<p> The ionospheric total electron content (TEC) is the integrated electron density across a unit area. TEC is an important property of the ionosphere. Accurate estimation of TEC and TEC spatial distributions are needed for many space-based applications such as precise positioning, navigation, and timing. The Global Positioning System (GPS) provides one of the most versatile methods for measuring the ionosphere TEC, as it has global coverage, high temporal resolution, and relatively high spatial resolution. The objective of this thesis is to develop an algorithm for accurate estimation of the TEC using dual-frequency GPS receiver measurements and simultaneously estimate the receiver hardware bias in order to mitigate its effect on the TEC. This method assumes the TEC in the portion of sky visible to the receiver can be represented as a two dimensional sheet with an absolute value and spacial gradients with respect to latitude and longitude. A code-phase multipath noise estimation algorithm is integrated with the TEC estimation process to mitigate environmental multipath contamination of the measurements. The integrated algorithm produces an approximate map of local TEC using a single dual-frequency receiver while minimizing both multipath induced errors and the receiver hardware bias. The goal of this method is to provide an accurate map of the ionosphere TEC, in the region local to the receiver, without the need for a network of receivers and in the absence of knowledge of the receiver hardware induced bias. This thesis describes the algorithm, its implementation, and attempts to validate the method through comparison with incoherent scatter radar (ISR) data from low, mid, and high latitude locations.</p>

In the spring of 2001 the Microwave Remote Sensing Laboratory (MIRSL) at the University of Massachusetts began the development of an advanced Multi-Frequency Radar (AMFR) system for studying clouds and precipitation. This mobile radar was designed to consist of three polarimetric Doppler subsystems operating at Ku-band (13.4 GHz), Ka-band (35.6 GHz) and W-band (94.92 GHz). This combination of frequency bands allows a measurement of a wide range of atmospheric targets ranging from weakly reflecting clouds to strong precipitation. The antenna beamwidths at each frequency were intentionally matched, ensuring consistent sampling volume. Multi-frequency radar remote sensing techniques are not widely used because few multi-frequency radars are available to the science community. One exception is the 33 GHz/95 GHz UMass Cloud Profiling Radar System (CPRS), which AMFR is intended to replace. AMFR's multi-parameter capabilities are designed for characterizing the complex microphysics of layer clouds and precipitation processes in winter storms. AMFR will also play an important role in developing algorithms and validating measurements for an upcoming generation of space-borne radars. The frequency bands selected for AMFR match those of several sensors that have been deployed or are under development. These include the Japanese Aerospace Exploration Agencies (JAXA's) Tropical Rainfall Measuring Mission (TRMM) satellite Ku-band (13 GHz) radar, the CloudSat W-band (95 GHz) radar, and the Global Precipitation Mission (GPM) satellite radars at Ku-band and Ka-band. This dissertation describes the AMFR hardware design and development. Compared to CPRS, the addition of one extra frequency band (Ku) will extend AMFR's measurement capabilities towards the larger particle sizes (precipitation). AMFR's design is based around high-power klystron amplifiers. This ensures complete coherency (CPRS uses magnetrons and coherent-on-receive technique). The partial loss in sensitivity due to lower output power of klystron amplifiers (comparing to magnetrons) is compensated by use of pulse compression (linear FM). The problem of range sidelobes (pulse compression artifacts) has been solved by using appropriate windowing functions in the receiver. Satisfactory sidelobe suppression level of 45 dB has been demonstrated in the lab. The currently best achievable range resolution of the AMFR system is 30 m (corresponds to 5 MHz receiver BW, set by the sampling rate of the Analog-to-Digital card). During the design stage, various polarization schemes have been investigated. The polarization scheme analysis showed the switching polarization scheme to be the best suited for the AMFR system. The AMFR subsystems were partially finished in the winter of 2005. Some preliminary tests were conducted in January 2006. Antenna platform was fabricated in summer 2006. The final assembly took place in the fall of 2006. Early results are presented in the dissertation. These results were helpful in revealing of certain problems in the radar system (i.e. immediate processing computer synchronization) that needed to be addressed during system development. Stratiform rain event occurred on December 18 2006 has been analyzed in detail. A number of commonly used theoretical particle size distributions is presented. Furthermore, it is shown that a fully calibrated multi-frequency radar system has capability of separating scattering and attenuation effects. This was accomplished by fitting the theoretical models into the measured data. An alternative method of estimating rain rate that relies on the dual wavelength ratios is also presented. Although not as powerful as theoretical model fitting, it has its merits for off-zenith observations. During January 2007, AMFR system participated in the C3VP experiment (Canadian CloudSat/CALIPSO Validation Project) in south Ontario, Canada. Some of the data obtained during C3VP experiment has been analyzed and presented. Analysis of these two weather events resulted in the development of the initial multi-frequency particle size distribution retrieval algorithm.

This thesis describes the Turbulent Eddy Profiler (TEP), a volume-imaging, UHF radar wind profiler designed for clear-air measurements in the atmospheric boundary layer on scales comparable to grid cell sizes of Large Eddy Simulation models. TEP employs a large array of antennas--each feeding an independent receiver--to simultaneously generate multiple beams within a 25$\sp\circ$ conical volume illuminated by the transmitter. Range gating provides 30 m spatial resolution in the vertical dimension. Each volume image is updated every 2-10 s, and long data sets can be gathered to study the evolution of turbulent structure over several hours. This thesis provides a summary of the design and operational principles of the Turbulent Eddy Profiler, including an analysis of the calibration and data processing. The TEP engineering tests are described, along with an analysis of precipitation data. Finally, the atmospheric experiments in North Carolina are presented, along with analysis of clear-air data.

This dissertation investigates the use of digital and adaptive beamforming techniques for remote sensing applications. While the theoretical foundations for digital and adaptive beamforming are well established, the application of these results to remote sensing imaging radar has seen little development in the literature. Practical radar systems suffer from a variety of component errors and simple logistical issues that complicate the direct application of theoretical results. The objective of this work is to investigate the limitations of the current theory, and to demonstrate the practical application of the theory where possible. Antenna hardware is a critical component in a digital beamforming system and the first part of this thesis details the design, fabrication, and testing of the antenna hardware for the Turbulent Eddy Profiler (TEP) radar system. A corrugated pyramidal horn antenna serves as the high power transmit horn, and a printed microstrip antenna is used as an element in a ninety-one element receive array. Detailed design procedures are given for both transmit and receive antennas, and a complete set of machine drawings are included. The antennas were fully tested, and measured results are given for the transmit antenna, a single receive element, and a seven element array. The second major section of this dissertation introduces digital and adaptive array processing principles, and investigates the impact of common system errors on the capabilities of these systems. A unified treatment of system errors is presented, and individual error sources are examined in terms of their impact on important performance indexes. A new result is obtained relating the achievable null depth to the cross-correlation terms of the array's correlation matrix. The final section of this dissertation examines the practical application of digital and adaptive beamforming theory to remote sensing imaging radar. Computer simulations and experimental data are used to demonstrate the application of theoretical results to signal environments typical of remote sensing radars. Limitations on antenna performance derived from the theory are discussed, and suboptimal array processing architectures are considered. Experimental results from the FOPAIR (linear array) and TEP (planar array) systems, both developed by MIRSL, form the basis of this section.

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.

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.

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.

A novel dual-mode microwave scatterometer system has been designed and fabricated for remote sensing. This dissertation describes the sensor and presents unique C- and Ku-band ocean surface radar backscatter measurements obtained with it during flights on NASA C-130 and P-3 aircraft. Anisotropic C-band normalized radar cross section measurements obtained for a limited range of ocean surface windspeeds with a spinning antenna are presented. These measurements are potentially free of errors that corrupt similar measurements made with fixed-azimuth airborne scatterometers during "circle-flights". Also presented, for the first time, are open-ocean observations of the electromagnetic (EM) bias at C- and Ku-bands.