Development of a Low-Cost, Multi-Beam, X-Band Reflectarray-Based Weather Radar System
Shaya Karimkashi, Blake McGuire, Robin Irazoqui, Matt McCord, Nafati Aboserwal, Michihiro Teshiba, Hjalti Sigmarsson, and Caleb Fulton
Better availability of smaller but low-cost and fast-scanning Doppler weather radars will result in new opportunities to replace expensive solutions near airports, fill gaps in coverage in hilly and mountainous areas, and open up new ground- and sea-based markets that could make use of early warning systems for severe weather. One of the primary roadblocks to the implementation of such a system has been the lack of availability of low-cost, low-power, and accessible microwave transceiver, antenna, and digital processing components. A team of researchers at the University of Oklahoma (OU) Advanced Radar Research Center (ARRC) is working with Weathernews Incorporated (WNI) towards the demonstration of a radar system that overcomes these barriers through the development of a low-cost, reflectarray-based, X-band, multi-beam antenna that is driven by a multi-channel electronics package, with each channel consisting of a microwave front-end and up/down-converter (UDC) module, an S-band digital transceiver and radar signal processing module (RSP), and a FPGA-based host controller to a simple human-machine interface (HMI). The antenna will provide more than four simultaneous elevations through a simple frequency multiplexing scheme. The overall system is being designed to support full volume scans on the order of 10 seconds with an eventual range of 30 km at sensitivities and range resolutions that are useful for quantitative precipitation estimation and detection of localized severe weather events (e.g. 25 dBz and 50m). The cost target for this system is aggressive, and is being pursued through a careful combination of off-the-shelf electronics, custom RF circuits, an open and accessible radar processing scheme, and a single-layer reflectarray driven by simple, 3D-printable feed structures.
The use of phased arrays to achieve multiple beams is inherently expensive, requiring thousands of individual components, careful calibration, and complex integration steps. At the same time, traditional dish-based with single beam systems are inherently limited in terms of scan times. The team is therefore pursuing an approach that uses a reflectarray with multiple feeds along a single structure to provide multiple beams, as depicted in the figure above, with a limited electronics support requirement. The reflectarray is being designed for a two degree beamwidth at low elevations, with naturally increasing beamwidths at higher elevations where less range is needed at higher altitudes. The reflectarray antenna consists of phase changing elements mimicking a parabolic reflector surface and creating collimated beams. Simple horns feed the patch elements on the surface of the reflectarray. Multiple beams are achieved by displacing the horn feeds from the focal point of the reflectarray. The main stages in designing the reflectarray include aperture design, phasing element design, and feed horn design and positioning. The antenna beamwidth dictates the aperture size of the reflectarray. A circular aperture will eventually be chosen to achieve higher aperture efficiency compared to a squared aperture. Square patch elements are selected as phasing elements, and both pyramidal and conical horn antennas are used as the feed. These will all be discussed in the paper/presentation.
Each feed is driven individually by its own X-band front-end and X-to-S-band UDC. This module makes maximal use of readily-available off-the-shelf parts, complemented by a flexible custom filtering and mixing scheme that can actually support Ku-band as well. The front-end is reconfigurable for both bands, utilizing 3W solid state amplifiers as HPAs/drivers, whereby the output connector and dedicated monitoring path will allow for a separate 20-100W module to be integrated for lower elevations, extending the range to the ultimate 30-km goal. The mixing, local-oscillator, and filtering scheme benefits from simplifications owing to the high S-band intermediate frequency enabled by the digital transceiver. The custom designed X/Ku band microstrip hairpin Chebyshev filters have high LO tone rejection to eliminate any leakage of that tone in the main RF chain after mixing. With the custom-designed power dividers and filters, the system achieves excellent filtering characteristics throughout the whole spectrum with at least 40-60dB attenuation of signals out of passband. The extra filtering provided by the antenna and S-Band receiver will provide a very clean signal to the RSP module.
Continued scaling of CMOS devices has led to advancements in mixed-signal, system-on-chip ICs over the last decade. One of the most promising technologies for low-cost radar is that of single-chip digital transceivers. The demonstrator will be making use of the Analog Devices 9361 dual-channel direct-conversion transceiver as the core S-band digital transceiver component, sacrificing overall system dynamic range for cost while waiting for next-generation parts to emerge within the next year. With this higher level of integration comes the need for low-cost solution to interface with these exceptionally complex devices. To this end, the transceivers will be interfaced to a low cost MicroZed board which will perform critical radar tasks including capturing and processing the received data, generating the transmit waveforms, and controlling components in the downstream RF signal chain. At the heart of the MicroZed is a Xilinx Zynq Z7020 System-on-Chip (SoC) which combines two ARM9 CPUs and low-power 7-series 28 nm FPGA fabric in a single chip. The Zynq also includes hardened silicon peripherals such as a DDR controller, Gigabit Ethernet PHYs and SPI interfaces. As a result, more design time can be focused on core system functionality rather than implementing common peripherals. On-chip DMA and dedicated high performance data ports enable the ADC samples captured by the Zynq to first be processed very rapidly in the dedicated DSPs and then transferred to the CPU where more complex algorithms can be applied prior to data offloading.
The paper/presentation will present each of these components in detail, and will also include the latest results from demonstrations and testing of the system. In particular, antenna patterns measurements for the reflectarray and a multi-channel demonstration of the full transceiver chain are scheduled in the September timeframe, using modules, electronics, and antennas that are representative of the conceptualized final system.