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A Brief Overview of Weather Radar Technologies and Instrumentation

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A Brief Overview of Weather Radar Technologies and Instrumentation A Brief Overview of Weather Radar Technologies and Instrumentation Mark Yeary, Boon L. Cheong, James M. Kurdzo, Tian-You Yu, and Robert Palmer eather radar technologies and instrumenta- networks, and spectrum sharing. Next, we look at several tion play a vital role in early warning of severe hardware and signal processing technology examples related W weather. For example, the annual impacts of to these lists. adverse weather on the U.S. national highway system and roads are staggering: 7,400 weather–related deaths and 1.5 Hardware and Signal Processing million weather–related crashes [1]. In addition, US$4.2 bil- Technologies lion is lost each year as a result of air traffic delays attributed Severe and hazardous weather such as thunderstorms, down- to weather. Research on high-impact weather is broadly mo- bursts, and tornadoes can take lives in a matter of minutes. To tivated by society’s need to improve the prediction of these improve detection and forecast of such phenomena using ra- weather events. The research approaches to accomplish this dar, one of the key factors is fast scan capability. Conventional goal vary significantly with the inherent predictability of the weather radars, such as the pervasive Next Generation Ra- weather system. For example, the current forecast approaches dar (NEXRAD) developed in the 1980s, are severely limited by for issuing warnings of short-lived events, such as tornadoes mechanical scanning with their large rotating dish. Approxi- and flash floods, are primarily based on observations with a fo- mately 168 of these radars are in a national network to provide cus on advanced Doppler radar measurements. the bulk of the United States’ weather information. In 2009, a Research is currently often focused on engineering to de- program was initiated to field test the feasibility of upgrading velop new radar systems, research to better understand these these radars with dual–polarization capability for hydrome- storm systems to develop new meteorological and hydro- teor classification and improved quantitative precipitation logical uses of radar measurements for prediction, and on estimation. All of the NEXRADs were upgraded to dual polar- the incorporation of radar measurements into atmospheric ization by 2013. and hydrological prediction models through a highly math- Under the development for weather applications, the ematical process known as data assimilation. New research electronically steerable beams provided by the phased array frontiers include moving prediction increasingly toward radar at the National Weather Radar Testbed (NWRT) in Nor- kilometer-scale atmospheric modeling and radar-based warn- man, Oklahoma can overcome the limitations of the current on-forecast as opposed to warn-on-observation systems. NEXRAD radar [2], [3] (Fig. 1). For this reason, the phased ar- Weather radar, owing to operational wavelengths of ap- ray radar was listed by the National Research Council as a proximately 3 cm to 10 cm (i.e., approximately 10 GHz to 3 candidate technology to supersede NEXRAD. By definition, GHz) is ideal for penetrating regions of precipitation while a phased array radar is one that relies on a two-dimensional providing meaningful returns for weather phenomenon array of small antennas. The apparent phase of each antenna characterization. Three levels of technology, as listed below, is controllable, thus allowing the overall system to instan- provide a trifecta of a strong foundation through a vibrant taneously and dynamically steer to interesting regions of future. Technologies that have already emerged are dual-polar- weather. The National Weather Radar Testbed is the nation’s ization, multiple wavelengths, phased array radar (military, first facility dedicated to phased array radar meteorology [2]. single polarization), and gap filling radars (non-adaptive). The phased array radar at the NWRT is approximately Emerging technologies are enabling technology: solid- 12 ft (3.7 m) in diameter, has a peak transmitter power of 750 state pulse compression, polarimetric phased array radar, kW, and operates at 3.2 GHz. This radar is a spare from the and imaging radar or ubiquitous radar. In the future, we ex- U.S. Navy. When the phased array radar at the NWRT first pect Digital-at-every-element phased array radar, passive radar, became operational in 2004, only its sum beam was instru- cognitive radar, multi-mission networks, ultra low-cost dense mented for comparison to a nearby NEXRAD. However, eight 10 IEEE Instrumentation & Measurement Magazine October 2014 1094-6969/14/$25.00©2014IEEE Fig. 1. The National Weather Radar Testbed and the multi-channel receiver yields a system that supports multi-mission capabilities. (© IEEE, M. Yeary, used with permission, [3]). other available channels were available for instrumentation. Recently, however, pulse compression has begun to be ex- These were: six sidelobe cancellation channels, a differential ploited for use in weather radar systems. Higher resolution, azimuth channel, and a differential elevation channel. These both spatially and temporally, is a largely desired attribute additional channels were later instrumented with a suite of RF in future weather radar platforms. By utilizing higher band- down-converters and digital receiver assemblies [3]. The sid- width, range resolutions up to an order of magnitude higher elobe cancelling channels are receive-only auxiliary channels than the NEXRAD are possible. Spatial resolution on the order that are separate from the main array. Six of these channels are of tens of meters has been utilized extensively in the analy- located around the periphery of the array. These channels have sis of severe convective storms and tornadoes with research been designed to have low gain and wide beam width to detect radars and could provide the ability for improved algorithm unwanted signals, e.g., clutter, interference, and non-friendly performance and data quality in future operational weather jammers. With the recent deployment of an eight-channel radar networks. multi-channel receiver on the NWRT, the sum channel and sid- Temporal resolution is a critical aspect in the trends of elobe channel data can be recorded and processed to research weather radar observations, especially for severe weather that different techniques for addressing the clutter contamination changes on the order of seconds rather than minutes. Cur- problem. The multi-channel receiver project is a collaboration rent NEXRAD update times of greater than 4 minutes are between the Oklahoma University Advanced Radar Research widely regarded as insufficient for rapidly evolving convec- Center (ARRC) and the National Severe Storms Laboratory. tive weather. Future implementations of operational weather radars could very well utilize phased array antennas to dras- Waveform Design for Emerging Radar tically improve update rates for forecasters, but the number Technologies of individual transmit/receive elements required at a high Pulse compression and other advanced waveform techniques enough power for long-range weather observations would re- have been used in military radar systems for decades but have sult in high costs. Pulse compression is an ideal candidate to been scarcely used in the weather radar community due to make future weather radar systems more affordable by uti- the distributed nature of hydrometeor targets. As defined by lizing low-power solid-state transmit/receive elements and the IEEE 686-2008, Standard Radar Definitions [4], pulse com- transmitting a frequency modulated pulse. However, from a pression is: cost perspective, it is critical to retain as much of the additional power yielded from a long pulse as possible. Typical pulse … a method for obtaining the resolution of a short pulse with compression waveforms often make use of heavy amplitude the energy of a long pulse of width T by internally modulat- modulation in order to achieve low range side lobes, even with ing the phase or frequency of a long pulse so as to increase its nonlinear frequency modulated designs. While this can work bandwidth B >> 1/T, and using a matched filter (also called a well for large point targets, the sensitivity of a weather radar pulse compression filter) on reception to compress the pulse of can be improved significantly by limiting transmit and receive width T to a width of approximately 1/B. windowing. October 2014 IEEE Instrumentation & Measurement Magazine 11 Recent work by Kurdzo and his research partners has enhance data quality especially in clutter (both stationary and led to the design of weather radar pulse compression wave- non-stationary) identification and suppression, gain more in- forms that have very high frequency flexibility, leading to the formation about the microphysics of precipitation, and obtain need for much less amplitude modulation while also achiev- environmental parameters such as turbulence. ing low sidelobe levels. This technique has been primarily Weather radar can measure the refractivity field using re- demonstrated on solid-state dish antenna weather radar sys- turns from ground targets, which can be a good proxy for tems (with peak transmit powers as low as 100 W) but can be the near-surface humidity field. This additional information directly translated to phased array systems. When the cost obtained by weather radar has the potential to gain under- considerations of lower-powered solid-state transmitters are standing and to improve prediction of convective storms. taken into account on a phased array
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