The utility of microwave remote-sensing of the ocean surface for current detection is well established. However, the advent of along-track interferometric synthetic aperture radar has provided a new, potentially powerful technique for current mapping. Although interferometric velocity measurements can be used to derive surface currents, the relationship between these two quantities is not always clearly defined. In response, this thesis presents comparisons between interferometric data collected by an X-band phased-array radar and in situ data, thereby demonstrating the relationship of interferometric velocity measurements to surface and subsurface currents. To develop a precise method of estimating the surface current from the interferometric measurements, the wave-orbital velocities and Bragg phase-speed are characterized. This analysis is extended to compare surface and subsurface water currents. Case studies are presented under varying environmental conditions for which the vertical current structure alters considerably. In these examples, analysis of the radar imagery yields both interferometric surface currents and subsurface current estimates derived from long-wave dispersion characteristics. A vertical profile of current in the water column is generated from the radar-derived velocities combined with coincident Acoustic Doppler Current Profiler measurements, revealing the sensitivity of X-band interferometric measurements to wind-drift and the near-surface current structure.

This thesis describes ground based measurements of the forward scatter characteristics of a variety of terrain. The system used to make these measurements is a truck based system of two radars at 35 GHz and 225 GHz. The 35 GHz radar is a coherent polarimeter, while the 225 GHz polarimeter is a non-coherent system. These radars are mounted on an antenna positioner in a truck based platform. These forward scatter measurements are made using a novel configuration where a trihedral corner reflector, located opposite a co-located receiver and transmitter, is used to reflect the transmitted pulse back to the receiver. Therefore the current monostatic configuration of the radars does not need to be modified and alignment problems are reduced. The measurements covered in this thesis are near-grazing measurements. These measurements are generally more difficult to make due to undulations of the ground. This thesis provides a summary of the radar systems, including recent improvements, and operational principles of polarimetry including an analysis of the calibration and data processing. Finally, the experiments in and around Amherst, Massachusetts are presented along with comparisons to a variety of scattering models. The most promising of which is an integral equation model which matches very well to the measured data. Also, the thesis describes a technique to determine the amounts of and separate the diffuse and specular components of forward scatter and furthermore separate the two components.

Railway track substructure maintenance management is the process of utilizing railroad resources to maintain and upgrade the track substructure. The process begins with a measure of the track condition to evaluate the substructure performance, determine locations along the track that require maintenance, and identify appropriate solutions. Ground penetrating radar (GPR) has been proposed as a potentially valuable tool for this purpose. The objective of the research was to develop GPR testing and data interpretation techniques suitable for use by railroad personnel in this application. The principle of GPR operation is the transmission of short electromagnetic waves into the subsurface and recording the resulting signal of the reflected waves. Electromagnetic waves are influenced most significantly by the dielectric constant of the soil. The dielectric constant is most affected by moisture content making GPR a valuable tool for locating trapped water that will cause increased track deterioration rates. GPR has the potential to evaluate the thicknesses and properties of the substructure layers on a continuous, non-destructive basis to improve the process of diagnosing substructure causes of track performance deterioration. GPR railroad research at UMass started with the construction of a test track. Different track structure components were tested to determine their effects on the GPR data. Approximately 200 tests were conducted. Subsequently, about 275 miles of data were collected on several U.S. railroads including Amtrak, Conrail, New England Central, and Burlington Northern Santa Fe, and in England on AMEC Rail. The data were studied to determine how well GPR can define substructure conditions, identify track problem areas, and provide an indication of the cause of the problem. The analysis included comparison of the GPR data to track geometry, subsurface stratigraphy, and ballast condition (fouling and moisture). GPR processing techniques were developed to simplify interpretation of the data. The results showed that GPR could locate zones of increased substructure degradation at over 75% of the sites.

This dissertation presents the design and initial experimental results of a second generation FOcused Phased Array Imaging Radar (FOPAIR-II) demonstrating its capability to measure areas on the order of 20 meters x 20 meters with 0.375 meter range resolution and 1° beamwidth. An analysis of the error budget for the given geometry is presented, yielding a worst case height bias of 4.5 cm and an expression for determination of the uncertainty given signal-to-noise ratio and the temporal lag between interferometer measurements. The processing algorithm is shown and a method for distortion removal is described based on basic assumptions of the properties of the ocean surface over a time average. Comparison of significant wave height measurements with the in-situ sensors shows a correlation of 0.92, with a slope of 0.97 and an intercept of −0.001 meters. Absolute height measurement comparison reveals a correlation between the radar estimated absolute height and a nearby tide gauge of 0.94, with a slope of 1.60 and an intercept of 0.75 meters.

This dissertation details the theoretical development, design, fabrication, and testing of two remote sensing antennas. The antennas operate in Ku and Ka bands and must support multiple beams, polarizations, and frequencies with a single aperture. The first antenna, developed for NASA's High-Altitude Imaging Wind and Rain Airborne Profiler, is a single, offset-fed reflector that supports dual-band beams incident at 30° and 40° off-nadir. The antenna uses two compact, dual-band feeds moved away from the reflector's focal point to meet the dual beam requirement. The radar is to be flown on the Global Hawk Uninhabited Aerial Vehicle which has a small payload bay requiring the feeds to be both rugged and compact. The second antenna, developed for Remote Sensing Solutions' Dual-Wavelength Precipitation Radar, is a dual-offset Gregorian reflector. The antenna supports a single, dual-band, beam with dual-polarization at each band. Additionally, the antenna has high polarization purity and matched half power beamwidths at Ku and Ka bands. The strict requirements of the antenna are met by precisely controlling feed radiation characteristics. The two antennas necessitated several advances in feed design. A foam sleeve is demonstrated as an effective method to reduce the beamwidth of a tapered dielectric rod antenna. The foam sleeve is an attractive design because it allows dual-band feeds where a corrugated horn is used to control radiation at lower frequencies and the sleeve corrected rod is used to control the upper band. By judiciously choosing sleeve material, independent control of the radiation pattern and phase center at each band is achieved allowing higher performance feeds. This dissertation also focuses on new developments in the backend design of feeds. Specifically, the use of tuning arms in the feed backend and double ridged waveguide to couple the signal into the feed allow more compact designs with greater bandwidth.

Although spheres and spheroids have been used extensively by researchers as convenient models to approximate "snowflakes" when computing their microwave scattering properties, recent research indicates that the scattering properties of more accurately simulated snowflakes are fundamentally different from the simplified models. To resolve this well-recognized discrepancy, a new snowflake aggregation model is developed in this study and the microwave single-scattering properties of the modeled aggregate snowflakes are characterized for use in radiative transfer modeling and remote sensing algorithm development. Three different aggregate snowflake types (rounded, oblate and prolate) are generated by random aggregation of 6-bullet rosettes constrained by size-density relationships derived from previous field observations. Additionally, they are further constrained to empirically determined aspect ratios (ar) and fractal dimensions (df) of aggregate flakes. Due to random generation, aggregates may have the same size or mass, yet different morphology, allowing for a study into how detailed structure influences an individual flake's scattering properties. Single-scattering properties of the aggregates were investigated using discrete dipole approximation (DDA) at 10 frequencies: 10.65, 13.6, 18.7, 23.8, 35.6, 36.5, 89.0, 94.0, 165.5 and 183.31 GHz. All of these frequencies are currently used in instruments (radar and radiometers) aboard satellites involved in the research of atmospheric ice particles. Results from DDA were compared to those of Mie theory for solid and soft spheres (with a density 10% that of solid ice) and to T-matrix results for solid and soft spheroidal cases with ar values of 0.8 and 0.6 dependent on flake type (rounded, oblate or prolate). Analyzing modeling results, it is found that above size parameter 0.75, neither solid nor soft sphere and spheroidal approximations accurately represented the DDA results for all aggregate types. The asymmetry parameter and the normalized scattering and backscattering cross-sections of the aggregate groups fell between the soft and solid spherical and spheroidal approximations. This implies that evaluating snow scattering properties using realistic shapes, such as the aggregates created in this study, is necessary in radiative transfer modeling and remote sensing studies. When examining the dependence of the single-scattering properties on each aggregate's detailed structure, morphology seemed of secondary importance. Using normalized standard deviation as a measure of relative uncertainty, it is found that the relative uncertainty in backscattering arising from the different morphologies caused by random aggregation is typically ~17%, 13% and 14% for individual particles and ~20%, 30% and 30% when integrated over size distributions for rounded, oblate and prolate flakes respectively. Relative uncertainties for other single-scattering parameters are less. These analyses indicate that a scattering database can be created to approximate the single-scattering properties of realistic aggregate flakes. A database of such aggregate flakes has been created based upon the research detailed herein, and made available for public use. In this work, it is found that flakes with similar size parameters can scatter differently. Ongoing research indicates that this is due to outer layer morphology of the flake (i.e. the arms of a dendritic snowflake) rather than any interior properties. When the interior of an aggregate flake is scrambled, the scattering results are nearly the same as the unscrambled interior whereas if the outer layer is altered, scattering results differ. Another interesting trend noted is that randomly oriented flakes with differing ar values have noticeably differing backscatter cross-sections and could have significant implications for future research. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2015. / April 23, 2015. / Includes bibliographical references. / Guosheng Liu, Professor Directing Dissertation; Eric Chicken, University Representative; Mark Bourassa, Committee Member; Robert Ellingson, Committee Member; Vasu Misra, Committee Member.

An atmospheric water vapor and non-precipitating liquid water profiling system is presented. Included are a review and performance characterization of the hardware, a description with results of the calibration procedure, and experimental confirmation of the profiling system with coincident radiosonde balloon comparisons. The hardware consists of a K-Band (20.5-23.5 GHz) Autocorrelation Radiometer (CORRAD), designed, built, and operated by the Microwave Remote Sensing Laboratory at the University of Massachusetts at Amherst. CORRAD measures the autocorrelation of thermal noise at K-Band generated by water in the atmosphere. The sensor represents a novel approach to microwave remote sensing of the atmosphere with regard to pre-detection bandwidth (3 GHz) and number of equivalent frequency channels (31). The complete system calibration procedure is presented, including frequency resolution (100 MHz) and accuracy, front end system noise debiasing, and absolute gain calibration. An algorithm is developed to recover the atmospheric profiles of water vapor and liquid water from the measured autocorrelation samples. The algorithm uses a constrained minimum squared error estimation procedure on a first order perturbation of the full equation of radiative transfer in the atmosphere. Water vapor lapse rates are estimated with better than $\pm$150 m accuracy. Profile results are in excellent agreement with simultaneous radiosonde balloon soundings by the National Weather Service. A complete system signal-to-noise analysis is performed, from the statistics of the raw data to the uncertainties in the estimated profile. Profile relative uncertainties are 5-10% in the lower troposphere with a 1.0 K standard deviation in the antenna temperature spectrum measurements.

The intention of this investigation is to investigate the various forms of water wave reflection to a high degree of accuracy. Close range photogrammetry is the technique that is used to measure the water surface profile, as it produces an accurate and comprehensive analysis of the water surface profiles. Conventional photogrammetry techniques (photography) were used in preference to near real time photogrammetry (digital). Although near real time photogrammetry has the advantage of a far higher rate of data acquisition, it does not achieve the same degree of accuracy as can be achieved by conventional photogrammetry, the technique finally used for this investigation. For the generation of the desired wave patterns, certain equipment was developed and modified. To test the various angles of incidence a moveable reflecting wall was built. Due to the small size of the wave basin, an efficient wave absorber was required to absorb the reflected wave generated when oblique wave reflection was investigated. It was observed that a very poor quality wave was being generated by the wave generator, as a result of its flexibility. Consequently the wave generator was stiffened considerably which improved the wave generated. Interesting information was obtained from the analysis of the standing wave. A coefficient of reflection of 1,6 at the reflection wall was obtained, this places new emphasis on the relationship between the standing wave and overtopping. The oblique wave reflection resu1ts corresponded well with theoretical predictions, while no well-defined trends were established for the mach wave. The data for the mace wave did however suggest that previously established trends for the mach wave may not be correct. Close range photogrammetry produced accurate results, and is an excellent method for water surface profile measurement. The results obtained showed that the wave generator was not generating a pure wave, which lead to unknown errors in the results of spot heights. Conventional photogrammetry is a slow process, so not enough data was acquired to adequately analyse the reflection trends. This suggests that in order for the trends to be well-established, near real time photogrammetry should be used once these systems have developed sufficient accuracy.

This thesis investigates the current knowledge of small space debris (diameter less than 10 cm) and potentially hazardous asteroids (PHA) by the use of radar systems. It clearly identifies the challenges involved in detecting and tracking of small space debris and PHAs. The most significant challenges include: difficulty in tracking small space debris due to orbital instability and reduced radar cross-section (RCS), errors in some existing data sets, the lack of dedicated or contributing instruments in the Southern Hemisphere, and the large cost involved in building a high-performance radar for this purpose. This thesis investigates the cooperative use of the KAT-7 (7 antennas) and MeerKAT (64 antennas) radio telescope receivers in a radar system to improve monitoring of small debris and PHAs was investigated using theory and simulations, as a cost-effective solution. Parameters for a low cost and high-performance radar were chosen, based on the receiver digital back-end. Data from such radars will be used to add to existing catalogues thereby creating a constantly updated database of near Earth objects and bridging the data gap that is currently being filled by mathematical models. Based on literature and system requirements, quasi-monostatic, bistatic, multistatic, single input multiple output (SIMO) radar configurations were proposed for radio telescope arrays in detecting, tracking and imaging small space debris in the low Earth orbit (LEO) and PHAs. The maximum dwell time possible for the radar geometry was found to be 30 seconds, with coherent integration limitations of 2 ms and 121 ms for accelerating and non-accelerating targets, respectively. The multistatic and SIMO radar configurations showed sufficient detection (SNR 13 dB) for small debris and quasi-monostatic configuration for PHAs. Radar detection, tracking and imaging (ISAR) simulations were compared to theory and ambiguities in range and Doppler were compensated for. The main contribution made by this work is a system design for a high performance, cost effective 3D radar that uses the KAT-7 and MeerKAT radio telescope receivers in a commensal manner. Comparing theory and simulations, the SNR improvement, dwell time increase, tracking and imaging capabilities, for small debris and PHAs compared to existing assets, was illustrated. Since the MeerKAT radio telescope is a precursor for the SKA Africa, extrapolating the capabilities of the MeerKAT radar to the SKA radar implies that it would be the most sensitive and high performing contributor to space situational awareness, upon its completion. From this feasibility study, the MeerKAT 3D distributed radar will be able to detect debris of diameter less than 10 cm at altitudes between 700 km to 900 km, and PHAs, with a range resolution of 15 m, a minimum SNR of 14 dB for 152 pulses for a coherent integration time of 2.02 ms. The target range (derived from the two way delay), velocity (from Doppler frequency) and direction will be measured within an accuracy of: 2.116 m, 15.519 m/s, 0.083° (single antenna), respectively. The range, velocity accuracies and SNR affect orbit prediction accuracy by 0.021 minutes for orbit period and 0.0057° for orbit inclination. The multistatic radar was found to be the most suitable and computationally efficient configuration compared to the bistatic and SIMO configurations, and beamforming should be implemented as required by specific target geometry.

High frequency microwave instruments are increasingly used to observe ice clouds and snow. These instruments are significantly more sensitive than conventional precipitation radar. This is ideal for analyzing ice-bearing clouds, for ice particles are tenuously distributed and have effective densities that are far less than liquid water. However, at shorter wavelengths, the electromagnetic response of ice particles is no longer solely dependent on particle mass. The shape of the ice particles also plays a significant role. Thus, in order to understand the observations of high frequency microwave radars and radiometers, it is essential to model the scattering properties of snowflakes correctly. Several research groups have proposed detailed models of snow aggregation. These particle models are coupled with computer codes that determine the particles' electromagnetic properties. However, there is a discrepancy between the particle model outputs and the requirements of the electromagnetic models. Snowflakes have countless variations in structure, but we also know that physically similar snowflakes scatter light in much the same manner. Structurally exact electromagnetic models, such as the discrete dipole approximation (DDA), require a high degree of structural resolution. Such methods are slow, spending considerable time processing redundant (i.e. useless) information. Conversely, when using techniques that incorporate too little structural information, the resultant radiative properties are not physically realistic. Then, we ask the question, what features are most important in determining scattering? This dissertation develops a general technique that can quickly parameterize the important structural aspects that determine the scattering of many diverse snowflake morphologies. A Voronoi bounding neighbor algorithm is first employed to decompose aggregates into well-defined interior and surface regions. The sensitivity of scattering to interior randomization is then examined. The loss of interior structure is found to have a negligible impact on scattering cross sections, and backscatter is lowered by approximately five percent. This establishes that detailed knowledge of interior structure is not necessary when modeling scattering behavior, and it also provides support for using an effective medium approximation to describe the interiors of snow aggregates. The Voronoi diagram-based technique enables the almost trivial determination of the effective density of this medium. A bounding neighbor algorithm is then used to establish a greatly improved approximation of scattering by equivalent spheroids. This algorithm is then used to posit a Voronoi diagram-based definition of effective density approach, which is used in concert with the T-matrix method to determine single-scattering cross sections. The resulting backscatters are found to reasonably match those of the DDA over frequencies from 10.65 to 183.31 GHz and particle sizes from a few hundred micrometers to nine millimeters in length. Integrated error in backscatter versus DDA is found to be within 25% at 94 GHz. Errors in scattering cross-sections and asymmetry parameters are likewise small. The observed cross-sectional errors are much smaller than the differences observed among different particle models. This represents a significant improvement over established techniques, and it demonstrates that the radiative properties of dense aggregate snowflakes may be adequately represented by equal-mass homogeneous spheroids. The present results can be used to supplement retrieval algorithms used by CloudSat, EarthCARE, Galileo, GPM and SWACR radars. The ability to predict the full range of scattering properties is potentially also useful for other particle regimes where a compact particle approximation is applicable. / A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2017. / March 21, 2017. / discrete dipole approximation, hydrometeors, microwave instruments, snow, T-matrix method, Voronoi diagrams / Includes bibliographical references. / Guosheng Liu, Professor Directing Dissertation; Max Gunzburger, University Representative; Jon Ahlquist, Committee Member; Robert Ellingson, Committee Member; Zhaohua Wu, Committee Member.