Remote Sensing using Coherent Multipath Interference of Wideband Planck Radiation Seyedmohammad Mousavi * , Roger De Roo † , Kamal Sarabandi * , Anthony England ‡ , and Hamid Nejati * * Electrical Engineering and Computer Science Department University of Michigan, Ann Arbor, Michigan 48109-2122 Email: mousavis@umich.edu, saraband@umich.edu, and hnejati@umich.edu † Climate and Space Sciences and Engineering Department University of Michigan, Ann Arbor, Michigan 48109-2143 Email: deroo@umich.edu ‡ College of Engineering and Computer Science University of Michigan, Dearborn, Michigan 48128-2406 Email: england@umich.edu Abstract— A novel microwave radiometric technique, known as wideband autocorrelation radiometry (WiBAR), is introduced as a direct method to remotely measure the layer thickness of low- loss terrain covers such as snow and ice. This is done by measuring the propagation time  ࢊࢋ from the autocorrelation function (ACF) of multipath microwave emission. We report measurements of the snowpack thickness using WiBAR at the University of Michigan Biological Station (UMBS) in winter 2015. The observations are done at frequencies from 1 to 3 GHz. At these frequencies, the volume and surface scattering are small in the snowpacks. This technique is inherently low-power since there is no transmitter as opposed to active remote sensing techniques. Keywords—microwave radiometry; remote sensing; snowpack I. INTRODUCTION Environmental changes such as global warming impose rapid changes upon the cryosphere [1]; as a result, the statistics which demonstrate the extent, timing, and Snow Water Equivalent (SWE) of seasonal snowpacks on prairie and alpine terrains are no longer stationary [2]. Effective management of this freshwater reservoir and adaption to variable risks of flooding can benefit from almost daily monitoring the spatial and temporal distributions of SWE and snowpack wetness. Thus a detailed understanding of the snowpack accumulation, metamorphism, and melting would be a beneficial outcome of near daily observations of the SWE and wetness of snowpacks. II. CURRENT REMOTE SENSING OF DRY SNOW PACK Current microwave remote sensing of dry snowpack is based on frequency dependent differential scattering by the ice grains that comprise snowpacks [3]-[4]. Differences between microwave brightness temperatures at two different frequencies, namely 19 and 37 GHz, are used to estimate the SWE of snowpacks. However, it is not robust since the scattering theory yields only the form of frequency dependent scatter darkening but not a reliable amplitude estimation. The algorithm should be empirically tuned to a region’s typical snowpack. Thus, it is highly dependent on the microscopic properties of the snowpack (e.g. grain size), which varies considerably from place to place and time to time. In addition, tuning algorithms become very complicated or even unworkable for complex terrains. III. WIDEBAND AUTOCORRELATION RADIOMETRY Wideband Autocorrelation radiometry makes use of the correlation that exists between the thermal radiation from the surface beneath or within the pack which travels upwards through the pack towards the radiometer, which will be referred to as the direct signal, and other portion of the radiation that reflects back from the pack’s upper interface then from its lower interface, before traveling towards the radiometer antenna, as shown in Fig 1. Thus, there are two signals received by the radiometer, the direct signal and a delayed copy of it. The vertical extent of the pack can be found from the microwave propagation time  ௗ௘ by measuring the time delay between these two signals. Fig. 1. Passive remote sensing of microwave travel time within the pack using WiBAR. The direct signal and the delayed signal arrive at the radiometer antenna with the time difference  ௗ௘ .  ௗ௘ = ʹ ௣ − ௥   ௣ is the one way travel time in the pack, and  ௥ is the travel time in the air between points P1 and P2 of the direct signal. Incidence angles  ௣ and   are related by Snell’s law, as shown in (2).  ௣   ௣ = ௥     where  ௥ =ͳ . In the case of homogenous and isotropic icepack with constant refractive index ( ௣ = ௖௘ = ͳ.775), (1) is simplified to (3). Support provided by NASA Terrestrial Hydrology program contract NNX15AB36G. 2051 978-1-5090-2886-3/16/$31.00 ©2016 IEEE AP-S 2016