Microwave Attenuation

1. ‘Scaling Down’ Remote Sensing Technologies for In Situ Monitoring of Snowpacks Jeff Frolik1, Chris Skalka2 and Beverley Wemple3 University of Vermont (UVM) 1School of Engineering, 2Department of Computer Science and 3Department of Geography < 1.25 dB/cm possible signal depolarization > 2dB/cm Gamma Photon Absorption • Determining SWE from the snowpack’s absorption of gamma-ray photons has been previously demonstrated. However, these systems employed high-cost, high-energy photomultiplier tubes and have considered specific energy levels well beyond 1 MeV. Our work is investigating the use of background radiation as a source, in particular a range of energies from 100-300 keV, and a nascent detector technology. • The above results were obtained utilizing a Cadmium-Zinc-Telluride (CZT) semiconductor gamma detector which was placed above the snowpack. Additional key results to date include: • Good correlation up to 40 cm SWE • Results independent of detector height • Results dependent on location Due to the time required for measurements (order of minutes), the gamma detector will require a significant amount of energy (in comparison to other station components). As such we are developing an adaptive, multi-modal (MM) sampling algorithm for the MM-SWE stations to ensure parsimonious use of limited energy. Motivation Airborne gamma radiation and microwave remote sensing are well-used tools for snowpack monitoring. However, these techniques have short-comings in terms of resolution (in time and space) and accuracy. As such, ground truth (i.e., in situ) data via snow courses and/or snow pillows/scales is used to complement remote sensing. Unfortunately, these existing in situ methods also have significant shortcomings. Snow courses are manually tasking, destructive and have poor time resolution and snow pillows/scales have poor spatial resolution, are invasive to the environment and are costly. Both methods are also limited to terrains that are relatively flat and hazard-free. Our work investigates ‘scaling down’ microwave and gamma radiation sensing by considering technologies that are lower in cost, lower in power requirements and operate at lower frequencies or energy levels. Our motivation is to ensure these ‘sensors’ can be integrated with wireless networking hardware in order to form a distributed, monitoring system that can be deployed in a variety of terrains and remotely accessed to retrieve data with arbitrary temporal and spatial resolution. Microwave Attenuation Our work has focused on the unlicensed 2.4GHz and 5 GHz frequency bands used by 802.11/WiFi systems. Due to the pervasiveness of such products, hardware for these bands are low in cost and energy efficient. Placing antennas above and below the snowpack, the radio pathloss change as snow accumulates can be captured using the RSSI functionality built into radio chipsets. In the data above, we find the 2.4 GHz and 5 GHz bands having attenuation rates of < 1.25 dB/cm-SWE and > 2.0 dB/cm-SWE, resp. (SWE 30%). The data below shows attenuation variability during conditions of snowmelt. The importance here is that the internal condition of the snowpack was ascertained in a non-invasive manner (in contrast to temperature probes). Synergistic Activities • We have also developed a robust wireless platform for harsh environments which enables multiple stations to be mesh-networked. This system (Snowcloud) is being tested in Sierra Nevada Range (Western U.S.) to monitor snowdepth (SD) through out the winter of 2009-10. Our long-term objective is to enhance these stations with MM-SWE capability. • Concurrent field work in the Appalachian Range (Eastern U.S.) is being conducted to examine forest canopy effects on snow distribution in complex terrains. • Acknowledgments • NASA / Vermont Space Grant Consortium • UVM SEED senior project teams • Sierra Nevada Aquatic Research Laboratory • Mammoth Mtn. Energy Balance Monitoring Site • Hubbard Brook Experimental Forest CZT-based detector and preliminary data SD deployment (Truckee, CA) and MM-SWE station concept Multi-band Field Investigation (Mammoth Lakes, CA) Snow course: Labor intensive, destructive and having poor temporal resolution Snow pillow: Costly, invasive and having poor spatial resolution Photos: U.S. Forest Service (L) & California Department of Water Resources (R) Site Plan for Study of Canopy Effects on SWE (Hubbard Brook Experimental Forest, NH) Adaptive Sampling Scheme Snowpack Attenuation During Freeze/Thaw Cycles American Geophysical Union (AGU) Fall Meeting, San Francisco CA, December 2009