X-band Dual Polarization Phased-Array Radar for Meteorological Applications

This dissertation details the development and operation of a novel dual-polarized Phase-Tilt Weather Radar (PTWR) designed for meteorological applications. The use of radar has a well-documented history in detection and classification of weather phenomena, but due to the limited mechanical scanning speed, its usage for severe weather observations remains far from ideal. The PTWR utilizes phased-array technology and provides unique capabilities such as smart scanning, fast scan update, and tracking. This technology is considered a candidate for a replacement and consolidation of the current US weather and surveillance radar networks. The dissertation can be divided into three parts. First, the hardware design of the radar is presented. Methods of an element and array calibration are discussed. The measured sidelobe level and pattern match exhibit satisfactory performance. The algorithms for signal processing in alternate transmit alternate receive mode of operation are described in detail. The PTWR weather detection capability is validated by an inter-comparison with a collocated X-band high-power radar. These tests showed correlation exceeding 90% for measurements of reflectivity in a convective storm system. The results support the hypothesis that phased-array technology poses an attractive solution for weather remote sensing. The second part addresses the radar waveform considerations. The sensitivity of the radar can be improved by several decibels by means of pulse compression techniques. This is necessary, since the PTWR utilizes low-power solid-state transmitters. The work discusses the trade-offs in waveform design and introduces a novel compression filter, which outperforms traditional window-based solutions. The pulse compression performance is validated using clutter data collected by the PTWR, proving that a deep sidelobe reduction in excess of 40dB can be achieved at the minimal penalty in signal-to-noise level (below 0.5dB). Finally, the third part focuses on the scanning geometry of a 1-D phase-tilt ar- chitecture. It is shown that as the elevation angle is increased, the measurements are affected by a self-induced apparent canting angle. The methods of polarization rotation correction are presented. The biases in typical weather radar products such as reflectivity, differential reflectivity, correlation coefficient, and specific propagation phase, are investigated. The analysis shows that for elevation angles below 15deg , the retrievals errors are acceptable

Design of an FPGA-based Array Formatter for Casa Phase-Tilt Radar System

Weather monitoring and forecasting systems have witnessed rapid advancement in recent years. However, one of the main challenges faced by these systems is poor coverage in lower atmospheric regions due to earth\u27s curvature. The Engineering Research Center for the Collaborative Adaptive Sensing of the Atmosphere (CASA) has developed a dense network of small low-power radars to improve the coverage of weather sensing systems. Traditional, mechanically-scanned antennas used in these radars are now being replaced with high-performance electronically-scanned phased-arrays. Phased-Array radars, however, require large number of active microwave components to scan electronically in both the azimuth and elevation planes, thus significantly increasing the cost of the entire radar system. To address this issue, CASA has designed a Phase-Tilt radar, that scans electronically in azimuth and mechanically in elevation. One of the core components of this system is the Phased-Array controller or the Array Formatter. The Array Formatter is a Field Programmable Gate Array (FPGA)-based master controller that translates user commands from a computer to control and timing signals for the radar system. The objective of this thesis is to design and test an FPGA-based Array Formatter for CASA\u27s Phase-Tilt radar system

BEAM STEERING CONTROL SYSTEM FOR LOW-COST PHASED ARRAY WEATHER RADARS: DESIGN AND CALIBRATION TECHNIQUES

Phase array antennas are a promising technology for weather surveillance radars. Their fast beam steering capability offer the potential of improving weather observations and extending warning lead times. However, one major problem associated with this technology is their high acquisition cost to be use in networked radar systems. One promising technology that could have a significant impact in the deployment of future dense networks of short-range X-band weather radars is the ``Phase-Tilt Radar\u27\u27, a system that uses a one-dimensional phase scanned antenna array mounted over a tilting mechanism. This dissertation addresses some of specific challenges that arise in designing and implementing air-cooled, low-cost, one-dimensional phased antenna arrays for phase-tilt radars. The goal of this work is to develop methods that can lead to reduce the cost and enhance the performance of this type of systems. Specifically, the thesis focuses on three concrete areas. The first one is on the development of a versatile low-cost beam steering system that can enable dual-polarimetric phased array radars to operate with high-frequency repetition pulses, difference pulsing schemes, and modern scanning strategies. In particular, the dissertation will present the development of critical components and describes the concept of operations of the beam steering system. The second area is to develop a calibration technique for small phased arrays. The work focused in finding the calibration settings for the array that best fit to the desired excitation. The technique provides lower random errors than conventional approaches, enabling the implementation of radiation patterns with sidelobes closer to the desired level. Additionally, the technique is extended to solve the gain-drift problem occurring in the two-way antenna pattern due to the temperature changes. The third area studies the use of mutual coupling as signal injection technique to maintain the calibration of both array and radar. Future air-cooled phased array radars will require the use internal circuitry to calibrate the aspect of the radar that tends to change over time. In particular, this work is focused on developing low-cost calibration techniques to correct the antenna gain and radar constant from effects of temperature changes and element failures

Phased-Array Antenna Calibration and Cross-polarization improvement of an X-band Weather Radar

Modifications and updates to an existing X-band dual-polarization phased-array weather radar. Efforts to improve the polarization isolation of the antenna. Characterization via near-field range and field measurements.Dual-polarized phased-array radars used for weather retrieval purposes is an emerging tendency over the last few years. The dual-polarization technology provides an expanded range of weather products and the reliability of these polarimetric products rely on the beam shape quality of the system under use and its polarization isolation. This thesis presents the calibration process of a one-dimensional scanning phased-array radar to assure its beamforming quality. A cross-polarization cancellation technique, with no additional hardware requirements, is tested and appeared to optimize the array settings for an improved isolation and therefore, for a better data quality.Radares formados por agrupaciones de antenas que trabajan con dos polarizaciones con la intención de tomar medidas meteorológicas es una tendencia a la alza en los últimos años. El hecho de utilizar dos polarizaciones ofrece una gama ampliada de productos meteorológicos y la fiabilidad de estos dependerá de la cualidad del haz radiado por el radar y del aislamiento entre las polarizaciones de éstos. Esta tesis presenta el proceso de calibración de un radar, formado por una agrupación de antenas que escanean en una dimension, con tal de garantir la calidad de los haces producidos. Una técnica para cancelar la polarización no deseada, sin la necesidad de ningún hardware adicional, ha sido probada y ha mostrado que puede mejorar la configuración de la agrupación para obtener una mejora del aislamiento y mejorar así, la cualidad de los datos.Radars formats per agrupacions d’antenes que treballen amb dues polarizations amb l’intenció de mesurar paràmetres meteorològics és una tendencia que va en augment en els últims anys. El fet d’utilitzar dues polarizations ofereix una gama ampliada de productes meteorològics i la fiabilitat d’aquests dependrà de la qualitat que tinguin els feixos radiats pel radar així com de l’aïllament entre les polaritzacions d’aquests. Aquesta tesis presenta el procés de calibració d’un radar, format per una agrupació d’antenes que escanejen en una dimensió, per tal de garantir la qualitat dels feixos produïts. Una tècnica per cancelar la polarització no desitjada, sense la necessitat de hardware adicional, ha estat provada i ha mostrat que pot millorar la configuració de l’agrupació per tal d’obtenir una millora en el l’aïllament i així millorar-ne la qualitat de les dades

Correcting For Terrain Interference, Attenuation, and System Bias for a Dual Polarimetric, X-Band Radar

This thesis outlines the procedure and theory used to calibrate the UMass eXperimental X-band Radar (UMAXX) for the purpose of monitoring meteorological events in the Pioneer Valley region. Due to the complex topography of the area, lower tilt angles are subject to partial or full beam blockage as well as ground clutter observed through the main beam or sidelobes. Additionally, there are biases internal and external to the system that impact the reflectivity and differential reflectivity measurements. These biases and corrections are addressed in this work. As the radar has been operational since September of 2018, there is ample data available to diag-nose and to perform the necessary corrections to the system. A variety of methods are employed to validate these corrections including comparing intersecting scan volumes between the UMAXX and nearby WSR-88Ds of the NEXRAD network as well as the use of membership functions. Key results of this work are: Ground clutter is principally identified through differential phase and secondarily through velocity and co-polar correlation Partial beam blockage is best estimated assuming a 1.67◦, 2-way beamwidth with an 8dB cutoff System differential phase and Differential Reflectivity bias are functions of az-imuth due to the radome panels. A linear relation between wet radome attenuation and rain rate is found Using an attenuation factor of a = .28 to determine path integrated attenuation improves correlation of reflectivity measurements between UMAXX and NEXRAD network. Ultimately, the goal is to establish UMAXX as a reliable and well understood benchmark with which to calibrate Raytheon’s dual-polarized phased array radar. The two radars operate in sufficiently close frequencies within X-band and collected data simultaneously while colocated. While phased arrays show great promise and potential in meteorologic observations, they come with many challenges that neces-sitate the use of a trustworthy baseline with which to validate its measurements. Additionally, UMAXX’s data is to be streamed to serve as a source to fill any gaps present in the National Weather Service’s network in the region

Applications in Low-Power Phased Array Weather Radars

Low-cost X-band radars are an emerging technology that offer significant advantages over traditional systems for weather remote sensing applications. X-band radars provide enhanced angular resolution at a fraction of the aperture size compared to larger, lower frequency systems. Because of their low cost and small form factor, these radars can now be integrated into more research and commercial applications. This work presents research and development activities using a low-cost, X-band (9410 MHz) Phase-Tilt Radar. The phase-tilt design is a novel phased array architecture that allows for rapid electronic scanning in azimuth and mechanical tilting in elevation, as a compromise between cost and performance. This work focuses on field studies and experiments in three meteorological applications. The first stage of research focuses on the real-world application of phased array radars in forest fire monitoring and observation. From April to May 2013, a phase-tilt radar was deployed to South Australia and underwent a field campaign to make polarimetric observations of prescribed burns within and around the Adelaide Hills region. Measurements show the real-time evolution of the smoke plume dynamics at a spatial and temporal resolution that has never before been observed with an X-band radar. This dissertation will perform data analysis on results from this field campaign. Results are compared against existing work, theories, and approaches. In the second stage of research, field experiments are performed to assess the data quality of X-band phased array radars. Specifically, this research focuses on the measurement of and techniques to improve the variance of weather product estimators for dual-polarized systems. Variability in the radar products is a complicated relationship between the radar system specifications, scanning strategy, and the physics governing precipitation. Here, the variance of the radar product estimators is measured using standard radar scanning strategies employed in traditional mechanical antenna systems. Results are compared against adaptive scan strategies such as beam multiplexing and frequency diversity. This work investigates the improvement that complex scanning strategies offer in dual-polarized, X-band phased array radar systems. In the third stage of research, simulations and field experiments are conducted to investigate the performance benefits of adaptive scanning to optimize the data quality of radar returns. This research focuses on the development and implementation of a waveform agile and adaptive scanning strategy to improve the quality of weather product estimators. Active phased array radars allow radar systems to quickly vary both scan pointing angles and waveform parameters in response to real-time observations of the atmosphere. As an evolution of the previous research effort, this work develops techniques to adaptively change the scan pointing angles, transmit and matched filter waveform parameters to achieve a desired level of data quality. Strategies and techniques are developed to minimize the error between observed and desired data quality measures. Simulation and field experiments are performed to assess the quality of the developed strategies

Deployment and Monitoring of an X-Band Dual-Polarization Phased Array Weather Radar

This thesis describes the deployment of MIRSL\u27s X-band dual-polarization Phase-Tilt Weather Radar (PTWR) at the University of Texas at Arlington during spring 2014. While this radar has been used to observe weather in Western Massachusetts, more observations of severe weather were required to determine the limits of its abilities in sensing more rapidly evolving weather systems. This site was chosen also for its proximity to the Dallas-Fort Worth Urban Testbed Network set up by the Center for Collaborative Adaptive Sensing of the Atmosphere (CASA), which provided the ability to compare and calibrate the PTWR data against another well-documented X-band weather radar. A data processing pipeline was developed for converting raw PTWR data to NetCDF format, which allows for easy sharing and mapping of weather data. Finally, this is the first in-depth documentation of the PTWR system and specifically the roof-mounted setup utilized for this deployment

CYLINDRICAL POLARIMETRIC PHASED ARRAY RADAR DEMONSTRATOR: PERFORMANCE ASSESSMENT AND WEATHER MEASUREMENTS

A desirable candidate for future weather observation is a polarimetric phased array radar (PPAR), which is capable of both using polarimetry for multi-parameter measurements and the fast-scan proficiency of the PAR. However, it is challenging to collect high-quality polarimetric radar data of weather with a planar PPAR (PPPAR), whose beam and polarization characteristics change with the electronic beam direction, causing geometrically induced cross-polarization coupling, sensitivity losses, and measurement biases when the PPPAR beam is steered away from the broadside. As an alternative to PPPAR, the concept of cylindrical polarimetric phased array radar (CPPAR) was proposed, which has scan-invariant beam characteristics in azimuth and polarization purity in all directions using commutating scan, thus enables high quality polarimetric weather measurements. To validate the CPPAR concept, a small-scale CPPAR demonstrator has been jointly developed by the Advanced Radar Research Center (ARRC) at the University of Oklahoma (OU) and the National Severe Storms Laboratory (NSSL) of NOAA. This dissertation presents the results of initial weather measurements, shows the performance of the CPPAR demonstrator, and evaluates the polarimetric data quality that has been achieved. The system specifications and field tests of the CPPAR demonstrator are provided, including system overview, waveform design and verification, pattern optimization and far-field tests. In addition, three methods of system calibration are introduced and compared, including calibration with an external source, calibration with weather measurements of mechanical scan, and calibration with ground clutter. It is found that calibration with weather measurements of mechanical scan has the best performance and it is applied on the CPPAR demonstrator for the first time, which effectively improved the beam-to-beam consistency and radar data quality in commutating beam electronic scan by minimizing gain and beamwidth variations. Performance of the CPPAR is assessed through system simulation and weather measurements. The CPPAR is evaluated through an end-to-end phased array radar system simulator (PASIM). The simulation framework, weather returns modeling, antenna pattern, channel electronics, and simulation results of CPPAR, as well as comparison with those that would be obtained with a PPPAR, are provided. Also, weather measurements of a few convective precipitation cases and a stratiform precipitation case made with the CPPAR, employing the single beam mechanical scan and commutating beam electronic scan respectively, are presented. First, a qualitative comparison is made between the CPPAR and a nearby operational NEXRAD. Then a quantitative comparison is conducted between the mechanical scan and electronic scan, and error statistics are estimated and discussed. In addition, a theoretical explanation of a feature of the commutating beam electronic scan in clutter detection that is different from mechanical scan is presented and verified by measurements in clear air conditions with the CPPAR. Moreover, clutter detection results based on multi-lag phase structure function, dual-scan cross-correlation coefficient, copolar correlation coefficient, and differential reflectivity obtained from both electronic scan and mechanical scan modes of the CPPAR are compared

TECHNIQUES AND INSTRUMENTATION FOR PHASED ARRAY CALIBRATION

Active phased arrays suffer the inherent problem of excitation errors, i.e., incorrect phase and amplitude excitation of the antenna elements. Excitation errors degrade critical performance parameters since they increase sidelobe level and reduce antenna gain and beam pointing accuracy. To ensure the correct operation of the array, it is necessary to quantify and compensate the phase and amplitude errors of each antenna element. The compensation is accomplished by calibrating the phased array radar. Calibration challenges include the quantification and compensation of errors initially, as well as maintenance of the calibration state once the system is fielded. This dissertation presents research on improving the calibration of the active phased array front-end for radar systems. A combination of custom-made instrumentation with initial and in-situ calibration techniques is proposed to calibrate an active array test-bed. The test-bed consists of an 8$\times$8 elements C-band array, and was developed in collaboration with NCAR-EOL to provide software and hardware features that enable the proposed calibration schemes. Different calibration techniques were experimentally tested. First, an initial calibration technique for phased array prototypes is proposed. The technique employs a planar NF scanner to sample the excitation of each antenna element, and also to scan the embedded element antenna patterns of the prototype. The novelty of the approach is that it combines the collected excitation data with the scanned embedded elements to allow the prediction of both the co- and cross-polar pattern components of the array. On the other hand, to explore techniques that do not rely on external equipment and use built-in feedback mechanisms instead, mutual coupling-based calibration is reviewed and implemented. Two techniques were tested: an initial type, proposed by Bekers et al., and a proposed in-situ type, conceived specifically for analog architectures, to track errors during fielded operation. It was found that mutual coupling calibration techniques are excellent options for in-situ applications, with a root mean squared error (RMSE) in phase and amplitude of 0.75$^\circ$ and 0.12 dB, respectively. Whereas, for initial type calibration, the tested mutual coupling-based technique yields a RMSE of 2.5$^\circ$ and $\geq$ 1 dB, respectively, which is not accurate enough to replace conventional park and probe for initial calibration of small arrays. Finally, to complement calibration theory, the required calibration instrumentation is reviewed, and more importantly, a novel scanner, designed exclusively for phased array front-end characterization, is introduced

Solid Earth science in the 1990s. Volume 3: Measurement techniques and technology

Reports are contained from the NASA Workshop on Solid Earth Science in the 1990s. The techniques and technologies needed to address the program objectives are discussed. The Measurement Technique and Technology Panel identified (1) candidate measurement systems for each of the measurements required for the Solid Earth Science Program that would fall under the NASA purview; (2) the capabilities and limitations of each technique; and (3) the developments necessary for each technique to meet the science panel requirements. In nearly all cases, current technology or a development path with existing technology was identified as capable of meeting the requirements of the science panels. These technologies and development paths are discussed