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

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

HIGH-PERFORMANCE ANTENNA ARRAYS FOR MULTIFUNCTION PHASED ARRAY RADAR (MPAR) APPLICATION

There are interest and practical value in utilizing polarization diversity for a radar to obtain more target information or for a communication system to carry more signal information without occupying more frequency band. This is because frequency bands are getting crowded in microwave frequencies due to the recent advancements in cellular communications. For example, the Spectrum Efficient National Surveillance Radar Program (SENSR) is started to study the feasibility of replacing the four radar networks that service the U.S with a single network of Multifunction Phased Array Radar (MPAR). Candidates being considered for future MPAR include Cylindrical Polarimetric Phased Array Radar (CPPAR), and Planar Polarimetric Phased Array Radar (PPPAR). To have the desired accurate weather measurements with a PPPAR or CPPAR, a high-performance phased array antenna with dual-polarization capability is required. The array antenna is required to possess matched main beams, high input isolation, and low cross-polarization level at broadside and scan angles up to 45◦. The beam mismatch should be within 5% of the beamwidth, the input isolation needs to be better than 40 dB, and to have ZDR bias of less than 0.2 dB, the cross-polarization level along beam axis needs to be lower than -20 dB and -40 dB for alternate and simultaneous transmission, respectively. These are very stringent requirements for antenna design and development. The primary objective of this dissertation is to propose high-performance dual-polarized antenna arrays with high input isolation and low cross-polarization level for multifunction phased radar application. To do so, four different types of dual-polarized microstrip patch antenna arrays are presented. In the proposed patch antennas, different feeding techniques such as, aperture coupling method, balanced feed method and the combination of these methods which is called hybrid feeding technique are used. The proposed antenna arrays in this dissertation are configured according to image configuration for improving the cross-polarization level. The issues and challenges of implementing image arrangement is discussed, and precise procedure for design and predicting the final array radiation characteristics is proposed. The CPPAR demonstrator antenna is redesigned to achieve matched horizontal and vertical polarization beam pointing angels. A method of beam matching between two linearly polarized radiation patterns of a dual-polarized frequency scanning antenna is proposed, implemented, and tested. A meticulous phase match process between the outputs of both individual cells and the whole corresponding horizontal and vertical feed lines is carried out. To verify the simulation results and to take the coupling effect into account, the radiation patterns of an isolated column, as well as those of three columns, are measured. In agreement with the design and simulation results, horizontal and vertical polarization beams with a pointing angle mismatch of less than ±0.2◦ within the resonant frequency bandwidth of 2.75–2.95 GHz are achieved

Simulation of Polarimetric Phased Array Weather Radars

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

Unmanned Aerial Vehicle-based Far-Field Antenna Characterization System for Polarimetric Weather Radars

The use of phased array radars for the US weather radar network (NEXRAD) has been proposed in lieu of the current mechanically steered dish-based systems, owing to its many attractive features, e.g., electronic steering and fast update rates, and others. Scatterer identification (hydrometeors and non-hydrometeors), accurate estimation of rainfall rates, and determination of propagation effects is possible in weather radars through polarimetry. However, the existence of cross-polarization, and co-polarization mismatch in the H- and V-polarization radiation patterns introduces biases in the polarimetric weather radar products, which can adversely affect the accuracy of the estimates of byproducts, thus imposing strict antenna requirements on the co-polarization mismatch of no greater than 0.1 dB, and cross-polarization levels of no greater than about -45 dB. Since the radiation characteristics of phased arrays are inherently dependent on the scanning direction, it becomes even more challenging to meet these requirements. Furthermore, ensuring that each system in this large network meets the requirements becomes an additional challenge where accurate characterization and calibration will be critical. Clearly, the system and instrumentation used for characterization also need to meet or exceed the system level requirements to provide reliable weather-radar-based estimates. Given that radar and other communications systems require in-situ calibration, it is hypothesized that a UAV-based antenna measurement system is able to replace conventional outdoor ranges in virtue of its low cost and flexibility of operation. The proposed solution is a UAV-based in-situ antenna characterization system with the necessary RF instrumentation to perform accurate measurements of a typical weather radar, along with general guidelines and procedures to ensure optimal results. This solution attempts to provide a portable and cost-effective alternative to conventional outdoor antenna ranges, which can be deployed in multiple sites with few to no modifications. While previous works in the literature have had successful results in the use of UAVs for far-field (FF) antenna measurements in a variety of operating frequencies, no other work has currently shown the RF performance needed to meet the stringent requirements expected in an application such as polarimetric weather radars. It is shown in this work, that the characterization and calibration of real polarimetric weather radar systems is possible to a high degree of accuracy set forth by the most critical requirements, i.e., co-polarization mismatch no greater than 0.1 dB and cross-polarization levels below -45 dB