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

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

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

Signal Processing Techniques and Concept of Operations for Polarimetric Rotating Phased Array Radar

The Weather Surveillance Radar 1988 Doppler (WSR-88D) network has been operational for over 30 years and is still the primary observational instrument employed by the National Weather Service (NWS) forecasters to support their critical mission of issuing severe weather warnings and forecasts in the United States. Nevertheless, the WSR-88Ds have exceeded their engineering design lifespan and are projected to reach the end of operational lifetime by 2040. Technological limitations may prevent the WSR-88D to meet demanding functional requirements for future observational needs. The National Oceanic and Atmospheric Administration (NOAA) has started considering radar systems with advanced capabilities for the eventual replacement of the WSR-88D. Unique and flexible capabilities offered by Phased Array Radar (PAR) technology support the required enhanced weather surveillance strategies that are envisioned to improve the weather radar products, making PAR technology an attractive candidate for the next generation of weather radars. If PAR technology is to replace the operational WSR-88D, important decisions must be made regarding the architecture that will be needed to meet the functional requirements. A four-faced planar PAR (4F-PAR) is expected to achieve the requirements set forth by NOAA and the NWS, but deploying and maintaining an operational network of these radars across the U.S. will likely be unaffordable. A more affordable alternative radar system is based on a single-face Rotating PAR (RPAR) architecture, which is capable of exceeding the functionality provided by the WSR-88D network. This dissertation is focused on exploring advanced RPAR scanning techniques in support of meeting future radar functional requirements. A survey of unique RPAR capabilities is conducted to determine which ones could be exploited under an RPAR Concept of Operations (CONOPS). Three capabilities are selected for further investigation: beam agility, digital beamforming, and dwell flexibility. The RPARs beam agility is exploited to minimize the beam smearing that results from the rotation of the antenna system over the collection of samples in the coherent processing interval. The use of digital beamforming is investigated as a possible way to reduce the scan time and/or the variance of estimates. The RPAR's dwell flexibility capability is explored as a possible way to tailor the scan to meteorological observations with the goal of improving data quality. Three advanced RPAR scanning techniques are developed exploiting these capabilities, and their performance in support of meeting the radar functional requirements is quantified. The proposed techniques are implemented on the Advanced Technology Demonstrator (ATD), a dual-polarization RPAR system at the National Severe Storms Laboratory (NSSL) in Norman, OK. Data collection experiments are conducted with the ATD to demonstrate the performance of the proposed techniques for dual-polarization observations. Results are verified by quantitatively comparing fields of radar-variable estimates produced using the proposed RPAR techniques with those produced by a well-known collocated WSR-88D radar simultaneously collecting data following an operational Volume Coverage Pattern (VCP). The techniques introduced are integrated to operate simultaneously, and used to design an RPAR CONOPS that can complete a full volume scan in about one minute, while achieving other demanding functional requirements. It is expected that the findings in this dissertation will provide valuable information that can support the design of the future U.S. weather surveillance radar network

Microwave remote sensing from space

Spaceborne microwave remote sensors provide perspectives of the earth surface and atmosphere which are of unique value in scientific studies of geomorphology, oceanic waves and topography, atmospheric water vapor and temperatures, vegetation classification and stress, ice types and dynamics, and hydrological characteristics. Microwave radars and radiometers offer enhanced sensitivities to the geometrical characteristics of the earth's surface and its cover, to water in all its forms--soil and vegetation moisture, ice, wetlands, oceans, and atmospheric water vapor, and can provide high-resolution imagery of the earth's surface independent of cloud cover or sun angle. A brief review of the historical development and principles of active and passive microwave remote sensing is presented, with emphasis on the unique characteristics of the information obtainable in the microwave spectrum and the value of this information to global geoscientific studies. Various spaceborne microwave remote sensors are described, with applications to geology, planetology, oceanography, glaciology, land biology, meteorology, and hydrology. A discussion of future microwave remote sensor technological developments and challenges is presented, along with a summary of future missions being planned by several countries

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

Microwave remote sensing from space for earth resource surveys

The concepts of radar remote sensing and microwave radiometry are discussed and their utility in earth resource sensing is examined. The direct relationship between the character of the remotely sensed data and the level of decision making for which the data are appropriate is considered. Applications of active and a passive microwave sensing covered include hydrology, land use, mapping, vegetation classification, environmental monitoring, coastal features and processes, geology, and ice and snow. Approved and proposed microwave sensors are described and the use of space shuttle as a development platform is evaluated

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

Applied Radar Meteorology

This is a textbook focused on operational and other aspects of applied radar meteorology. Its primary purpose is to serve as a text for upper-level undergraduates and graduate students studying meteorology, who wish to work as professional operational meteorologists in the U.S. National Weather Service or the Air Force Weather Agency. In addition to a detailed description of operational weather radar systems operating in the United States, this text also provides a brief historical overview of the subject as well as a basic review of the physics of electromagnetic radiation and other theoretical aspects of weather radar. The last two chapters discuss a sample of other radar systems (such as the Doppler on Wheels and the Canadian and European operational networks), and future directions of weather radar, including its use as an input for high-resolution, rapid refresh computer models

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