WEATHER RADAR NETWORKS
National Weather Surveillance Radar Network
The WSR-88D, or the Next Generation Radar (NEXRAD), weather radar network comprises 156 long-range Doppler radars sited on a grid covering the contiguous United States (CONUS) as well as portions of Alaska, Hawaii, Puerto Rico, and Guam. The radars operate unattended according to selected scanning patterns, and radar data (reflectivity, mean radial velocity, and Doppler spectral width) and products derived from these data are disseminated to offices of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD), as well
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Evaluation of the Multifunction Phased Array Radar Planning Process 2 Overview of the Current National Radar System The US government civil radar infrastructure comprises four separate radar networks: the Weather Surveillance Radar 1988-Doppler (WSR-88D) and the Air Route Surveillance Radar (ARSR) networks that support national-scale weather and aircraft surveillance, and the Terminal Doppler Weather Radar (TDWR) and the Airport Surveillance Radar (ASR) networks that support regional-scale weather and aircraft surveillance in the vicinity of medium and large commercial air terminals. Table 2.1 summarizes key features of these networks, and Figure 2.1 shows the locations of the 510 radars that comprise the WSR-88D, TDWR, ASR, and ARSR networks. TABLE 2.1. US Civil Radar Infrastructure. Source: John Cho, Massachusetts Institute of Technology. Network No. Radars Deployment Freq. (band) Range Antenna Size Tx Power Update Rate WSR-88D 156 National grid S 460 km (Z); 230 km (V,W) 8.5 m dia 750 kW 4-6 min TDWR 45 45 largest airports C 460 km (Z); 90 km (V,W) 7.6 m dia 250 kW 1-5 min ARSR 101 Nationwide L 370 km (ARSR-1,2,3); 460 km (ARSR-4) 9 m × 7 m 4 MW (ARSR-1,2); 5 MW (ARSR-3); 60 kW* (ARSR-4) 12 s ASR 233 Commercial airports S 110 km 5 m × 3 m 1.1 MW (ASR-9) 20 kW* (ASR-11) 5 s *Uses pulse compression. (Z = radar reflectivity factor; V = Doppler velocity; W = spectrum width) WEATHER RADAR NETWORKS National Weather Surveillance Radar Network The WSR-88D, or the Next Generation Radar (NEXRAD), weather radar network comprises 156 long-range Doppler radars sited on a grid covering the contiguous United States (CONUS) as well as portions of Alaska, Hawaii, Puerto Rico, and Guam. The radars operate unattended according to selected scanning patterns, and radar data (reflectivity, mean radial velocity, and Doppler spectral width) and products derived from these data are disseminated to offices of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD), as well
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Evaluation of the Multifunction Phased Array Radar Planning Process as to the private sector and the public. Crum and Alberty (1993) summarize the timeline for the creation of this system, beginning with the establishment of the Joint Doppler Operational Project at the National Severe Storms Laboratory (NSSL) in 1977, proceeding through the award of competitive pre-production contracts to Raytheon and Unisys in the period from 1983 to 1989, followed by the award of a full-scale production contract to Unisys in 1990. The system was deployed as a joint program between the Departments of Commerce, Transportation, and Defense (DOD) during the early to mid-1990’s as a replacement for the 1957 (WSR-57) and 1974 (WSR-74) predecessor radar networks. The National Research Council (NRC, 1995) evaluated coverage of the NEXRAD system in comparison to that of the predecessor systems. FIGURE 2.1. Locations of U.S. operational weather and air traffic control radars. Source: Office of the Federal Coordinator for Meteorology. Source: OFCM, 2006. Terminal Doppler Weather Radar Network The Terminal Doppler Weather Radar (TDWR) network comprises 45 Doppler weather radars deployed at major commercial airports near medium-to-large-sized US cities with greatest wind-shear risk. This system addresses the FAA’s requirement for surveillance of weather close to airports with higher sensitivity and faster updates than can be provided by the WSR-88D system (Whiton et al., 1998). The TDWR system was developed in the late 1980s after low-altitude wind shear events caused a series of commercial aircraft accidents. It operates at C-band to avoid interference with the ASR radars, the WSR-88D, and other systems operating in the 2.7 - 2.9 GHz band. The system was manufactured by Raytheon on the basis of specifications developed by the FAA, Lincoln Laboratory, and the National Center for Atmospheric Research (NCAR).
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Evaluation of the Multifunction Phased Array Radar Planning Process The network is owned and operated by the FAA, but the NWS is developing a means for its forecast offices to access data from TDWR radars having coverage in specific metropolitan areas. AIRCRAFT SURVEILLANCE RADAR NETWORKS National Air Route Surveillance Radar Network The ARSR network is the nation’s primary radar means of detecting and tracking aircraft throughout the national airspace. There are four ARSR generations and a total of 101 L-band radars in this network. The ARSR-1, ARSR-2, and ARSR-3 systems were developed in the 1960s and deployed in the nation’s mid-section and inland areas. The more recent ARSR-4 system is deployed around the perimeter of the CONUS; this system was built by Westinghouse. These newer radars also have a capability for providing limited weather surveillance data. The DOD and DHS recently assumed responsibility for support of the operation, maintenance, and upgrades of this system. Airport Surveillance Radar Network The ASR network comprises 233 radars deployed at commercial airports near medium-to-large US cities. These radars are used principally for tracking commercial and private aircraft in the vicinity of the air terminal locations; they also provide some indications of precipitation, and a subset of the radars can detect low-altitude wind shear. SITING, MAINTENANCE AND LIFECYCLE ISSUES Leone et al. (1989) describe the site selection procedure for WSR-88D radar installations. The system is jointly owned by the NWS, DOD, and FAA, and each of these agencies established criteria for siting the radars based on factors including population distribution, climatology, approach and travel directions of severe weather, locations of airports and airways, and the location of NWS forecast offices and high-priority military and civilian facilities. The radars in the WSR-88D network, like those in the TDWR and ARSR networks, are long-range (> 400 km) high-power systems. In order to penetrate through heavy precipitation, long-range operation requires the use of wavelengths not subject to substantial attenuation. This in turn necessitates the use of large antennas to achieve km-scale spatial resolution throughout the coverage region. The radars use high-power transmitters1 and mechanically-rotated antennas that require 1 The latest-generation ASR and ARSR systems are being fielded using lower-power solid-state transmitters and pulse compression techniques to achieve the needed sensitivity. Service-life extension programs are in place to convert the early-generation transmitters in these networks to
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Evaluation of the Multifunction Phased Array Radar Planning Process dedicated land, towers, and other support infrastructure. The large physical size of these systems, combined with potential environmental impacts and possible interference by and to other radars, limits the availability of potential sites. Scanning of the WSR-88D antenna below 0.5° elevation is prohibited owing to public concerns about radiation safety. The acquisition cost of each radar unit (including radar equipment as well as land and other installation costs) in current dollars would be approximately $10 million. The annual per-radar operating and maintenance cost has been estimated by Lincoln Laboratory to be $500,000 per radar (JAG/PARP report, Appendix C). WEATHER RADAR COVERAGE The coverage of the WSR-88D network has been reviewed in several NRC reports (NRC, 1995; 2002; 2005). The protection of lives and property requires weather radar coverage from the height of cloud tops (~ 18 km or 60,000 ft.) down to near-ground level, where damaging storm features such as tornadoes, hail, and downbursts impact both the public and low-flying aircraft. The Weather Service Modernization Act of 1992 established a criterion that the network should provide complete coverage over the CONUS at a height of 3.05 km (10,000 ft.) above ground level (AGL) without degradation in service, compared with the WSR-57 and WSR-74 predecessor networks. Coverage of the WSR-88D network is nearly complete at 10,000 ft AGL over the eastern United States, while gaps exist in the mountainous regions of the Midwest and Pacific Northwest (Serafin and Wilson, 2000). A 1995 National Research Council study (NRC, 1995) investigated the adequacy of WSR-88D coverage relative to the detection and warning of a variety of weather phenomena (e.g., landfalling hurricanes, supercells, mini-supercells, mesocyclones, tornado vortices, microbursts, macrobursts, and various types of precipitation and snowfall). This study found that WSR-88D coverage over the nation was generally excellent in terms of providing superior forecasting and warning capability compared with the predecessor radar systems. The improved coverage and performance of the WSR-88D network has led to a significant improvement in the short-range forecasts and warnings of severe thunderstorms, tornadoes, and flash floods (Serafin and Wilson, 2000). Nevertheless, the report found a significant lack of low-level coverage that limits the detection of the full range of hazardous weather conditions over large expanses of the CONUS. This issue is discussed further in Chapter 8. solid-state as well. The WSR-88D and TDWR radars use high-power microwave tube transmitters.