, due south. Black is 7-day smoothed GPS results. Red is terrestrial laser scanner, co-located with the GPS. Blue are manual snow depth measurements collected approximately every week. See poster C33A-0620 for additional information on this study. Not GoodGood Peak frequency returned by the periodogram is converted into distance to snow surface and snow depth. Reflection points indicate spatial coverage. This figure shows all possible L2C satellites observed over one day at GPS station ASHL. Each satellite is viewed at a different time of day, creating temporal coverage as well. MN-DOT GPS Network MN Dept of Transportation network are sensors of opportunity – continuously operating, free data, installed for use by surveyors and geodesists 33 stations are useful for GPS-IR, out of about 125 stations in full network COOP Observing Network Volunteer weather observers record daily values of temperature, liquid precipitation, snowfall, and snow depth Manual measurements, records sometimes incomplete 141 stations in MN, with variable sampling in SNODAS / National Snow Analysis Modeled quantities with daily semi- objective & manual assimilation of observations Observations include MODIS snow cover, SNOTEL's, COOP depths ~4km resolution over the CONUS Operational system at NOAA-NOHRSC since Oct Looking South EMLY MRTA ASHL SD = SNOWDAS, shown both at the COOP location and at the location of the GPS station. COOP observations are shown as black squares, with the distance to COOP station given in plot legend. GPS snow depths are colored circles; we provide the daily mean value of several satellites present at a given azimuth. Colored vertical bars show the range of individual satellite observations contributing to the mean. Looking West ASHL N Location map for COOP observing stations (grey triangles) and GPS-IR stations (colored circles). Circle size and color indicate when L2C data streams for the GPS-IR stations were activated in All stations are operational for the winter season. Google Earth imagery is overlain with the first Fresnel zone for satellites used to generate the GPS-IR timeseries. The zones assume an average 2-m antenna height abive snow-free ground. The largest zones are for satellite at 5 deg elevation, whereas the smaller zones represent the area sensed by satellite at 10 deg elevation. EMLY N MRTA N Looking WestLooking North GPS-IR time series accurately capture snow information, in locations where these data are not normally available: Local variability in snowpack Time and magnitude of snowstorms Snow melt rates These two signals interfere, and the composite signal is recorded by the GPS receiver. Although this interference affects all GPS measurements (phase, pseudorange), this study uses the signal-to-noise ratio (SNR) measurement. summer spring