Caglar Yardim

An intercomparison of models for predicting bistatic scattering from rough surfaces

This paper investigates the algorithms that are used to predict the full polarimetric bistatic normalized radar cross-section of rough surfaces. These include small perturbation method (SPM), the physical optics (PO) approach, the small slope approximation (SSA) and the integration equation method (IEM) and its derivatives improved IEM and advanced IEM. The methods are then compared to ground truth values obtained from multiple Monte Carlo runs a numerical Method of Moments (MOM) code using the same surface statistics. Effects of using band-limited exponential instead of a true exponential correlation function for surface statistics are also explored. Mean L1-norm error values integrated over the hemisphere are given between AIEM and MOM and SSA and MOM.

CASPER: Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research

CapsuleCASPER objective is to improve our capability to characterize the propagation of radio frequency (RF) signals through the marine atmosphere with coordinated efforts in data collection, data analyses, and modeling of the air-sea interaction processes, refractive environment, and RF propagation.

500–2000-MHz Brightness Temperature Spectra of the Northwestern Greenland Ice Sheet

An ultra-wideband radiometer has been developed to measure subsurface properties of the cryosphere including ice sheets and sea ice. The radiometer measures brightness temperature spectra from 0.5 to 2 GHz using 12 channels, each of which measures scene brightness temperatures over an ~88-MHz bandwidth resolved into 0.24-MHz intervals. The instrument was flown over northwestern Greenland in September 2016 and acquired the first, wideband, low-frequency brightness temperature spectra over the ice sheet and coastal region. The results reveal strong spatial and spectral variations that correlate well with the physical properties of the surface encountered along the flight path, which started over ocean, then passed the rock near the coast, and then up onto the ablation, wet, percolation, and dry snow zones of the interior ice sheet. In particular, strong spectral responses in percolation and dry snow zones are observed and plausibly explained by varying the distribution of horizontal density layers and isolated icy bodies in the upper portion of the firn. The success of the airborne deployment of the instrument and subsequent implementation of algorithms to limit radio frequency interference in unprotected bands is motivating continued airborne investigations as well as stimulating research into the feasibility of a spaceborne instrument.

Effects of ionospheric scintillation on V and W band signals

Crowded frequency bands are forcing Earth to space applications to investigate frequency bands that are higher than those currently used. While natural ionosphere effects will be negligible on frequencies above 30 GHz, amplitude scintillation caused by a high altitude nuclear explosion (HANE) could degrade communication links for a period of time. Amplitude scintillation is modeled using the multiple phase screen method. The results show that for a short duration of a few minutes after a HANE that amplitude scintillation will have an impact on signal frequencies below 60 GHz.

Range-dependent evaporation duct height estimation from a versatile ship-mounted X-band receiving array

As low-altitude atmospheric refractive conditions significantly affect the shipboard radars and radio communication conditions in the maritime environment, characterizing marine atmospheric boundary layer properties, ducting in particular, is thus crucial for air-sea interaction research. The CASPER-East at-sea experimental campaign conducted off the coast of Duck, NC, during October-November of 2015, provided an opportunity to measure EM propagation. A key part of the EM measurements is a specially designed X-band receiving array, which is deployed to measure the one-way propagation loss between emitters and receivers as a function of antenna height and range. The collected data from the receiving array at multiple ranges is be used to invert for the range-dependent evaporation duct height (EDH) by comparing with a library of propagation curves generated using the parabolic wave equation. The estimated EDH vs. range is compared with predictions from NAVSLaM using concurrent environmental measurements as input. The results show that EDH varies with range and time during data collection.

The Ultra-Wideband Software Defined Microwave Radiometer (UWBRAD) for Ice sheet subsurface temperature sensing: Calibration and campaign results

The Ultra-Wideband Microwave Radiometer is a novel pseudo-correlation radiometer design measuring scene brightness temperatures from 0.5–2 GHz created under NASAs Instrument Incubator Program. This document analyzes the design and operation of the radiometer, the accuracy and stability of the brightness temperatures it produces, and presents initial results from a field campaign conducted in Greenland in September 2016.

An efficient implementation of the nonlocal small-slope approximation for bistatic scattering from two-dimensional rough surfaces

We implement the second-order nonlocal small-slope approximation (NLSSA) to compute bistatic scattering from deterministic two-dimensional rough surfaces. Due to a fewer number of function evaluations in the NLSSA kernel versus the (local) second-order SSA kernel and NLSSA scattering amplitude characteristics that enable integral evaluations by fast Fourier transforms, the computational cost of NLSSA can be slightly reduced versus that of second-order SSA. For the rough surfaces and parameter space considered here, the results do not show significant differences among the techniques.

Variability of index of refraction from Large Eddy Simulations

Large Eddy Simulation (LES) is able to resolve the variability of the elevated trapping layer on top of the atmospheric boundary layer and hence can be used as a tool to study the uncertainties of using refractive profiles from a mesoscale model that is normally volume averaged within the mesoscale model grid. The variability within a stratocumulus-topped boundary layer is quantified here using LES simulated thermodynamic fields. Comparisons are also made between the LES field and aircraft measurements.

Evaporation duct refractivity inversion from EM propagation measurements and NAVSLaM predictions

An important application of air-sea interaction research is in characterizing marine atmospheric boundary layer (MABL) properties, ducting in particular, in order to predict radar and radio communication conditions in the marine environment. This ongoing research project, conducted off the coast of Duck, NC, during October-November of 2015, provided an opportunity to measure EM propagation during the CASPER at-sea experimental campaigns. The measured signal is used to invert for the evaporation duct refractivity profile based on Terrain Parabolic Equation Model (TPEM) and compared to the profile predicted from NAVSLaM using concurrent meteorological and oceanographic measurements as input.

UWB lower atmospheric propagation (LATPROP) measurement system

The ultra-wide band lower atmospheric propagation (UWB-LATPROP) measurement system is designed for remote sensing of marine atmospheric boundary layer (MABL) from propagation loss measurements over multiple frequencies from 2-40 GHz. The propagation loss is measured as a function of range and frequency and is used to invert for the evaporation duct heights. The system was deployed during the CASPER 2015 East Campaign. The evaporation duct parameters are estimated by minimizing the error function between the measured and simulated loss. A parabolic equation propagation model is used for EM propagation inside MABL with non-homogenous refractivity. The duct height matched well to the values recorded by the meteorological sensors.