Remote Sensing of Soil Moisture using S-band Signals of Opportunity: Model Development and Experimental Validation

Root zone soil moisture (RZSM) is a vital aspect in meteorology, hydrology, and agriculture. There are currently some methods in passive and active remote sensing at L-band, but these methods are limited to a sensing depth of approximately 10 cm. Observing RZSM (water in the top meter of soil) will require lower frequencies, thus presenting significant difficulties for a spaceborne instrument, because of the required antenna size, the presence of radio-frequency interference (RFI), and competition for spectrum allocations (in the case of active radar). Bistatic radar using Signal of Opportunity (SoOp) (e.g. digital satellite transmitters) provides an opportunity for remote sensing using powerful signals, which already occupies bands allocated for communications. Recently, a tower experiment has been conducted at Purdue Agronomy Center for Research and Education (ACRE). Linearly polarized measurements were made over bare soil, observing a strong reflected signal. Corn is being planted on the field and measurements will be made throughout the growing season. These measurements focus on measuring the soil properties, such as the weight and dielectric constant, as well as the vegetation characteristics

Refinement and Validation of a Real-time Airborne System for Remotely Sensing Ocean Surface using Communication Satellite Signals

The ability to remotely sense ocean wave heights and wind speed by measuring the reflected Radio Frequency (RF) signals from the ocean’s surface has been demonstrated in previous research projects. The recording systems for these research projects collected and stored unmodified RF signals and then analyzed the data through post-processing. Several disadvantages to this approach include large requirements for data storage and lengthy post-processing time. To assist in the creation of a suitable platform for an airplane-based application, a new system was designed which features real-time processing of the RF signals. This system captures two RF signals in the 2.4 GHz regions (direct and reflected), calculates the cross-correlation between the two signals and then outputs the result to a PC. Due to the time-consuming nature of the cross-correlation algorithm, a FPGA based implementation of the system was chosen to conform to the real-time constraints of the system. In this project, previously created Verilog source code for the system was debugged, improved and verified. This project also developed a method to test the system by using several 110-foot sections of RG6 Coaxial Cables. These cables induced a physical delay in the reflected channel, simulating the application’s conditions, in order to cause a shift in the correlation peak. The results are discussed as well as suggestions for future improvements

Remote sensing using I-Band and S-Band signals of opportunity

Measurement of soil moisture, especially the root zone soil moisture, is important in agriculture, meteorology, and hydrology. Root zone soil moisture is concerned with the first meter down the soil. Active and passive remote sensing methods used today utilizing L-band(1-2GHz) are physically limited to a sensing depth of about 5 cm or less. To remotely sense the soil moisture in the deeper parts of the soil, the frequency should be lowered. Lower frequencies cannot be used in active spaceborne instruments because of their need for larger antennas, radio frequency interference (RFI), and frequency spectrum allocations. Ground-based passive remote sensing using I-band(0.1-1GHz) signals of opportunity provides the required sensing depth and solves the problems that come with the spaceborne remote sensing instruments using I-band reflectometry. A dual monopole antenna setup was used with one on the ground for direct signal and one 30m above ground for the reflected signal. The reflectivity and therefore the soil moisture was obtained from the differences between direct and reflected signals. Initially, an S-band (2-3GHz) signal was used as a proof of concept and its ease of implementation because of its higher transmitted power and stationary satellite. This experiment provides conclusions about the root zone soil moisture based on our observation and comparison of direct and reflected satellite signals of two different frequency bands and determination of reflectivity

Using P-band Signals of Opportunity Radio Waves for Root Zone Soil Moisture Remote Sensing

Retrieval of Root Zone Soil Moisture (RZSM) is important for understanding the carbon cycle for use in climate change research as well as meteorology, hydrology, and precision agriculture studies. A current method of remote sensing, GNSS-R uses GPS signals to measure soil moisture content and vegetation biomass, but it is limited to 3-5 cm of soil penetration depth. Signals of Opportunity (SoOp) has emerged as an extension of GNSS-R remote sensing using communication signals. P-band communication signals (370 MHz) will be studied as an improved method of remote sensing of RZSM. P-band offers numerous advantages over GNSS-R, including stronger signal strength and deeper soil penetration. A SoOp instrument was installed on a mobile antenna tower in a farm field at Purdue University in West Lafayette, IN. An additional half-wave dipole antenna, as well as corresponding modifications to the experiment’s front-end box, was included to capture horizontally-polarized reflected P-band signals throughout a corn growth season. By measuring the reflected signal power off the soil over time, soil moisture and above-ground biomass can be measured. Soil moisture and vegetation biomass change the soil’s dielectric reflection coefficient and thus affect its reflectivity properties. It is expected that there will be strong correlation between reflected signal strength and soil moisture. Data will be compared against soil moisture measurements from in-situ soil sensors. The data obtained will be used to verify existing analytical soil moisture and above-ground biomass models. In addition, these results will be used to build an airborne and/or space-based remote sensing instrument

Ocean Wind Speed Measurement Using Wideband GNSS-R Signals

The use of Global Navigation Satellite System-Reflectometry (GNSS-R) for remote sensing of ocean wind speeds has been explored for nearly two decades and has become an important passive remote sensing technique for verification of hurricane formation models. To obtain wind speed measurements, reflected GNSS signals are first cross-correlated with locally generated copies to construct delay-doppler maps (DDMs). Through statistical models, DDMs can provide information about the mean-square slope (MSS) of the ocean surface, which is related to wind speed. Previous studies have focused on legacy signals such as GPS L1. However, it is expected that the MSS relationship with ocean wind field can be better understood by using similar methods with wider bandwidth signals, such as the GPS L5 or Galileo E5 signals. For this research purpose, a prototype data collection system was developed to handle the necessary data sampling speeds for the L5 and E5 signals. This system used a Universal Software Radio Peripheral X300 device configured to sample at 100 MHz. A receiver was designed to generate DDMs with the L5 signal through zero-padded FFT techniques for cross-correlations. Lab tests showed that the resulting DDMs have the proper sinc function behavior with respect to doppler frequency and triangle function behavior to time delay. For future field experiments, data will be obtained by mounting the prototype system to the National Oceanic and Atmospheric Administration (NOAA) Hurricane Hunter aircraft during the 2017 hurricane season, and wind speed retrieval will be performed by analyzing the DDMs generated by the receiver

Nanoscale fluid structure of liquid-solid-vapour contact lines for a wide range of contact angles

We study the nanoscale behaviour of the density of a simple fluid in the vicinity of an equilibrium contact line for a wide range of Young contact angles θY ∈ [40◦ , 135◦ ]. Cuts of the density profile at various positions along the contact line are presented, unravelling the apparent step-wise increase of the film height profile observed in contour plots of the density. The density profile is employed to compute the normal pressure acting on the substrate along the contact line. We observe that for the full range of contact angles, the maximal normal pressure cannot solely be predicted by the curvature of the adsorption film height, but is instead softened – likely by the width of the liquid-vapour interface. Somewhat surprisingly however, the adsorption film height profile can be predicted to a very good accuracy by the Derjaguin-Frumkin disjoining pressure obtained from planar computations, as was first shown in [Nold et al., Phys. Fluids, 26, 072001, 2014] for contact angles θY < 90◦ , a result which here we show to be valid for the full range of contact angles. This suggests that while two-dimensional effects cannot be neglected for the computation of the normal pressure distribution along the substrate, one-dimensional planar computations of the Derjaguin-Frumkin disjoining pressure are sufficient to accurately predict the adsorption height profile

Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory

We examine the nanoscale behavior of an equilibrium three-phase contact line in the presence of long-ranged intermolecular forces by employing a statistical mechanics of fluids approach, namely density functional theory (DFT) together with fundamental measure theory (FMT). This enables us to evaluate the predictive quality of effective Hamiltonian models in the vicinity of the contact line. In particular, we compare the results for mean field effective Hamiltonians with disjoining pressures defined through (I) the adsorption isotherm for a planar liquid film, and (II) the normal force balance at the contact line. We find that the height profile obtained using (I) shows good agreement with the adsorption film thickness of the DFT-FMT equilibrium density profile in terms of maximal curvature and the behavior at large film heights. In contrast, we observe that while the height profile obtained by using (II) satisfies basic sum rules, it shows little agreement with the adsorption film thickness of the DFT results. The results are verified for contact angles of 20, 40 and 60 degrees

Nonequilibrium molecular dynamics simulations of nanoconfined fluids at solidliquid interfaces

We investigate the hydrodynamic properties of a Lennard-Jones fluid confined to a nanochannel using molecular dynamics simulations. For channels of different widths and hydrophilic-hydrophobic surface wetting properties, profiles of the fluid density, stress, and viscosity across the channel are obtained and analysed. In particular, we propose a linear relationship between the density and viscosity in confined and strongly inhomogeneous nanofluidic flows. The range of validity of this relationship is explored in the context of coarse grained models such as dynamic density functional-theory

The vicinity of an equilibrium three-phase contact line using density functional theory: Density profiles normal to the fluid interface

The paper by Nold et al. [Phys. Fluids 26 (7), 072001 (2014)] examined density profiles and the micro-scale structure of an equilibrium three-phase (liquid–vapour–solid) contact line in the immediate vicinity of the wall using elements from the statistical mechanics of classical fluids, namely density-functional theory. The present research note, building on the above work, further contributes to our understanding of the nanoscale structure of a contact line by quantifying the strong dependence of the liquid–vapour density profile on the normal distance to the interface, when compared to the dependence on the vertical distance to the substrate. A recent study by Benet et al. [J. Phys. Chem. C 118 (38), 22079 (2014)] has shown that this could explain the emergence of a film-height-dependent surface tension close to the wall, with implications for the Frumkin–Derjaguin theory

Generalized dynamical density functional theory for classical fluids and the significance of inertia and hydrodynamic interactions

We study the dynamics of a colloidal fluid including inertia and hydrodynamic interactions, two effects which strongly influence the non-equilibrium properties of the system. We derive a general dynamical density functional theory (DDFT) which shows very good agreement with full Langevin dynamics. In suitable limits, we recover existing DDFTs and a Navier-Stokes-like equation with additional non-local terms.Comment: 5 pages, 4 figures, 4 supplementary movie files, I supplementary pd