82 research outputs found

    Theoretical analysis of and bias correction for planar and cylindrical polarimetric phased array weather Radar

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    Planar or Cylindrical Phased Arrays are two candidate antennas for the future polarimetric weather radar. These two candidate antennas have distinctly different attributes when used to make quantitative measurements of the polarimetric properties of precipitation. Of critical concern is meeting required polarimetric performance for all directions of the electronically-steered beam. The copolar and cross-polar radiation patterns and polarimetric parameter estimation performances of these two phased array antennas are studied and compared with that obtained using a dual polarized parabolic reflector antenna. Planar Polarimetric Phased Array Radar (i.e., PPPAR) creates biases in observed polarimetric parameters when the beam is pointed off broadside. The biases of polarimetric parameters with a PPPAR are presented, and it is unacceptably large. Thus, a bias correction matrix needs to be applied for each beam direction. A bias correction matrix is developed for array elements consisting of either dipole, waveguide apertures or patches. Correction matrices are given for both the Alternate Transmission and Simultaneous Reception mode and the Simultaneous Transmission and Simultaneous Reception mode. The PPPAR, however, has significant deficiencies for polarimetric measurements, as well as other limitations, such as increases in beamwidth, decreases of sensitivity, and high geometrically projected cross polar fields when the beam scans off its broadside. The Cylindrical Polarimetric Phased Array Radar (i.e., CPPAR) is proposed to avoid these deficiencies. The CPPAR principle and potential performance are demonstrated through theoretical analysis and simulation. It is shown that the CPPAR has much lower geometrically induced cross-polar fields and less bias of polarimetric parameters than those of PPPAR. Array lattices, element separations, and error effects of CPPAR are examined

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

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    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

    Simulation of Polarimetric Phased Array Weather Radars

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    Polarimetric phased array radars (PPARs) are a rapidly developing area of research interest in weather radar. However, they present intrinsic challenges for calibration and operation. Foremost among these are the adverse effects of copolar radiation pattern mismatch as well as cross-polar fields on polarimetric measurement accuracy. Characterization of the impact these effects have on weather radar observations and the effectiveness of proposed methods for mitigation of those impacts can be time-consuming and costly if conducted using radar hardware. Furthermore, few operational PPARs exist to serve as testbeds. Alternatively, the effects of copolar and cross-polar fields can be studied using numerical simulations. In that regard, this work outlines a simulation method that allows for the characterization of PPAR performance and the prototyping of techniques to mitigate cross-polar biases. To achieve this, a simulation volume is populated by thousands of scattering centers, whose movement and scattering characteristics at any point in space and time are governed by a high-resolution numerical weather prediction model. Each of these scattering centers has its own individually calculated Doppler spectrum in both the horizontal (H) and vertical (V) polarizations. These spectra are used to determine instantaneous scattering parameters that are combined with a highly flexible radar system model in order to compose time-series signals in H and V. This simulation method is used to evaluate and compare the performance of several bias mitigation techniques that have been previously proposed

    The QUIET Instrument

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    The Q/U Imaging ExperimenT (QUIET) is designed to measure polarization in the Cosmic Microwave Background, targeting the imprint of inflationary gravitational waves at large angular scales (~ 1 degree). Between 2008 October and 2010 December, two independent receiver arrays were deployed sequentially on a 1.4 m side-fed Dragonian telescope. The polarimeters which form the focal planes use a highly compact design based on High Electron Mobility Transistors (HEMTs) that provides simultaneous measurements of the Stokes parameters Q, U, and I in a single module. The 17-element Q-band polarimeter array, with a central frequency of 43.1 GHz, has the best sensitivity (69 uK sqrt(s)) and the lowest instrumental systematic errors ever achieved in this band, contributing to the tensor-to-scalar ratio at r < 0.1. The 84-element W-band polarimeter array has a sensitivity of 87 uK sqrt(s) at a central frequency of 94.5 GHz. It has the lowest systematic errors to date, contributing at r < 0.01. The two arrays together cover multipoles in the range l= 25-975. These are the largest HEMT-based arrays deployed to date. This article describes the design, calibration, performance of, and sources of systematic error for the instrument

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

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    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

    Frequency diversity wideband digital receiver and signal processor for solid-state dual-polarimetric weather radars

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    2012 Summer.Includes bibliographical references.The recent spate in the use of solid-state transmitters for weather radar systems has unexceptionably revolutionized the research in meteorology. The solid-state transmitters allow transmission of low peak powers without losing the radar range resolution by allowing the use of pulse compression waveforms. In this research, a novel frequency-diversity wideband waveform is proposed and realized to extenuate the low sensitivity of solid-state radars and mitigate the blind range problem tied with the longer pulse compression waveforms. The latest developments in the computing landscape have permitted the design of wideband digital receivers which can process this novel waveform on Field Programmable Gate Array (FPGA) chips. In terms of signal processing, wideband systems are generally characterized by the fact that the bandwidth of the signal of interest is comparable to the sampled bandwidth; that is, a band of frequencies must be selected and filtered out from a comparable spectral window in which the signal might occur. The development of such a wideband digital receiver opens a window for exciting research opportunities for improved estimation of precipitation measurements for higher frequency systems such as X, Ku and Ka bands, satellite-borne radars and other solid-state ground-based radars. This research describes various unique challenges associated with the design of a multi-channel wideband receiver. The receiver consists of twelve channels which simultaneously downconvert and filter the digitized intermediate-frequency (IF) signal for radar data processing. The product processing for the multi-channel digital receiver mandates a software and network architecture which provides for generating and archiving a single meteorological product profile culled from multi-pulse profiles at an increased data date. The multi-channel digital receiver also continuously samples the transmit pulse for calibration of radar receiver gain and transmit power. The multi-channel digital receiver has been successfully deployed as a key component in the recently developed National Aeronautical and Space Administration (NASA) Global Precipitation Measurement (GPM) Dual-Frequency Dual-Polarization Doppler Radar (D3R). The D3R is the principal ground validation instrument for the precipitation measurements of the Dual Precipitation Radar (DPR) onboard the GPM Core Observatory satellite scheduled for launch in 2014. The D3R system employs two broadly separated frequencies at Ku- and Ka-bands that together make measurements for precipitation types which need higher sensitivity such as light rain, drizzle and snow. This research describes unique design space to configure the digital receiver for D3R at several processing levels. At length, this research presents analysis and results obtained by employing the multi-carrier waveforms for D3R during the 2012 GPM Cold-Season Precipitation Experiment (GCPEx) campaign in Canada

    Electronic scan weather radar: scan strategy and signal processing for volume targets

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    2013 Fall.Includes bibliographical references.Following the success of the WSR-88D network, considerable effort has been directed toward searching for options for the next generation of weather radar technology. With its superior capability for rapidly scanning the atmosphere, electronically scanned phased array radar (PAR) is a potential candidate. A network of such radars has been recommended for consideration by the National Academies Committee on Weather Radar Technology beyond NEXRAD. While conventional weather radar uses a rotating parabolic antenna to form and direct the beam, a phased array radar superimposes outputs from an array of many similar radiating elements to yield a beam that is scanned electronically. An adaptive scan strategy and advanced signal designs and processing concepts are developed in this work to use PAR effectively for weather observation. An adaptive scan strategy for weather targets is developed based on the space-time variability of the storm under observation. Quickly evolving regions are scanned more often and spatial sampling resolution is matched to spatial scale. A model that includes the interaction between space and time is used to extract spatial and temporal scales of the medium and to define scanning regions. The temporal scale constrains the radar revisit time while the measurement accuracy controls the dwell time. These conditions are employed in a task scheduler that works on a ray-by-ray basis and is designed to balance task priority and radar resources. The scheduler algorithm also includes an optimization procedure for minimizing radar scan time. In this research, a signal model for polarimetric phased array weather radar (PAWR) is presented and analyzed. The electronic scan mechanism creates a complex coupling of horizontal and vertical polarizations that produce the bias in the polarimetric variables retrieval. Methods for bias correction for simultaneous and alternating transmission modes are proposed. It is shown that the bias can be effectively removed; however, data quality degradation occurs at far off boresight directions. The effective range for the bias correction methods is suggested by using radar simulation. The pulsing scheme used in PAWR requires a new ground clutter filtering method. The filter is designed to work with a signal covariance matrix in the time domain. The matrix size is set to match the data block size. The filter's design helps overcome limitations of spectral filtering methods and make efficient use of reducing ground clutter width in PAWR. Therefore, it works on modes with few samples. Additionally, the filter can be directly extended for staggered PRT waveforms. Filter implementation for polarimetric retrieval is also successfully developed and tested for simultaneous and alternating staggered PRT. The performance of these methods is discussed in detail. It is important to achieve high sensitivity for PAWR. The use of low-power solid state transmitters to keep costs down requires pulse compression technique. Wide-band pulse compression filters will partly reduce the system sensitivity performance. A system for sensitivity enhancement (SES) for pulse compression weather radar is developed to mitigate this issue. SES uses a dual-waveform transmission scheme and an adaptive pulse compression filter that is based on the self-consistency between signals of the two waveforms. Using SES, the system sensitivity can be improved by 8 to 10 dB

    Breaking the Practical Performance Barriers of Polarimetric Phased Array Weather Radars

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    Phased array radars (PAR) are being proposed as an alternative to replacing the Next Generation Weather Radar (NEXRAD) network, which has been in service for more than 30 years, reaching the end of its life cycle. The PAR can improve the temporal resolution of weather coverage compared to reflector antennas (currently implemented on NEXRAD). Temporal resolution is crucial for severe weather detection and surveillance, especially rapid-evolving phenomena such as tornadoes and hail storms. An all-digital PAR design is presently being explored based on their performance and flexibility improvement. Nevertheless, even all-digital PARs are not free from limitations. This work proposes two signal processing solutions to mitigate two significant limitations observed in those radar systems, i.e., blind range resulted from pulse compression technique and cross-polar contamination inherent in the patch antenna implementation, which is currently the only viable solution to an all-digital PAR system. The mitigation techniques to these two limitations are called Progressive Pulse Compression and Cross-Polar Canceler, respectively. The Progressive Pulse Compression (PPC) technique is proposed to mitigate the blind range problem observed in radars using a frequency modulated waveform and pulse compression. The blind range is caused by the strong leak-through coupled into the receive chain during the transmission cycle. The PPC technique is based on partial decoding. It uses a portion of the uncontaminated received signal in conjunction with pulse compression to estimate the target characteristics from the incomplete signal. The technique does not require using a fill pulse or any hardware modifications. The PPC technique can be divided into three steps. First is to apply a smooth taper to discard all the contaminated samples in the received signal that corresponds to the transmission cycle. The second step is to perform pulse compression using the so called matched filter. Finally, the third step is to calculate and apply a calibration factor to compensate for the progressively changing return signal (affected by the tapering) to recover the proper reflectivity values. This technique is implemented on the PX-1000 radar. In the near future, PPC will be implemented on the Horus phased array radar system. The PX-1000 and Horus radar systems have been designed by the Advanced Radar Research Center (ARRC) at the University of Oklahoma (OU). Nevertheless, PPC has some limitations caused by the different frequency content between the modified (tapered) return signal and the matched filter used for compression. This difference causes a shift in the mainlobe peak and an asymmetrical increase in the sidelobe levels producing a “shoulder” effect. This work proposes improving PPC by compressing the modified return signal with amplitude-modulated versions (range dependent) of the original matched filter. The improved PPC is termed PPC+ and is planned as a software update from PPC. The PPC+ has been tested using data from the PX-1000 and will be presented in this dissertation. The Cross-Polar Canceler (XPC) technique is proposed to mitigate the cross-polar contamination observed on phased array radars. The cross-polar contamination is especially problematic when steering the beam away from the broadside. It is defined as a leakage from the intended polarization observed in the perpendicular one. In the XPC technique, the elements on the array are divided into two groups: main elements and canceler elements. The main elements transmit without any modification. However, the canceler elements transmit a modulated version of the inverse (i.e., the mathematical negative) of the original waveform in the perpendicular polarization. After integration, the field radiated by the canceler elements cancels the cross-polar contamination produced by the main ones. The XPC technique involves calculating the correct number of canceler elements, their location in the array, and the complex scaling factor that better mitigates the cross-polar contamination. This technique has been designed for polarimetric radars transmitting in simultaneous transmission and simultaneous reception of H/V polarization (STSR). The XPC technique will be implemented on the Horus radar system, currently under development. For polarimetric radars, the difference in the element patterns on each polarization produces an angular mismatch between the peaks on the H and V array patterns. This angular mismatch affects the maximum performance achievable with the XPC. Calibration is included as part of XPC to mitigate this effect. Iterative calibration is necessary in the XPC technique. Additionally, calibration is performed before and after XPC is implemented on an operational PAR system. This enhanced version of XPC (including calibration) is termed improved XPC. Like the XPC, the improved XPC is intended to be implemented on the Horus radar system

    Final report for the CSU-CHILL radar facility

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    Submitted to the National Science Foundation, Division of Atmospheric Sciences.6 May 1996.Cooperative agreement no. ATM-8919080
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