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Evaluation of the Multifunction Phased Array Radar Planning Process 8 Family of Systems The Joint Action Group/Phased Array Radar Project (JAG/PARP) report outlines a comprehensive research plan to investigate the capability of a Multifunction Phased Array Radar (MPAR) system to replace the current radar network comprising the WSR-88D, Terminal Doppler Weather Radar (TDWR) and Airport Surveillance Radar/Air Route Surveillance Radar Version (ASR/ARSR) aircraft and weather surveillance radars. However, the entire set of surveillance needs described by the various Federal agencies as part of the JAG/PARP investigation and summarized in the JAG/PARP report, which are not all being satisfied by the current network, likely cannot be met economically with any one network of widely-spaced surveillance radars. An MPAR network may be able to economically replace the current radars and enhance some capabilities while lowering life-cycle costs. However, to fully meet the Nation’s surveillance needs, any future MPAR surveillance network must be viewed as a member of a family of sensing systems. The MPAR system as conceived in the JAG/PARP report would provide nearly complete coverage of the National Air Space (NAS) at and above 5000 ft above ground level, and low-level coverage of the atmosphere in the vicinity of the 334 radar sites. However, neither that MPAR system nor any architecture based on widely spaced radars (including the current system) can provide comprehensive vertical coverage of the NAS down to the surface. As discussed in Chapter 2 of that report, the MPAR concept does not address a key aspect of the DOD’s strategy for Homeland Defense & Civil Support (2005), namely, “the nation will need to develop an advanced capability to replace the current generation of radars to improve tracking and identification of low-altitude airborne threats.” Also, it does not address current deficiencies related to gaps in boundary layer coverage or meet the future needs for low-level radar coverage identified in numerous places in the report, such as the Federal Highway Administration (FHWA) needs articulated in Where the Weather Meets the Road (NRC, 2004). A 1995 National Research Council (NRC) study, Toward a New National Weather Service—Assessment of NEXRAD Coverage and Associated Weather Services (NRC, 1995) investigated the adequacy of WSR-88D coverage relative to the detection and warning of a variety of weather phenomena, including landfalling hurricanes, supercells, minisupercells, 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 WSR-57 and WSR-74 systems that preceded the WSR-88D. It is generally agreed that 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 incomplete low-level coverage limits the detection of the full range of hazardous weather over large expanses of the Continental United States (CONUS).
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Evaluation of the Multifunction Phased Array Radar Planning Process Table 8.1 shows the percent of vulnerable CONUS land-mass over which the WSR-88D system is incapable of detecting various types of weather events. These percentages, taken from calculations performed for the 1995 National Research Council study, reveal that the WSR-88D is able to observe certain hazards in only 29-69 percent of vulnerable regions. The NWS does make use of TDWR and ASR data to address some of the gaps in populated regions where these radars provide additional coverage. TABLE 8.1. Fraction of Vulnerable CONUS Land Mass Where WSR-88D Coverage is Inadequate to Detect Specific Weather Events. Source: NRC, 1995. Event Insufficient Coverage Fraction Supercell 29% Mini-supercell 69% Macroburst 42% Lake effect snow 46% Stratiform snow 31% Westrick et al. (1999) assessed the impact of limited WSR-88D coverage for detection and quantitative estimation of precipitation amounts over the US west coast regions. This study concluded that, as a result of significant terrain blockage in that region combined with shallow depth of precipitation during cold seasons and low melting levels, 67-75 percent of the land surface in the region has inadequate radar coverage to support quantitative precipitation estimation. Figure 8.1 shows the coverage provided by the combined WSR-88D, TDWR, ASR, and ARSR systems at 1,000 ft Above Ground Level (AGL). The white spaces reveal that the majority of the airspace at the 1000 ft level is not observed by these radar networks. This fundamental limitation of the ability of any widely-spaced network of ground-based radars to observe close to ground level results from both the curvature of the earth and blockage of the radar beam by mountainous terrain. An MPAR network like that envisioned in the JAG/PARP report may be able to economically replace the current weather and aircraft surveillance system, and possibly enhance its capabilities while lowering life-cycle costs, but it will not be the entire national weather and aircraft surveillance solution. A number of DOD and DHS systems are currently used to help meet NAS surveillance requirements and likely will continue to be a key part of the NAS surveillance system. Weather surveillance is supplemented by a variety of independent radar and non-radar systems. Other new sensing systems which could address portions of the national surveillance needs are also being developed, including low-power, low-cost boundary layer radars (see Box 8.1) and acoustic and lidar systems.
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Evaluation of the Multifunction Phased Array Radar Planning Process Consequently, any proposed MPAR system must be designed and developed as part of a larger family of systems. Economic and design tradeoffs must be considered across the entire family of systems in order to meet national surveillance requirements by the most economic means. FIGURE 8.1. CONUS coverage at 1000 ft AGL for the combined WSR-88D, TDWR, ASR, and ARSR systems. Source: Weber, 2007; printed with permission from the American Meteorological Society. Recommendation: MPAR system design studies and analysis of alternatives should consider the MPAR system as a candidate member of a family of systems, carefully considering design and mission tradeoffs with existing and new surveillance capabilities under development. Agencies must define clearly the role that MPAR will play toward meeting their needs and identify the supplemental sensing networks required to fully meet their needs.
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Evaluation of the Multifunction Phased Array Radar Planning Process BOX 8.1 Low-altitude coverage A fundamental limitation of any network comprising widely-spaced radars is the inability to comprehensively cover the lowest regions of the troposphere, owing to both the curvature of the earth and terrain blockage. The solid curves in Figure 8.2 show the percentage of the volume in a thin layer at various heights above ground level covered versus radar spacing, assuming a smooth earth. At 230 km separation, which is the approximate spacing of the NEXRAD radars in the eastern half of the United States, coverage is nearly complete at a height of 3000 m but decreases to less than 10 percent at 300 m above ground level. Denser radar placement can overcome this limitation, but larger numbers of radars would be needed in the network, as shown in the dashed line in the figure. Achieving comprehensive coverage down to 300 m, for example, would require a network of several thousand radars spaced tens of kilometers apart. Realizing such a network cost effectively would require substantial reductions in radar acquisition, siting and recurring costs compared to today’s radars. The Center for Collaborative Adaptive Sensing of the Atmosphere (CASA), an Engineering Research Center chartered by the National Science Foundation, is investigating the feasibility of small low-cost radars and the associated software architecture and data handling issues that would enable future deployment of such networks (McLaughlin et al., 2007). FIGURE 8.2. Volume coverage at different heights (solid lines) and number of radars needed for CONUS coverage versus radar spacing (calculations based on smooth earth).